Aligning-- Polar Concerns--Page 1

       

Subject: Accurate Polar Alignment--Drift & Iterative   

From: Philip Perkins <philipastrocruise.com>

Here's a reminder about two articles that deal specifically with polar alignment of the LX200:

   <www.astrocruise.com/polarnew.htm> Detailed description of the DRIFT method of polar alignment, complete with Field Document.

   <www.astrocruise.com/polarold.htm>  Detailed description of the ITERATIVE method of polar alignment
   (clarifies and expands upon the method described in the Meade manual)

Subject: LX200 Polar Alignment Routine --part 1 of 2    

From: Chris Vedeler <cvedelerix.netcom.com>

This may be nothing new, but it occurred to me tonight that I could polar align my LX200 Classic within it's pointing accuracy (about 2 - 5 arc minutes) simply going back and forth from Polaris to other known stars using the LX200's internal map of the night sky. There is no need to let any time elapse between iterations as the scopes pointing accuracy will tell you immediately how far off you are. This is what I did:

I centered the scope on Regulus and synched on it. Then I hit GoTo star 19 (Polaris) and the LX200 moved the scope to where it though Polaris was located. Using the Superwedge adjustments I then centered Polaris. (Note -- if you remove only half the error at the Polaris step, it will converge faster--Bob Denny, see part 2 following.) Then slewed back to Star 100 (Regulus). Then using the hand controller I centered Regulus and synched on it again. By repeating this process about 5 times I was able to get the scope to slew from Polaris to Regulus with less than 30 arc seconds of error. I then checked the pointing accuracy to other stars all across the sky, and they were all within 2 to 3 arc minutes (within the field of view of my 9mm reticle eyepiece). Spica was the only star which did not end up well within the field of view of my 9mm reticle (250x) eyepiece, but it was close enough to the edge that I could easily tell which way to move the scope to center it.

To test the new polar alignment I let the scope track on Spica for 30 minutes. When I returned to the scope to see how much Dec movement there was at 250x, I was surprised to find none what so ever! In less than 10 minutes of polar aligning, I aligned my scope to show zero Dec movement in 30 minutes!

Synching on Regulus sets the LX200's computer to think that it is pointing at Regulus (as it is in reality). Slewing to Polaris, the LX200 moves to where Polaris would be if the scope was perfectly polar aligned (assuming perfect pointing accuracy and perfect time and date settings). By manually moving the wedge to center Polaris, one is in effect reducing the polar alignment error. By slewing back to Regulus it permits a test of the new alignment. Doing this a number of times will widdle away at the polar alignment error until the only error remaining is no more the error in pointing, or no greater than about 5 arc minutes on a typical LX200. This accurate of polar alignment is probably about as good as could be realistically obtained with the LX200 regardless of the method used.

Using this method is quicker and easier than using the drift method, and from my experience tonight, it is more accurate as well (assuming your scope is pointing well, and your clock is set accurately).

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Subject: LX200 Polar Alignment Routine --part 2 of 2     

From: Bob Denny <rdennydc3.com>

Right. This is what Astronomer's Control Panel's Alignment Wizard does, with a couple of refinements. It knows about Sigma Octans for the Southern hemisphere. It picks the "other" alignment star for you, and gives you a selection of alternates in a droplist if you find the primary is obscured. It also does a 2-phase Dec-axis bias calibration as a first step.

One thing to note -- if you remove only half the error at the pole star step, it will converge faster.

Assuming your scope is sited and timed "right", the results are comparable to drift alignment and it takes much less time.

Subject: Southern Polar Alignment Suggestions    

From: Doug McEachern <applejuiceozemail.com.au> Date: Nov 2003

----- Original Message -----
From: Peter Narkauskas<wunilimira.net>
> I don't know why, but polar alignment has me stumped! I'll try and explain
> what happens on my 12" LX200 on Meade (unmodified) Superwedge.
> I am in the Southern Hemisphere (Birregurra, Australia)

> Okay, scope placed in 'Pole Home' position and tripod level. Checked and
> found stars rotating around centre of EP. Autostar in Polar mode and then
> tried easy and 1 star alignment, both times went to Sigma Octantis - was a
> bit difficult 'cause of the Moon brightness, adjusted wedge to align that
> star. Then Autostar chose Canopus, I aligned with keypad arrows.
>
> I went on to try drift alignment for the first time, but, and this has
> happened a couple of times previously to me at this point, when I did a
> GoTo, the object was well and truly off the mark. Anywhere from about 1-5
> degrees out and to the left of the object (when looking straight up at the
> sky). Even a GoTo canopus was out like this. So something is obviously
> wrong with my setup at this point. I tried this Polar alignment about three
> times last night and each time my GoTo's were way out.

> Nevertheless, I still tried a drift alignment. Chose a star (with the help
> of Astroplanner) to the North, near the Meridian etc. The star drifted down
> in the EP, so I adjusted the horizontal axis of the wedge. But no matter
> which way I went (and I mean I went to the extremes both ways), I could not
> make any difference at all to the star drift. It did not improve or change at all.

> I have read as much as I can find on the web. But find that confusing. For
> example the Meade manual says leave the motors running when polar, aligning,
> but I read elsewhere to leave motors off. I have often found conflicting
> advice. (I kept the motors running last night).

> I know this is not really that hard, but like I said, I am extremely
> frustrated and lost as to what I am doing wrong. In normal Alt/Az my
> GoTo's are pretty okay and have always been. I have the 1.7k version of
> Autostar. What I find particularly weird is my GoTo's in polar mode are way
> off. Perhaps I am not picking Sigma Octantis properly...it is a hard star
> to pick...we are not so lucky to have a Polaris down south here! But I have
> studied star charts to get an idea of the area and I have slewed in Alt/Az
> mode a few times to Sig Oct to get to know it. Even if I picked a wrong
> star close by, surely my GoTo's wouldn't be that far out?? Or would they??
> Once again, I am sure my initial setup in Polar home is correct. And yes, I
> allow for pointing true south etc.

Just a couple of suggestions for you--

Firstly just to be absolutely certain that you are picking true south, you might want to do a quick Alt setup which you say is ok. Then when you are happy with that, GoTo sigma Oct. It is star #351. When you have it you now know exactly where true south is both in elevation and Azimuth. As for Azimuth, mark a point (maybe on your fences, buildings) so that you have a permanent line running N/S. Refit your wedge, make sure you have the polar angle set to your latitude. Note that you already have a good idea from your altaz setup what the angle should be. No insult intended but it might be possible to have set your wedge to the compliment of your latitude angle. What I mean is as latitudes approach 45 deg the difference between 40 and 50 is not as obvious as say between 30 and 60 (just a thought). I'm not sure what your latitude is down there in Vic, up here around Sydney it is 34 deg, that is why I guess you are getting up around Lat 40. In my set up I can't even see Sigma Oct. thanks to a wonderful. neighbor that thinks everyone should live in pine forest. I just press enter as soon as my 12" LX200 thinks it has found Sigma; why disillusion it? And then centre the next star it selects. I have found though that if I want really good GoTo I have to turn on the High precision feature. Not bragging but in case you are wondering, I can track unguided for several hours in Polar so don't confuse accurate GoTo with accurate polar alignment.

The other thing to make sure of is that you set the N/S switch to the south position. I think you say that you do, but I'm not sure. That switch has no effect in Alt/Az mode that I could ever find, but it a must for polar, but I guess you would know if that was the problem.

Subject: Explanation for the Polar Alignment Problem --Classic   

From: Ralph Pass <rppassrppass.com>

The problem is easily understood (see below for explanation). The bottom line is that you should avoid using stars with RA close to that of Polaris (2h 15m) or close to 12h away (14h 15m). The closer the 'sync' star is to 2h 30m the more likely you will converge to a bad alignment (mathematically there is a singularity at RA 2h 15m with this procedure, also known as bad geometry). The closer the 'sync' star is to 14h 15m the more likelihood of the near perpetual oscillation. This is also because of bad geometry. See below for more details.

The polar alignment procedure provided by Meade is for convenience. If you manually do the procedure (where you pick the second star), then you can control the geometry and can avoid frustration and waste of time. The closer the second or 'sync' star to RA 8h 15m or 20h 15m the better and faster the alignment.

In the northern hemisphere, Deneb (Alpha Cyg, star 232) is close to optimal for the 20h 15m area. Pollux (Beta Gem, star 81) or Regulus (Alpha Leo, star 100) are about as well as you can do for bright stars around 8h 15m.

In the southern hemisphere, Avior (Epsilon Car, star 86) is close to optimal for 8h 15m and Alpha Ind (star 231) is close to optimal for 20h 15m.

1. Stop reading now if you are not interested my attempt to explain the bad geometry problem.
   Suppose the local sidereal time is that of the RA of Polaris, 2h 15m. Suppose that by chance your mount is perfect in azimuth and off by 10 degrees in altitude. You go to a star with RA 2h 15m besides Polaris, center it and sync on it. The LX200 has now adjusted for a presumed bias in the dec axis readout of 10 degrees. If you now slew to Polaris, the LX200 will move in Dec by the distance between the two stars and amazingly Polaris is centered and you think the alignment is perfect. The 10 degree error in elevation was corrected by the 10 degree bias introduced in the dec readout. This happens because the RA does not change. With a little thought you can see that if the Polar axis is misaligned but points to a point on the RA 2h 15m semi great circle then the problem is there, although with a combination of AZ & ALT errors and Dec & RA readout 'corrections' applied by the LX200. You cannot beat this system with the second or 'sync' star being at RA 2h 15m.
2. Suppose the local sidereal time is that of the RA of Polaris plus 12 hours, 14h 15m. Suppose that by chance your mount is perfect in azimuth and off by 10 degrees in altitude. You go to a star with RA 14h 15m, center it and sync on it. In doing so you will introduce a bias in dec readout of 10 degrees, just as
   in the first case. You now slew to Polaris. In doing so, the RA axis rotates by 180 degrees (12h). and is now pointing 20 degrees from Polaris (10 degrees for the altitude error and 10 degrees for the bias in dec readout introduced when you synced on the star). You correct by 20 degrees. Now suppose we slew back to the first star. We will find an error of 20 degrees. If we center and sync and repeat, then we are in an infinite loop. It is this observation that has led people to suggest making mechanical adjustments of half the observed error at Polaris. If you do this when the LX200 picks a star close to 14h 15m, then you will see a rapid convergence to polar alignment. If you do this with a 'good' star like Deneb, then you are making the process take longer.
   

Subject: Polar Alignment -- Iteration Method 

From: Chris Heapy <Chrisheasynet.co.uk>

Consider:
If you set the scope to Ohr RA and 90°DEC, as described for Polar Aligning, then the LX200 will slew around to the offset required to center Polaris (about 89.2°). You will then centre Polaris using the wedge adjustments. However, this step is not quite as simple as it seems. All the following assumes that you have correctly set the time, the time zone and the observing site.

There are three sources of error in the Polar-Align routine which will prevent the LX200's 'Polaris-finding' function from working with anything like the precision required:

1. The OTA may not be *exactly* orthogonal to the mount's axes (this is a factory setting - see Michael Hart's DEC bearing mod for details of resetting this). This error cannot be corrected by polar alignment.
2. The setting circles may not be correctly registered with the mount's Polar and DEC axes (example., the circle reads 89 deg, but the scope is pointing at 90 deg).
3. You would not have set the mount to *exactly* 0hr RA and 90° DEC, you just made the best judgment you could by lining up the marks on the circles, something of a crude estimate at best.

Normally, the second stage of the Polar-Align routine then slews the scope to an alignment star (e.g., Altair), you move the scope to center it in the FOV using the handset controls, and finally press 'Enter' synchs on it. This gives the computer the two datum points it requires from which it can then calculate the position of all other objects in the sky.
However, after this last step, if you do a 'GoTo' to Polaris you will most likely find it's not in the center of the FOV (maybe not even *in* the FOV). Why?? Well, the amount by which Polaris is misplaced is the summation of the errors listed above.

What can you do about it? - Perhaps the easiest is the 'interation' routine. I use it myself and it works well. Fine tuning can subsequently be done using the 'drift method' which will get your alignment very close indeed. The latter is only really necessary for astrophotography and not visual use.

For the Iteration Method:

1. After the last step mentioned in the example above (synching on Altair), do the GoTo back to Polaris. This position is where the LX200's computer 'thinks' Polaris is.
2. Use the wedge's adjusters to move Polaris back into the center of the FOV (using a 9mm reticle eyepiece is better than guessing the center of the 26mm's field).
3. After that, press STAR, and select Altair again, and press GoTo. The telescope will slew back to where it thinks Altair is. Of course this will now be misplaced too (because you moved the wedge).
4. The next step is to move Altair's position in the FOV (using the hand controller only) until it is *half-way* from where it is now, to where it should be meaning the center of the FOV.
5. After moving it hold ENTER down for a couple of seconds to synch on it.
   Next step is to select STAR again, choose Polaris and do a GoTo. Polaris should not have moved much from the center of FOV - if it has, bring it back to the center. Do NOT hold ENTER down at this stage - you don't want to re-synch Polaris' position.
6. The next step is a repeat - press STAR, select Altair, do a GoTo. Now, this time you will see that the positional error is less (about half actually), and once again you want to move Altair half the distance toward the center of the FOV and synch on it.
7. The rest of the process is to repeat the cycle, Altair to Polaris and back again, each time shuffling Altair closer to the center of FOV. After a half-dozen cycles (iterations) the error will be very small.

Notice that at no time in this process have you re-synched the position of Polaris, only Altair. This is important. At the end of this process any further GoTo's should be correct.

It's worthwhile at this stage refining the alignment using the drift technique, and then checking the exact positioning of your DEC circle (the manual mentions this step). Use the handcontoller to point to 90 deg and ensure the scale also reads 90. If it doesn't, you will need to loosen the center knob and turn the circle until 90 lines up with the mark. Correcting any error in the DEC will help speed up polar alignment next time out.

Subject: Polar Alignment--Drift Method   

The polar alignment description in the LX200 manual is pretty good, but try this description from Chuck's Astrophotography Page and Scott Tucker's Guide to Astrophotography site:

      <www.aa6g.org/Astronomy/Articles/drift_align.html>
        Note: should open a new browser page.

Chuck's Home Page has links to other interesting articles:
      <www.aa6g.org/astro.html> Note: should open a new browser page.

Also a good polar (drift) alignment description is available on Scott Tucker's site:
      <www.darkskyimages.com/guide.htm>

Subject: Drift Alignment Procedure     

From: Bruce Johnson, Date: Jan 2001

I did a writeup on 'drift alignment' for SCTs. It just might help some of the people having trouble with it.

<www.mapug-astronomy.net/ccdastro/drift-align.htm>  Note: should open a new browser window.

Subject: Polar Alignment with CCD? --part 1 of 3 

From: John Teel <mapuglist2yahoo.com> Date: Jan 2002

Basically just you the autoguide feature to track declination drift. Of course, you need to turn off guiding corrections in declination but leave RA corrections on. I use CCDOPS on my SBIG camera and autoguide with only RA corrections. It shows what the current declination drift is but also allows you to log the error into a text file that allows easy importation into Excel so I can create drift graphs. This is much more accurate than just eyeballing the drift in an eyepiece. See next...

--------------------------------

After spending a couple nights trying to absolutely perfect my polar alignment I'm beginning to think that using a CCD to track for drift alignment is NOT the best way. I've read others making this same statement. I think the biggest problem is that it requires absolutely perfect seeing because the CCD isn't smart enough to tell the difference in seeing and drift because of poor alignment. Maybe I'm not either but I think the human eye can differentiate between seeing and drift better than a CCD. I've found it very very difficult to get absolutely zero drift according to the CCD. About the best I can get it 1-1.5 arc-sec every 10 min. If I make the smallest azimuth/altitude change I can (I watch star movement to monitor how much I changed it) it seems to overshoot the ideal alignment and I get drift in the other direction. The seeing lately hasn't been very good so I think this may be a factor. All I know is that I've got it good enough that my GoTo accuracy is nearly dead-on everytime (i.e. object within 20% of CCD center) and my periodic error dominates for unguided exposures under 30 minutes or so. I tried desperately to achieve perfect polar alignment but I'm getting tired of making adjustments. I may try one more iteration using my old reticle eyepiece. I think they same problems apply when using a CCD to guide while training PEC and I think the human eye may be a better choice for this.

----------------------------

Subject: Polar Alignment with CCD? --part 2

From: Jim Seargeant <JimSargeattbi.com>

I use a CCD to polar align because I don't want to de- and re-mount the CCD every time. My CCD scope is pretty well fixed in place, but the mount isn't very stable - got'a get a pier installed.

I use dial indicators to measure wedge movement in R.A. and Dec. A .001" measured movement equals to about 34 arc sec and I can interpolate to about half or a quarter of that. (I just got a dial test indicator that will measure to .00005", but don't know if I can move the wedge in such small increments - haven't tried it yet.) By measuring actual movement of the wedge, it's possible to adjust based on drift/time and really home in on 0 drift. Unless you measure wedge movement, it seems to me very difficult to avoid overshooting.

I've got it down to 0 pixel drift over 20 min, but it takes me the better part of an hour to get there in both axes.

Did a previous reply on this topic suggest using CCD images compared to a high definition star field - such as USNO A2.0 and the DSS- to point the mount at true North? Sounds worth a try - I'll do that when it clears off.
Editor's note: see part 3 below.

----------------------------

Subject: Polar Alignment with CCD? --part 3 of 3

From: Berto Monard <LAGMonarcsir.co.za> Pretoria, South Africa

Hi Mapug readers, as promised, herewith my approach to polar alignment using a CCD camera which, in my case is a dedicated one, permanently attached to the scope, an LX200 12" f/10. As a matter of interest I have mounted the ST-7E CCD camera via a Meade focal reducer f/3.3 and the (with it) delivered short spacer piece with T-ring fitting. This combination gives a very sturdy connection which allows the CCD camera to even travel through the fork (if necessary). The result of this set-up is an image size of 19.2 x 12.8 arcminutes. The telescope is then effectively a LX200 12" f/4.2 instrument. I have also made the flat top part (with the attachment platform for power supply and PC connectors) of the CCD camera parallel to the declination axis. The flat top is also on the side where the finder scope is. This positioning produces images with North up, and East to the left, the way that DSS images show it. I think it must be different in the Northern hemisphere.

I hereby wish to thank all those that contributed towards my earlier queries on focal reducers and adapters.

I am a mechanical engineer since 1974 but never really practised engineering as a profession. Sometimes I feel sorry about that. However I have always used my acquired engineering problem solving approaches based on my training to reach high standards in whatever I did (at least I like to think so...). One such approach is the 'back to basics' approach.

What is the aim of a polar alignment?
Answer: we want the RA rotation axis to point exactly to the North (or South) pole. The only way we can check that is if we are able to point the OTA in the same direction as the RA axis and check it then via the eyepiece or a CCD image.

The procedure to align the OTA accurately to the RA axis can be done in the day by viewing a distant reference point. But you have to wait until night to do polar alignment.

I will here describe the procedure to do both at night. This is the way most applicable for those with permanently housed telescopes. It only requires a coarse polar alignment (within a degree), or an established method to do so. Obviously you need an open/clear sky towards the pole...

The first task: align the OTA roughly in the direction of the RA axis, i.e. set the declination to 90 degrees. It would be nice if we could do that by relying on the DEC setting circle, but for that it needs calibration first. In my case it is a full degree off!

Actually we do not need exact alignment in DEC, just to within some 10 arcminutes depending on the actual FOV depicted on the CCD chip. The alignment must be preferably better than half the FOV. But then even that is not required initially.

We can use the CCD images (from 'dim' exposures of 3 seconds in focus mode (CCDOPS)) to tell us where we stand. All we must do is to rotate in RA manually (with the LX200 switched off!!) or, if we are reasonably aligned to the pole, by pressing E or W in slew mode during the exposure time. The image will show concentric arcs around a centre position. That centre position is the apex of the RA axis and will not be at the centre of the CCD image. What we want to achieve is to align the centre of the arcs to the centre of the CCD image, not? The only ready way to do that is to fine tune the Declination. The continuously updating exposures will show an eventual improvement by having the two centres coming closer. However, what you will notice is that the two centres will never match. They'll come closer up to a point and then start moving away from each other again. Possibly (in a bad case, or with a narrow FOV) the rotation centre (RA apex) might not even enter the image...

In my case it stays at best some 7 arcmin away from the image centre. I can't get it closer. What this means is that my DEC setting is now optimized (90 degrees) but there is a misalignment from the Dec. axis itself. It is not 90 deg. to the RA axis, but some 7 arcmin off that. Note: Try to get this information (RA apex position and the approx. size of the misalignment) with a reticle eyepiece!!??

What can we do about it?
I don't know the specifications claimed by Meade on the OTA alignment accuracy. The only possible adjustment is to slide the OTA where it is mounted onto the fork. There are Allen screws holding the OTA connected to the fork mount. Since there is no system for fine adjustments (at least I didn't see them), it is very likely, unless one has an optical workshop, that after loosening those screws it will be difficult to even get back to the previous accuracy. But then I claim significant practical ignorance on this matter. I just have a bad feel about doing this at home... (Editor's note: see OTA Alignment Procedures topic in the Topical Archive.)

Next point: to align your RA apex to the pole by using the wedge adjustments..
Keep the CCD focus mode going, consult the 30'x30' or 15'x15' DSS image of the polar region, centered on the actual (2000 pole) and try to match the RA apex from the CCD image with the centre of the DSS image (2000) using the alt and az movements of the wedge. You can possibly derive the pole position for 2002, but I don't think it will make much difference. The wedge movements are far too coarse to worry about that. But then others have taught you how to improve the Superwedge... (Editor's note: see Wedge/Tripod Modifications topic in the Topical Archive.)

Once I had aligned to the South pole to the best of my ability and I pressed E or W in slew mode during a 3 second exposure, I saw concentric star trails around a point which was within 2 or 3 arcmin of the South pole which is shown at the centre of the DSS pole image. As mentioned above, this centre is some 7 arcminutes from my CCD image centre. I can't do much better without taking a risk..

In conclusion I would like to put things in perspective. The well known tracking problems in RA, the backlash, the sagging of the OTA and mount (in RA and DEC) and other trade-offs in the construction of LX200 telescopes will always limit expectations for precise pointing and tracking. I suggest one should polar align just good enough so that its influence is only a minor factor in the error budget.

Subject: Polar Aligned Start-Up Position 

From: Vince Gardiner

Regarding a way to avoid the polar alignment 2-star routine, we know that in Alt/Az mode the 'scope needs to be in a certain position at boot up so it knows its orientation. I thought it may also have a position in polar mode. It appears so. You do need to have your declination setting circle accurate. (Pointing straight up it should read your latitude). Align the two RA pointers together, point the scope to zero declination (the equator), and power on. If necessary, GoTo an object and 'synch' on it and bingo! Simple. Of course the scope must be in polar mode first and accurately polar aligned.

Subject: Polar Alignment Questions Answered --- Part 1 of 2    

From: Philip Perkins <philipastrocruise.com>

> I have a few questions about polar aligning my 8" f10 LX200 v3.34 Classic

> on a standard wedge. Until now I have only used it in alt-az.

> 1. Can anyone explain why the polar alignment routine requires the finder

> (and eyepiece unless you rotate the diagonal) to be UNDERNEATH? It's

> driven me to distraction tonight, the first clear night in weeks, trying to

> use the iterative method (a la Philip Perkins) as I keep on having to get

> on my knees and look upwards through the darn things. I have a 9x60 RA

> finder, BTW which is usually very convenient in use; as far as I can see

> this technique would be impossible with a straight through finder as you

> could not get to the finder eyepiece to look through it. Related to this

> question, the scope will point to, e.g. M81 & M82 with the eyepiece and

> finder the right way up, but then a slew to anywhere near the pole puts the
> scope upside down again. Why???

This is because Polaris is not at exact celestial north, it is about 0.8 degrees away. This means that Polaris rotates around the celestial pole just as other stars do. In order for the LX200 to land on Polaris it therefore has to match the rotation of Polaris by an equivalent rotation of the RA axis. It just so happens that in winter the amount of RA rotation required positions the LX200 'upside down'. This means that the finderscope is upside down which I agree, can be a pain in the neck. It's somewhat easier if you use a Telrad or suchlike.

There is another reason why the Iterative method can be a problem in winter months, and that is because you're likely to be Synching on a star in the same sky quadrant as the zero hour angle and the closer you are to 0 RA the bigger the error that will creep into the Sync.

> 2. I found that successive iterations between Polaris (centered using wedge)

> and Aldebaran (centered using keypad, and synched) gave me a similar error

> each time, with adjustments in altitude always being in the same direction,

> which implies that I am driving the alignment further out each time. When

> I finally got reasonable alignment so that it would go from one to the

> other repeatably, pointing accuracy on other stars was off by about half a

> degree. Synching on a new star would then give reasonable pointing in that

> part of the sky. I guess this means my polar alignment was not accurate.

> Pointing in alt-az has always been good.

I have experienced similar problems. I believe this is due to either pointing inaccuracy or Synching on a star near to 0 RA (as described above) or both.

> 3. I used to use my 9x60 finder on an LX50 for polar alignment, as it has

> a polar alignment reticle, but never checked by drift and didn't use it for

> prime focus photography, so don't know if it was really accurate. I tried

> this method instead of the above, and then tried a GoTo Polaris, which was

> off by over half a degree. Any suggestions?

Nigel, the Iterative method can work very well, however it is *totally* dependent on pointing accuracy. Therefore it can only work as well as the pointing accuracy of your scope. It worked very well for me when I first got my LX200 because I was lucky enough to have one with good drives (initially); however as the gear trains started to wear I got more and more errors creeping into the Iterative method. I soon got into a never ending 'to and fro' mode - no matter how many times I slewed between Polaris and the Second Star, Polaris would never converge. Recently there has been evidence of quality problems in Meade drives, and even new scopes can have problems with pointing accuracy.

There is a solution to all of this -- the Drift Method. The Drift Method is much easier than you might imagine and can also be quite quick once you get the feel of it. You may want to access:
   <www.astrocruise.com/polarnew.htm>  Note: this should open a new window over this page.

There you will find a complete description of the Drift Method as it applies to the LX200. The Drift Method has the following advantages:

1. It will yield very accurate polar alignment - the degree of accuracy depending upon how long you are prepared to drift for. No drift for 5 minutes per axis is enough to achieve sufficient accuracy for almost all astrophotography applications.
2. It is simpler to understand and implement.
3. Has no dependency on pointing accuracy at all. It will work equally well on a scope that does not even have pointing ability.
4. No continual slewing between Polaris and the Second Star, and this saves the drives from wearing out prematurely.
5. Has no dependency on atmospheric refraction for stars that are close to the horizon.
6. Has no dependency on pointing errors induced by Synching on a star that is close to 0 hour angle.
7. The drift process is an ideal time to become familiar with sky conditions, degree of turbulence / transparency, behavior of the drives (periodic error, declination backlash, etc.) -- I find this information an essential precursor to an astrophotography session.

There is just one disadvantage -- initially it is likely to take you longer than the Iterative method, however after a little experience you will find that it takes no longer, and may even take less time. This is especially true if, like me, you were at it with the Iterative method for half an hour or more and still did not have polar alignment at the end of it.

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Subject: Polar Alignment Questions Answered -- Part 2 of 2    

From: Michael Richmann

This problem's easier to solve than you might think. I typically use my 8" LX200 for astrophotography and have to transport it to a dark site. So quick setup is crucial to maximize the amount of time spent getting photos and a big part of that is getting a precise polar alignment in a minimal amount of time. My general procedure is as follows: Setup your scope normally (wedge facing due south, etc.). Align the scope on a reasonably bright star and use it to align your finderscope with respect to the main scope as accurately as possible. Next, start the polar alignment procedure as you normally would. When the scope slews to the position where you would align the equatorial wedge on Polaris, use the finderscope to align the wedge, not the 90 deg. diagonal/9-12 mm eyepiece combo on the main scope. It's much easier this way and hair splitting alignment on Polaris is not necessary at this point.

Next, continue on with the alignment and allow the scope to do its slew to the bright overhead (celestial equator) star. Use a high power eyepiece at this point *without* a diagonal. Otherwise, there's a very good chance you'll introduce an offset unless the diagonal is very precisely constructed.

Now, instead of following Meade's instructions to use the keypad to center the star, use the wedge adjustments. The first adjustment on Polaris was just a crude one to make sure your bright overhead star would definitely be in the field of view. At this point, you're ready to go ahead and follow the standard Meade instructions for drift aligning the scope.

So far, this technique has worked like an absolute charm and virtually never requires that I move the drift testing stars more than about a full viewing field's worth in azimuth w/a 9.7 mm eyepiece and no more than about a half field in latitude. Drift align till you see no apparent motion in either the azimuth or latitude cases for at least 5 minutes (I prefer 10) and you should be good to go. Also, since you'll end up tweaking the various adjustments, it's not a bad idea to slew to a known star and SYNC the computer's position to that star.

As for your specific questions, in order:

1) Polaris is about 0.8 degrees away from the true north pole at about RA 2:28. Depending on the local time and the particular time of year, your scope may slew to an inconvenient position. Not pleasant but you'll learn to live with it. The trick of using the finderscope alone was partially an attempt to make things easier as well as remove the error introduced by using a diagonal in the alignment process. When the finder scope is impossible to use in this case, I skip the initial alignment on Polaris step and continue on to the bright overhead star and use the finderscope and then high power eyepiece combined with the wedge adjustments to achieve the "normal" or non-precision polar alignment, then I drift align.

2) The misalignment might be due to the use of a diagonal or another good possibility is the fact that the mount is not perfectly rigid. For this reason, I use the high precision option when doing the critical alignment of the camera/scope before taking a photo.

3) Without knowing whether the polar alignment finder scope is carefully aligned during installation on the LX200, it's difficult to say what the problem most likely is.

Subject: Polar Drift Alignment Described   

From: Michael Hart

Typically, we use Dec drift methods to tweak polar alignment in part because this eliminates RA drive clock and worm/worm wheel errors. Three things happen when we are NOT polar aligned:

1. The stars appear to drift in RA.
2. The stars appear to drift in declination.
3. The field rotates.

The term "declination drift" itself implies a polar aligned scope will drift in declination only. In fact, a scope that is not polar aligned drifts in RA as well, however, we ignore any RA drift during the process as Ric states and watch "declination drift". As we approach polar alignment, RA drift slows as well. The reason for RA drift is easy to visualize. If our mount is not quite aligned with the celestial poles, the amount of misalignment formed causes the RA to appear to drift. The greater the misalignment, the faster the RA drift. The idea is to line up our scopes to turn parallel with the earth's rotation. Anything less than parallel produces RA drift. The result is a drive tracking at perfect sidereal time will appear to drift in RA.

Those experiencing unusual RA drift should assure polar alignment is quite accurate and then erase and reprogram their smart drive which adds or subtracts pulses to correct clock crystal drift and worm errors in the RA drive. Erasing the smart drive also removes any "extra" pulses programmed in to correct RA drift.

Why accurate polar alignment BEFORE smart drive programming? Because poor polar alignment produces changes in the RA drift as well. With smart drive programming, in addition to clock frequency errors and worm errors (which are repeatable), we introduce another error (RA drift) which is corrected by the smart drive as well. The next day, we set up and roughly align again and find our RA drive doesn't track as well. This is likely because we programmed in the RA drift produced by a the previous night's poor polar alignment. The previous night's rough polar alignment is usually more difficult to replicate than accurate polar alignment.

How do I tell which way is drifting north in the eyepiece? Align the reticle eyepiece crosshairs with the RA axis by moving the telescope in RA. If the star drifts ABOVE the RA axis, the star is drifting is north. When in doubt, move the telescope in RA to determine which reticle lines to use. What if I prefer NOT using a diagonal (SCT) or live in the southern hemisphere? Reverse the action performed as a result of drift.

Perhaps polar alignment is more difficult to visualize than it would seem. Wallis has abandoned the "mean photographic declination" suggested in his book to that of using the celestial equator which I have found improves the speed of drift alignment.

Subject: Leveling Accuracy and Polar Alignment? -- Part 1 of 3   

From: John Mahony, Date: Dec 2005

> > I can understand that in most cases leveling isn't
> > needed for AltAz, (although I can think of situations
> > where it's useful) but I don't understand for polar?

> Technically, all that matters for a polar align, is that the RA axis, is
> pointing exactly at the celestial pole. It doesn't care what is happening
> below this bearing, to actually 'achieve' this. So it would (for instance),
> be possible to mount a scope on a plate at an angle of 30 degrees from the
> horizontal, north-south, and then just adjust the latitude setting to still
> get the RA axis at the right line!. That having been said, the sky 'model'
> in some scopes, can get slightly confused as to where the meridian line
> actually 'is', if the scope is sitting on a base that is well out of level
> east-west (it appears that they may actually shift the internal
> relationship to sidereal time, to compensate for this, and result in errors
> here). Hence it is usually safer, and easier, to start from reasonably
> close to 'level', but it is not usually anything like as necessary as some
> people would have you believe....

On the classic, if you're using the standard polar alignment method in the manual, the method starts on the meridian (0 hour angle). But 0 hour is determined by matching two marks on the moving and fixed parts of the scope. If the tripod is unlevel in the E/W direction, then the mark on the fixed base will be off.

If you use advanced methods such as the iterative or drift method, it will correct this error, although if you use the standard method to start, and then refine with the iterative or drift method, it will take more iterations or more drift checks, since your starting alignment is less accurate. But the error from E/W level is slight to begin with. Roughly speaking, each degree of E/W level error will cause only 1/2 arcminute of polar alignment error when using Meade's standard polar alignment method.

Your theory is correct, but in the real world, check the numbers to see what is practical. As I stated earlier, each degree of E/W level error causes about 1/2' error when using the basic polar alignment method. Given the effects of atmospheric refraction, there is no "perfect" polar alignment that will give perfect tracking for all parts of the sky. Fork flex is another unavoidable error (and in most parts of the sky, this error is in the same direction as the atmospheric refraction error, so these errors add rather than cancel).

Given these errors, I consider 1' to be the practical useful limit for desired accuracy in polar alignment, so levelling to better than 2 degrees accuracy doesn't really help. That works out to roughly a 1 inch adjustment of the length of a tripod leg, if the legs are not extended too far. If the tripod is taller, then the adjustment is even less sensitive.

So you only need to adjust the legs to 1" accuracy, which you should be able to do with the usual leg adjustment.

Keithnk wrote:
> Thanks guys that makes sense. So here's what I'm thinking
> now. Reasonably good leveling is not necessary but
> facilitates the drift method of AltAz wedge
> adjustments. (I do have to set up from scratch most
> sessions, so reducing set up time without sacrificing
> tracking accuracy is always a goal) If level is way off it
> may take several sets of drift alignments to get dialed in,
> but you can. However, I'm thinking at least a good E/W level
> would be useful because if for example it's dead on, then
> the azm adjustment would leave you pointed true north and
> now you're done with that adjustment. Any out of level N/S
> can be taken care of in the alt adjustment.
> If I'm understanding correctly, 5 or even 10 min. spent
> getting a good level could reduce the drift check/adjust
> sets to as little as one. A common recommendation for drift
> check/adjustments is about 10-15 min per axis, (10 min. is
> usually my longest exposure).Each drift check/adjust saved
> is therefore worth at least 20-30 min time. So, I can still
> see where a device to quicken and get good level would be a
> time saver.

------------------------------------------------------

Subject: Leveling Accuracy and Polar Alignment? -- Part 2 of 3

From: Randy Marsden <jmarsden_at_san.rr.com>

John and Keith, and Roger,
You are all correct. There is another factor to consider when doing polar alignment - the coupling between altitude and azimuth adjustments if the platform is not level. If the platform is not level, then when an altitude adjustment is made, it also changes the azimuth angle and vice versa.

It is this coupling that causes many people to have to iterate back and forth between the altitude and azimuth adjustments when doing polar alignment. But if the platform is level to start with, then as long as the altitude and azimuth axes are truly orthogonal, then making an adjustment in one axis will not change the setting of the other.

So, while starting out with an out-of-level base does not prevent obtaining good polar alignment, it can make the alignment process more difficult. It is much faster to get the tripod or pier level than it is to iterate an extra three or four times to get good alignment.

------------------------------------------------------

Subject: Leveling Accuracy and Polar Alignment? -- Part 3 of 3   

From: John Mahony

Keith, and Dan Simpson, both raised the issue of AltAz interaction with an E/W tilted wedge. Again my answer is, do the math, to see if it's really a practical concern. I've said many times that the big obvious weak point in Meade's basic method is that it starts by relying on the dec setting circle to set the tube parallel to the RA axis (90 dec), but the circle is crude and prone to slipping, and any error here will translate to similar sized pointing/alignment error. A quick way to cure this is to rotate the scope in RA while at 90 dec, to see if the image rotates around the center of the FOV. If you _don't_ use that, error in the circle (or in reading it) may be a significant fraction of a degree.

So let's look at a hypothetical case where the tripod E/W levelling error is substantial, say 10 degrees, and the initial alignment is off by 20', mostly due to dec circle error. You start a drift alignment by aiming at a star near the meridian/equator intersection, to adjust the wedge az. Making a 20' az correction with an E/W tilted wedge has about a 3.5' effect on alt, but at this point we haven't even adjusted the alt yet, which we expect to be off by a significant fraction of a degree, so the 3.5' alt change isn't significant. But then we aim at a star near the E or W horizon to do a drift check, to adjust the wedge alt. Now if we need a ~20' adjustment in alt, on a 10 degree E/W tilted wedge, this will have about a 5' affect on az, which is not within the 1' limit I mentioned earlier as the practical limit for polar alignment accuracy, so we have to do a second iteration.

The second time around, making a 5' correction in wedge az has somewhat less than a 1' affect on alt, so we are done. An extra half-cycle of drift alignment was needed, not three or four. If you use the "rotate in RA while at 90° dec" method to confirm the initial alignment, then if done carefully enough, the initial alignment may be so accurate that you won't need the drift method at all. Done a little less precisely, the initial error may be on the order of a few arcminutes. Then the effects of the AltAz interaction will be less than 1'.

Subject: Meade Polar Alignment Method -- Avoid Second Star with the Same RA as Polaris    

From: Ralph Pass

When you are using the Meade polar alignment procedure picking the second star with the same RA as Polaris will result in convergence with an arbitrary offset. This is due to the mathematics of the alignment. To see this assume you have a second star with the same RA as Polaris. Suppose your mount is now 5 degrees (or any value) from the pole on either Polaris' RA or 12 hours from that. Suppose you loosen the dec axis lock, mechanically move the OTA so it points to Polaris and relock (so you have moved through the NCP (North Celestial Pole) to get to Polaris). Tell the LX200 you centered on Polaris, and then when it slews to the second star, it does not need to move the RA axis!. The bias that was introduced at Polaris will be applied to the next star and the LX200 will travel exactly the proper distance to get the second star and it will be centered. You press enter and you think you are polar aligned but you really are not! The star to really avoid here is Diphda, star 8.

If, on the other hand, you have Polaris below the NCP (so the telescope has to rotate through 180 degrees to get the second star, then any bias in Dec at Polaris will be reflected as twice than error at the second star. You correct twice the error and go back to Polaris and you have twice the error. Which you correct and then go to the second star and you have twice the error. Repeat until you get tired and you have not improved your accuracy. Again this is a limit in the mathematics of the algorithm. It is this scenario that prompts some people to suggest correcting half of the error. Slower convergence but it does converge.

Subject: Keypad (classic) Enter Key Problem During Alignment Routine Solution    

From: Ed Stewart

>I think I'm having a problem with my keypad. Each time I go through my 2
>star alignment routine I find that when I'm done centering the star (whether
>it's the first or second star), I have to press several time the ENTER key
>for it to finally register or beep. Is this normal? What I don't understand
>it that I only get this problem during the alignment routine. Otherwise the
>ENTER key works great !

This is a know "feature" of the alignment routine. The answer--> a quick press or almost a tap of the Enter key. Why Meade decided to make it this way is unknown.

Subject: Better Alignment Accuracy for the LX200    

From: Ralph

The retical is definitely not necessary for accurate 2-star alignment. I center my alignment by eyeballing my alignment stars with a 20mm Nagler and objects are "ALWAYS" in the field of view in the 20mm Nagler afterwards even without using HPP. Remember the 20mm Nagler is a fairly wide angle eyepiece. This is an advantage.

I have found 5 areas that are critical to perfect centering:

1. Don't refocus on the second alignment star once you have picked the first one. The image shift of the primary will be enough to throw off your centering. Even if the second star is blurry and not focused, don't refocus until the alignment process is completed.

2. The exact time entered into the hand controller is very important. I tried my telescope with the time off by over 20 minutes and accuracy definitely degraded. I recommend setting the clock on the hand controller to the exact WWV time before every outing.

***Counterpoint: WWV is overkill. Setting your wristwatch to the time from the Net or the phone company or the TV news and setting your LX200 from that is sufficient. There's no need to get paranoid about milliseconds here.

3. Perfect leveling of the scope has much more effect on centering than some people realize or are willing to admit even though the manual says its not that critical. Spend the extra time to level the scope perfectly and make sure the scope is on a hard surface and wont sink into the ground over time.

***Counterpoint: My experience is different. One day I deliberately set up with my tripod very badly out of level. After 2-star alignment everything worked perfectly. If I recall the discussions here correctly, the 2-star alignment algorithm corrects for leveling errors as well as encoder biases. The *amount* of level correction shouldn't matter much.

4. Exact latitude and Longitude. Also contrary to what is believed, your exact latitude and longitude is very important to centering. This is especially important when viewing objects close to the horizon where there is a parallax effect of the atmosphere as you view lower in the horizon. Take the time for either borrow or buy a GPS unit. They cost as much as a good eyepiece and are well worth the expense for the advantage you will get from its precision. With out precise lat and long, your scope will be off at the horizons, as noted in the manual.

***Counterpoint: A cheap GPS is also a good time source. But again, GPS accuracy is way overkill. You can enter the lat/long only to 1 minute accuracy anyway.

5. Finally, pick your pointer stars carefully. Learn the sky enough to always pick stars low on either side of the horizon, east and west, north and south or at least opposite areas in the sky. Keep your stars low on both ends of the horizon. The farther apart your alignment stars will be, the more accurate your scope will be.

***Counterpoint: Wrong. The alignment stars should be between 30 and 60 degrees elevation. Refraction correction is NOT applied during the alignment process. If you use stars too close to the horizon refraction will foul it up. See freeware BestPair II in Software & Computer Issues, page 1 topic.

Don't expect bullseye accuracy every time if you are not using HPP, but you should always be able to keep any star or object in the field of view of a good wide angle low power eyepiece, even without the HPP active. You will definitely not get the centering accuracy if you go to a higher power eyepiece, for example, something over 200x, even using the Naglers tremendous field of view. Keep your searching to lower powers and wider field of views and when the object is there, then up your power if you like.

Sometimes if you use a high power eyepiece with HPP, the alignment star may not be in the field of view of your eyepiece. Just check your finder for the brightest star. That will be the one you are trying to center on. Use the slowest slew speed to recenter the star, then when you press GoTo, the object you were searching for will be right there. This is usually never necessary unless you leave a higher power eyepiece in you scope when moving to the next object.

My opinion based on my experience is that if you follow these guidelines, you will be amazed at the accuracy of your scope.

Subject: Polar Axis Balance with Equatorial Mounts  

From: Richard Davis

What follows is some discussion on the subject of balance of the LX200 around the POLAR AXIS, under the condition that the scope is mounted on a wedge that places the base of the scope in plane which is parallel to the earth equatorial plane. I believed there is a strong case for working with a balanced fork assembly on the LX200 under these conditions.

==============================

Recently, <wjmsarnoff.com>, William Murray, has submitted the following comments, together with a quote from another authority:

"I'm not sure what the internal workings of the RA motor assembly on LX200 scopes are but the idea of letting the motor "guide the fall" is old astrophotography wisdom. In his book "Astrophotography", page 74-5 Barry Gordon states:" (quoting Mr. Murray.)

"Note that, for the polar axis, perfect balance is NOT what we want. If we achieved such balance, then any play in our right ascension tracking mechanism would allow the instrument to wander back and forth between the limits of that play. We are generally better off with slightly imperfect balance, so that the instrument has slight tendency to fall behind or, even better, to fall forward in its tracking."

==============================================================

I do wonder what is really meant by "...slightly imperfect balance..."?

While the sentiment is respected coming from this astronomy authority, such terms are next to useless in evaluating a given situation. Given any echanical configuration could be very different from all the rest of our LX200's. But, for any given LX200, what is the degree of imperfect balance recommended by Mr. Gordon, or any other person on this list who has recommended some degree of imbalance?

Well, how could the degree of imbalance be stated in quantitative terms that each of us could reproduce with our own systems? I suppose that Mr. Gordon could have stated the degree of imperfect balance in terms of torque, say 345 gram meters, or 14 inch pounds. But then we'd need to have some consistent way of measuring this torque.

But why even bother? What is this "west" torque designed to do? The idea that this torque will place a "load" on the RA drive train so that the gearing is under a "constant" load--a load which keeps the drive gears under some constant force. This will promote a consistent advance of the RA drive. If this is done, then the guiding process only needs to remove the constant error rate in the RA drive which is either leading or lagging the local sidereal rate.

Now suppose that you decide to place your LX200 pointing due south so that the forks are aligned in the east-west direction. Now, suppose that you add some weight to the west fork arm that makes it heavier than the east arm. Well, why do that? You have already had that done for you! Meade has placed a quite heavy weight on the west fork arm--the declination motor assembly.

Notice that this weight is exerting the MAXIMUM westerly torque when the fork is aligned along the east--west line. As the fork turns in RA, either to the west or the east, the declination motor weight moves toward the north-south line, and the moment arm about which the weight is applying torque to the scope is getting shorter. This means that the force, which was loading up the RA drive train at the maximum level when the fork was aligned east-west, will decrease steadily as the fork is rotated toward the north-south line. When the fork becomes aligned with the north-south axis, this force will be reduced to ZERO. At that point, the benefit of an "imperfectly balanced" mount will VANISH!

Moreover, when ever the scope axis passes through the east-west line, the DIRECTION OF THE IMPERFECT BALANCE TORQUE WILL NOT JUST CHANGE IN AMOUNT, IT ALSO CHANGES IN DIRECTION! If you work at higher declinations and follow objects year round, then this happens to you frequently.

It seems to me, that when deviations in RA tracking are critical, it is best to have the RA drive mechanism under consistent, constant load. This condition can only be realized when the scope is balanced about the polar axis, so that at every RA setting, there is not any net torque on the scope due to weight distribution.

There may be other variations in actual RA movement, which need to be addressed. These variations may be so large that they require special attention. But, at least, once the weight of the scope has been balanced around the polar axis, then this source of RA deviation can be eliminated.

Remember that Mr. Gordon suggested a " ...slightly imperfect balance..." to guarantee that the RA drive train was under load, and thus would be constrained to work in the 'loaded condition.' This would reduce the effects of loose play in the RA drive train from Mr. Gordon's view. Perhaps the loaded condition suggested by Mr. Gordon can be found even when the fork is balanced around the polar axis.

Well, in your LX200, when you are using the GUIDE rates of RA adjustment, and if you ask for an easterly move to correct the RA track, the scope DOES NOT MOVE EAST IN RA. Instead, the LX200 drive system is slowed down to 1/2 the sidereal rate. This lets the 'sky' move on to the west, thus the stars run away from the optical axis of the scope. The net result is an 'eastward' apparent movement.

Because this is the way guiding works on the LX200, the RA drive train is ALWAYS LOADED UP IN THE DIRECTION REQUIRED TO MOVE THE SCOPE TO THE WEST. It is NOT necessary to unbalance the scope around the polar axis to obtain a continuously loaded RA drive train.

I do not believe that the RA drive train of the LX200 is completely free of friction. Having inspected the gearing and RA suspension of this scope, I believe that there is substantial friction in the mechanism when the scope is mounted on a wedge and polar aligned. This friction is quite sufficient to provide the load that is needed to keep the RA drive up against its mechanical limits.

What remains to produce deviations in RA tracking is probably mostly related to the main, large, 8 minute gear in the LX200 RA drive, and the 'worm' gear which drives the 8 minute gear with their individual imperfections.

When the scope is balanced around the polar axis, and my drive hits that really big imperfection on the 8 minute gear (which it does every 8 minutes), at least the mechanical load due to torque around the polar (RA, that is) axis is consistent--not varying due to RA position. Well, that's just my opinion, and I could be wrong! If you want to do the right thing.....don't believe me, or Mr. Gordon, or any of the rest who are readily giving you free advice. Do the following study, and you'll know for sure:

Use a wedge mounted, polar aligned LX200. Use precisely the same scope and mount configuration throughout the study. Do a careful drift alignment. Get both declination and RA drift down to the smallest possible levels.

1. Take a pleasant drive to your local hobby store that supports the model airplane hobby. There you can buy strips of lead, 6 inches long, with an adhesive backing. The strips are scored very 1/4 inch so you can vary the weight in small amounts. You'll need just under two pounds.
2. Apply these strips to the arm of your LX200 that is opposite the declination motor. Apply just two or three strips to the arm itself, and then stack strips thereafter. This will minimize the number of strips you apply to the scope itself. Add enough weight to allow the scope fork to remain on east-west line without moving. Use weight on the OTA, aligned with the optical axis, to balance when the scope is pointed due east and west. Check that the scope doesn't rotate in RA when placed in any arbitrary RA position.
3. Take one night to work on training the LX200 Periodic Error correction system. Do this slowly and carefully. Repeat the training process at least 8 times. Use your highest magnification possible with an illuminated and graduated reticule eyepiece. I use a 2x or 3x barlow with the Meade 9mm illuminated eyepiece. Remember to work with stars that are near the celestial equator, and about one hour before crossing your local meridian (due south).
4. Establish some method of observing a star and noting the RA deviations at periodic intervals, say every 5 minutes for an extended period, say all night. Just let the scope "free-run". You'll nee a good drift alignment and PEC training session to make this possible. Plot these deviations for the initial "balanced telescope" night. Make sure you work from far easterly RA over far into the westerly RA. A set of CCD images works best for this purpose to get accurate and consistent measurements of RA deviation.
5. Add one pound of lead weight to the 'east' arm of your scope and repeat step 3.
6. Shift the added one pound of lead weight used in step 4 to the 'west' arm of your scope and repeat step 3.

An eye-ball inspection of the results plotted together on the same graph should answer the question for you, for once and for all.

If you really want a surprise....repeat this study at a declination of around 60 degrees. Use stars that are about 7 hours east of your local sidereal time, or 5 hours west. This will show you the effect of the CHANGE IN DIRECTION of the load torque as the fork moves thru the north- south line.

So, you can see that this is a lot of work... Ask who has done this study, if its not worth it to do it for yourself. I have not done precisely this study myself. But I've done portions of the procedure, and the results have convinced me to work with a balanced fork on my LX200 when mounted in the polar aligned mode.

Subject: Fork Weights & Tracking --part 1 of 2  

From: William Murray

> From: <jonhpcvlx.cv.hp.com>
> I've noticed that my wedge mounted 12" LX200 lists to west. This happens
> even when the tube is balanced in declination, and is in fact independent
> of declination setting. By "west list" I mean that when the RA axis is
> un-clutched the scope tries to rotate such that the heavy fork arm (the one
> with the dec motors) ends up on the bottom.
> I'm considering gluing/nailing/bolting a weight into the light fork arm.
> Anybody have a design that they like? (Or do you all think I should worry
> about real issues?) I would like things to stay put when un-clutched.

Jon -I use a set of exercise wrist weights I got at a sporting goods store on my 10" LX200. These things come with a set of 1/2 pound ingots so that you can vary the weight from 1/2 to 5 pounds total. They come with a covering that is meant to wrap around your wrist (or ankle) and secure with velcro. I just vary the weight as needed and wrap them around the fork arm near the handle, they are very secure and do not slip.

Since I do a lot of astrophotography I try to adjust the total weight so that the scope falls slowly to the west through the meridian. This allows the motor to control the fall of the scope instead of having to push it around. I find that this is a better solution than having the scope perfectly balanced.

----------------------------------------------------------------------------------------------------------------

Subject: Fork Weights & Tracking --part 2 of 2 

From: William Murray

> Bill and Ric,
> Both of you are advocating letting the motor "guide the fall" rather than
> push. Sorry about further MAPUG time on this but am very curious why
> you've chosen to go this way. Aren't there thrust bearings and the like
> in the RA motor assembly that would be happier pushing? This would
> speak to over weighting in the other direction wouldn't it?

Jon -I'm not sure what the internal workings of the RA motor assembly on LX200 scopes are but the idea of letting the motor "guide the fall" is old astrophotography wisdom. In his book "Astrophotography", page 74-5 Barry Gordon states:

"Balance is a problem when heavy telescopes sit atop tilted- over-to-one-side mountings. Hanging a significant weight in eccentric fashion results in that weight trying to occupy the lowest position available. It is our wish, however, that it will tend to remain exactly where we place it. This requires careful balancing of the equipment about each active axis - that is the polar axis and the declination axis - ignoring any now-locked axes used only for initial polar alignment. While balancing is often provided for in the telescope's design, it may also require attaching additional weight. Whatever the method, balancing must be done, particularly if the equipment is to be driven mechanically. To make matters worse, the job cannot be done once and for all; it must be redone when a camera is added or removed or even shifted from one location to another. Fortunately, this is a simple chore. Note that, for the polar axis, perfect balance is NOT what we want. If we achieved such balance, then any play in our right ascension tracking mechanism would allow the instrument to wander back and forth between the limits of that play. We are generally better off with slightly imperfect balance, so that the instrument has slight tendency to fall behind or, even better, to fall forward in its tracking."

I have a tendency to hang a lot of equipment on my 10", ( I mount a 5.5" Celestron Schmidt camera on top for wide field astrophotography. ) this increases the scope's tendency to fall forward rapidly. I use about 2 to 3 pounds of counterweights on the eastern fork arm to reduce the rate of fall. To make the scope fall behind would require more weight than I care to hang on it. I have gotten very good results with my setup.

Subject: One-Star Alignment for Wedge-Mounted LX200 Classics  

By: Robin Casady

To do a one-star alignment:

If your wedge is polar aligned, you can do a one star alignment on any object in the LX200 catalog. The accuracy of subsequent GoTos will depend on the accuracy of your polar alignment of the wedge. For visual observing, a precise alignment is not necessary.

1. Make sure the keypad is set to Polar.
2. Do a keypad GoTo of an object in the sky.
3. Align the telescope on that object with the NEWS keys or by loosening the clutches and hand slewing.
4. When the object is aligned, press and hold the ENTER key until it beeps and says coordinates matched.

Subject: Aligning the Classic -- Polaris is Visible -- 2nd Alignment Star Isn't   

From: Ralph Pass <rppassmediaone.net> Date: July, 2000

Just press Enter, select a visible star, press GoTo, use the keypad buttons to center, and then press and hold enter until it beeps.

I am of the opinion that using Polaris and the same star does not converge as fast as it could. I use this technique to alternate the 'second' star to get better convergence.

You should be aware that there are two conditions where the alignment technique has a problem. The first is when the selected second star is around RA 2h 30m (the technique can converge to an arbitrary solution). The second is when the selected second star is around 14h 30m (where the technique can oscillate and never converge). Each of this is due to the geometry of the stars and not the alignment technique. The solution is very simple. Do as suggested above. Simply press enter and then go to a star away from these bad RAs, center, and sync.

Subject: Polar alignment When Polaris is not Visible --part 1 of 3   

From: John Mahony <jmmahonyhotmail.com> Date: Mar 2003

Syncing the second star only synchronizes for that area of the sky and doesn't improve the overall alignment. For visual work, AltAz is better. Besides saving a little set-up time and effort, when using the polar method, the computer assumes the mount is accurately polar aligned and doesn't try to compensate for errors.

For long exposure imaging (polar required) when you can't see the north star, there's a (probably very old) method that will get you very close quickly. Since you already have the latitude pretty close, you can simplify this even more:

1. Aim at a star near the intersection of the meridian and the celestial equator. Sync on this star.
2. Now go to a star that's either far to the east or far to the west, and still roughly near the equator. Use the keypad to "GoTo" this star. Check the error. If the star is in the western sky and the scope aimed too far north, that means your wedge azimuth is too far east. Similarly if the scope missed to the south, the wedge azimuth is off to the west. Reverse all this if the star is in the east.

The amount of wedge azimuth error is roughly the same as the north/south pointing error for this star. This all assumes the latitude is accurate.

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Subject: Polar Alignment -- Polaris Is Not Visible --part 2 of 3

From: Keith <keithnkprodigy.net>

-------- Original Message ----------
Within the next few weeks, my school's robotic observatory will be installed (LX200 10" classic in a Robodome) at its rooftop location. Due to the nature of the location, the view to the north is blocked. This probably means that I won't be able to align the 'scope on its meridian using the Pole Star.
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If you're just trying to get a better idea of where north is compared to what a compass will give you, here's a method I'll use to get me quite close.

If you have an astronomy program like TheSky, or something similar, then - assuming you have it set up for your longitude and latitude and the program clock is correct - check out the minute or even the second when the sun will be due south. Then look at the shadow of a vertical pole up in your observatory at that minute or second. The shadow will point due north. If you set it up right, you can get the shadow to fall across the center of your telescope and onto the opposite wall. You can mark the opposite wall (or note a landmark) where the due north spot is. You can surprisingly close just standing in front of your telescope and at the appointed time when your shadow is pointing straight at the center of your wedge or scope, taking note of a landmark or spot that is straight away to the north. This is not nearly as accurate as a tall vertical pole's shadow, but may be close enough to give you an idea. If you miss that time, you can also check when it's due east or west to find that line, and then can work off it. This should get you close enough so that the methods also located here on this Archive page should enable you to be able to easily dial it in the rest of the way.

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Subject: Polar Alignment -- Polaris Not Visible --part 3 of 3   

From: Michael Gerszewski <mgerszewantaressol.com>

I am assuming that you have a wedge, and that you are attempting to do polar alignment. There are several good resources and methods that have already been mentioned. I am also assuming that with a RoboDome, you will be using a CCD. Here is a link for a nice CCD method that I found a couple of years back: <www.ucihs.uci.edu/pandb/hall/polar.htm>

I used this method with a 16" LX200, and it worked quite well. I think it will be useful to you, especially with a RoboDome. You'll probably want to use a drift method of some sort, especially with a permanently mounted scope, and are probably forced to because of your inability to use Polaris as a first-order starting point.

For a first-order starting point, you'll probably have to use magnetic north, corrected to offset, for your azimuth, and your latitude for altitude. Remember to iterate between meridian and horizon when doing your drift align to increase your accuracy.

Just a warning with a RoboDome and a 10" scope w/CCD. Watch the CCD with your system set up in equatorial. If you add any extra accessories (focal reducer, adaptive optics, filter wheel, motorized crayford focuser, etc.) the CCD may get dangerously close to brushing the wall of the dome, possibly binding the scope. I know they say it will fit, but it can be a pretty tight fit (at least with a RoboDome I had some experience with a couple years ago)

Subject: How Close is "Close Enough" OTA Alignment?

From: Joe Diregolo <JOE.DiREGOLOkla-tencor.com> Date: Aug 2000

-----Original Message-----
From: Gregory Pyros <gpyroshome.com>

Thanks to the MAPUG Topical Archives, after I first brought the scope (classic) home and had alignment problems with it, I returned it to Meade for an adjustment. What happened was that when I polar aligned the scope on the SuperWedge with everything turned off, and did a 360 in RA, I would get a nice tight little circle of Polaris, but it was nowhere near the center of the 26mm eyepiece. Imagine drawing a 3" diameter circle on a piece of paper, then drawing a 1" circle just about touching the outer edge of the first circle. Meade agreed that it was an OTA alignment problem, and adjusted it.

Now I have it back, and trying the same test, the two circles are perfectly concentric, but if the full 26mm eyepiece is a 3" circle, Polaris will, at best adjustment, form a 2" circle within it. So there is no setting where I am perfectly aligned that I can have Polaris in the center of my eyepiece, and not even in the field of view of the 9mm illuminated reticle.

For visual observing it would probably be fine, but my goal is to do CCD photography, and I'm sure I will have enough problems there without worrying about mechanical scope alignment causing tracking problems!
-----End Original Message-----

Answer to question:
I noticed the same problem with my scope. But to check it requires that you point the scope to a fixed object. You are correct in using Polaris since it is close enough to the celestial pole that it move so slow that it would not effect your test. What you are seeing is the mismatch of the Optical axis and the RA axis. Meade seems to do a good job getting RA and DEC axis orthogonal. But they fail to get the all three axis's to intersect each other at the same point in space.

If the error is large enough one would see dec drifting even after perfect drift polar alignment.

There is another test that you can perform that will show what direction the error is. Rotating about any fixed object at least 100 feet away and adjust the dec to 90 deg or to the exact location that you achieved the smallest orbit of the object.

Without moving the dec axis point the scope at a diffuse light source. (A florescent fixture should work fine). Remove the cover at the bottom of the fork arm. This will expose the upper RA bearing. Using a 12.5mm or smaller eye piece project the light on to the screw and washer holding the RA bearing. Adjust the focus so that the light circle is slightly larger that the screw. If the light is centered then its as good as it going to get. If not then one of two thing are still out of alignment. If its off in the Fork direction it indicates that the OTA is not centered between the forks.

If its off in the other direction this indicates that the forks are tilted and that the dec axis does not intersect the RA axis.

Now how to fix this error...
If the error is in the fork arm direction one could theoretically add different size trust washers to the dec assembly to offset the OTA until it is concentric with the RA axis.

I would be very interested in knowing if anyone has done this adjustment. I would like to know about it because this is exact error I have with my scope. I'm off center about 1/4" in the fork direction. Still I get good tracking of unguided CCD images for about 5 min.

If the error is in other direction this would indicate that both forks arms are tilted and would need to be adjusted. This adjustment can be found at OTA Alignment here in the Mapug Topical Archives.

Subject: Polar 'GoTo' Adjustment Web Page  

From: Bruce Johnston Date: May 2002

I've completed a write-up on analyzing problems with Polar 'GoTo' operations. Hopefully, it will help people to find more of what's wrong when their 'GoTo' is off, and some ways of adjusting things accordingly. It's written with the LX200 in mind, but it actually applies to pretty much any 'GoTo' scope.

Also, inside the info, is a new method of Polar aligning that I've found to be very accurate, and much, much faster than drift aligning. Just go to: <www.mapug-astronomy.net/ccdastro/goto-tests.htm> Note: should open a new browser window.