AltAz Aligning Concerns --Page 1

       

GoTo: Polar Aligning 

Subject: LX200 Classic Longitude Calculator

From: Clark Williams <S.I.G.Hix.netcom.com>

Someone had a question about the Classic longitude values a few days back. Several years ago I wrote a WinTel based LX200LatLon calculator (enter your latitude and longitude and it displays what you should enter for the LX200 Classic). It is available at: <http://www.s-i-g-h.com/lx200lon.zip>

Known Issues:
None that I know of. I put together a Java applet to do the same at: <http://www.niteis.com/NILX200LatLon.html>

Known issues:
I put this into an applet from an application. The application used a call to the system exit() function to terminate. This will of course throw a java exception in the applet world. So when you click on the Quit button you'll see an exception on your java console. Just close the browser window and the world will exit gracefully.

I'm sure you can 'fool' the thing, especially at the 180/360 degree boundaries. So if you are on GMT you enter 000 00 00 Lastly---if you can't figure out how to use it---don't. As always with RAD code, no flames, no law suits USE AT YOUR OWN RISK!!!

Subject: Procedures for Obtaining Best Alignment--Classic    

From: James Burrows <burrjawearthlink.net> Date: June, 2000

Don Tabbutt wrote:
>Didn't we conclude years ago (a lot of it through your work) that the net
>effect of tilt (lack of level) was that errors could be introduced in the
>refraction correction algorithms for objects near the horizon, and that the
>actual horizon itself could be offset?

Meade's AltAz alignment, as I mentioned, has second order errors in the modelling of tilt, trying to correct tilt about the level NS axis by altering sidereal time. I'm not quite sure what the result of that error would be; probably not much effect on the refraction correction, and the tilt would have to be pretty ferocious to get a star out of the field of view.

Review of ancient (3 years ago!) notes on AltAz alignment (polar mode guys can delete the rest of this message):

1) Get the best out of Meade's algorithm, beyond leveling and a reticle EP. I get about 3' whole-sky (scope dependent).

a) Run my program BestPair II:
<http://home.earthlink.net/~burrjaw/lx200/odyframe.htm> to choose the best pair of alignment stars visible from your location. Rules of thumb don't seem to work too well. Also see Software & Computer Issues topic for more on BestPair II.

b) Be backlash-aware centering the stars. This means the last motions centering a star must use the same drives as used during tracking. To see which way that is, point near the star, push W in GUIDE mode - that's the tracking direction.

2) Ralph Pass and I developed a Kalman filter n-star alignment - choose more than 2 stars around the sky. I found that it doesn't help much, maybe down to 2' whole-sky for my scope (still scope dependent).

3) The most important errors not covered in the above algorithms are gear errors: runout and scale factor. I wrote a program to estimate those errors and alter the RA/DEC pointing commands. I understand the POINT program has this capability. The trouble is that this desynchronizes the laptop and keypad, so you can't use Meade's object databases, but I did get down to the 1' whole-sky goal.

Subject: LX200 Initial Alignment --part 1 of 2 

From: Ralph Pass <rppassrppass.com> Date: Jan., 2001

astrovikingjuno.com wrote:
> If your using zero Dec and zero hour angle are you referring to the
> computer readout on the hand controller? Or the mechanical scale on the
> scope, i.e. RA ring and Dec scale? If your referring to the mechanical
> scale for Dec the LX200 tells you to move it to 90 deg. after bootup
> and doesn't make much difference if you move it to 90 deg. before boot or
> after boot. Give it a try and you will see that it makes no difference
> which method you choose. Setting the RA hour wheel makes no difference
> where it is set because it not connected to anything that can change,
> just about totally useless on the LX200. Maybe I'm missing something in
> your reason for setting to 0 Dec and 0 HA.

I use the mechanical scale. After aligning once I slew the telescope to 0 deg. and 0 azimuth (yes that is azimuth since that will give me 0 hour angle). I then note the offset on the mechanical scales. I set the scope to this when I start and given accurate time and location, the scope is darn close to aligned, see below for the caveat.

If you loosen the Dec axis and manual move the scope to 90, then it makes no difference where the Dec axis started. However, setting a 0 hour angle and slewing to Polaris (select star 19 and press GoTo). will get you closer on your initial align (you can also GoTo a star, sync on it and then slew to 19).

Now for the caveats:

1. Before I got my permanent pier I would take my scope to one of several locations I used. I had marked each after a trip there so I could put the scope back in the same location. If I went to the same location for consecutive evenings, I could use the method described above to be able to start the scope during daylight and have Venus comfortably in the field of view of the finder, and many times in the field of view of the 26 mm eyepiece. Final alignment was always quick. NOTE THAT I WAS USING A MEADE SUPERWEDGE and had little problem with it once I learned not to overshoot on corrections (backlash it its major problem in my opinion).
2. Once I got my permanent pier, I found that the SuperWedge was stable enough that using the method above would put a star in the field of view of my 416 on my 10" F/6.3. I stopped trying to tweak the alignment more than a year ago. Now I simply tell the software to center the star and sync on it and I am set for the evening.

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

Subject: LX200 Initial Alignment --part 2 of 2

From: Bob Denny <rdennydc3.com> Date: Jan., 2001

Due to ACP's (Astronomer's Control Panel software) being attributed as starting alignment at 90 deg. DEC. I feel the need to comment. The correct starting point is 0 deg. Dec and 0 hour angle. If you start the LX200 (in polar alignment mode) in this position, it knows where it is pointing immediately. If you park it in this position, you can turn it on and it will be ready to use without even syncing. There is no other start position for which this is true. This proves that the firmware designers meant it to be started in this position.

Note that the firmware designers went further... if you put the scope into LAND mode and slew to the Alt/Az location corresponding to 0 Dec and 0 hour angle then shut off the scope... then later you turn on the scope (still in LAND mode) allowing it to slew off from the perfect parking location as it does when it starts, then put it back into Polar mode... it will be lined up perfectly! Even if you wait for an hour before switching to polar mode!!! The firmware designers had it together when they did the LX firmware.

ACP slews the LX200 to 90 Dec as part of the Alignmemt Wizard process. It's part of the process of removing Dec calibration errors. For the Alignment Wizard to work, you must start the scope at 0 Dec and 0 HA.

By the way, ACP is controlling the scope that has discovered 5 supernovae in the last 3 months, the 24" Ritchie at Puckett Observatory. In addition, ACP was just brought online at the Tenagra Observatory, on the Tenagra 3 20" scope, again for Supernova searching. There are not LX200's (!!) -- ACP is being used with ASCOM telescope drivers, and is serving as a scripting object host and script controller.

Subject: LX200: Backlash-Aware 2-Star Alignment    

From: James W. Burrows <burrjawearthlink.net>

Altaz users: There has been a lot of discussion lately about DEC backlash, which I have been ignoring as a polar problem, but it's at our (altaz users) peril. The first step was to measure the RA and DEC backlash on my 8" (during the day in LAND mode - no use wasting good night time on such as this). Results: RA, .21 mrad (44"); DEC, .32 mrad (67"); width of reticle guidance box, .16 mrad (34"). I have devised a "backlash-aware" 2-star altaz alignment which prevents backlash from messing up the alignment. I assume a reticle EP.

1. Center the alignment star in the Telrad (or finder).
2. In the EP & center speed, get the star approximately on a line from the EP center toward the NCP. Using the star diagonal, this would be in the upper right quadrant of the FOV for stars E of the meridian, upper left for stars W of the meridian (hope I've got that right).
3. lip the W button (still on center speed).
4. Switch to GUIDE speed, and without touching the S button (i.e., hold down N button and use E or W buttons as necessary), run the star toward the guidance box.
5. Release buttons right when the star first enters the guidance box and hit ENTER (no hurry) to say the star is centered.

If the star can't be gotten into the guidance box in step 4, go back to center speed and step 2. The critical part of the procedure is step 5; stars W of the meridian will drift out of the box after the button(s) are released, drift which would be cured by hitting the S button - DON'T DO IT! - this is where backlash would introduce alignment error.

The interesting part of this alignment mod is that it is faster - you don't waste time chasing stars W of the meridian. As I said, backlash isn't fixed, a slew to an object should do an intermediate GOTO a position N of the object, then GOTO the object; objects W of the meridian will drift to the "other side" of the DEC backlash.

Results (errors evaluated by centering as above, not including backlash):

A1: Meade(Altair, Alkaid); errors.

AZM ALT: ERR, mrad

E: 79 62: 0.75 @ Deneb

E: 189 55: 0.82 @ Rasalhague

E: 297 49: 0.27 @ Alkaid

A2: SYNC; errors.

AZM ALT: ERR, mrad

Y: 192 54: 0.85 before SYNC @ Rasalhague

E: 297 48: 0.90 @ Alkaid

E: 194 54: 0.11 @ Rasalhague

E: 81 64: 0.81 @ Deneb

SAO 44752 13:47:32 49d18'48" 1.86: 85 eta UMa, Alkaid

SAO 49941 20:41:26 45d16'49" 1.25: 50 alf Cyg, Deneb,

SAO 102932 17:34:56 12d33'36" 2.08: 55 alf Oph, Rasalhague

This particular trial shows approximately 2' whole-sky pointing accuracy, which could probably be improved by a better choice of alignment stars; Alkaid and Altair are 84d apart, best I could do from my site last night. Note that, as usual, SYNC improves accuracy near the SYNC star at the cost of reduced whole-sky accuracy.

Subject: Selecting Slew Speeds for Alignment       

From: James Burrows <burrjawearthlink.net> Date: Aug 2001

FLag wrote:
>I had no idea that the 7,4,1 and 0 key
>effected more than the rate of speed that you slew
>(sensitivity). I usually set the 4 key for hand slewing and then sometimes
>switch to the 1 key for final centering (which it says the 1 key is for
>centering). I didn't know that the 0 key turned off tracking though.

We're talking altaz here. The 7 (SLEW), 4 (FIND), 1 (CNTR) keys just set the rate of speed as you say. Moreover, for these three speeds, the N,S buttons cause rotation about a horizontal axis (up and down); E,W buttons rotate the telescope about a vertical axis (the buttons should say L(eft), R(ight), rather than E, W (I always hesitate - which way is the E button going to move the telescope?)). The 0 (GUIDE) speed rotates slower and the axes are different - E,W now rotates about a parallel to the Earth's polar axis, N,S changes the declination the telescope is pointing toward. If you're at GUIDE speed, the tracking motors are turned off only when you push the E button, and the stationary telescope rotates E relative to the stars.

>Anyway, you are basically telling me that once I get my second alignment
>star centered and it goes drifting off before I can sync on it is due to
>backlash, right? Why do I not have this problem with the first alignment star?

Oops, I forgot one very important point relative to tracking the first alignment star. When you turn on the telescope, Meade starts immediately tracking, but this only tracks the real stars if you put the telescope in the position Meade assumes before you turn it on: OTA level and pointed south. We've called this the 0-star alignment. It works fairly well for daytime viewing if your time and position settings are good.

>Any further information on this would be appreciated......

OK, here's almost the full dope on backlash-aware centering. As I said, the main thing is that the star has to sit still and centered after you push the last button(s). If you grab it on the run, Meade won't have the right readings from the encoders for the alignment algorithm.

 Star      Azimuth    center        GUIDE    

 0-90        NW      (NW W SW)        E 

 90-180     NW        (N NW W)        E 

 180-270    SW        (NW W SW S)     E 

 270-360    SW        (N NW W)        E 

The first line says if you're trying to center a star to the NE, the last buttons pushed at center speed have to be N, W, or N and W simultaneously. If you push E or S last, the star will drift for a while. For the final centering at GUIDE speed, push W or N&W or S&W last (and you can always push E last).

To end this long dissertation, the way I work it is to have the reticle crosshairs vertical and horizontal as I look in the eyepiece. If I'm centering a star to the east, I hit SE simultaneously at center speed to push the star almost to the edge of the field of view, then get it close to the center using only N and W buttons. Switch to GUIDE speed and put the star in the reticle box using only the allowed buttons. The star had better stay there while you pause a moment before the push-and-hold ENTER.

Subject: Choosing Stars for 2-Star AltAz Alignment     

From: James Burrows <burrjawearthlink.net> Date: Aug, 2001

In the following table, the column labeled "Star 1" has the 20 alignment stars visible at latitude 45 deg last night, "Az" and "Alt" their azimuths (0=N) and altitudes. The column labeled "Star 2" is the best star to use with "Star 1" in a 2-Star alignment, best in the sense of whole-sky pointing accuracy, expressed by the number "RMS", described later.

The first fact that can observed in the table is "don't use low altitude stars." None of the freely chosen stars in column "Star 2" are below 35 deg. All the alignments which are forced to contain low-altitude stars in column "Star 1" (e.g., the last 5) have the poorest pointing performance. It is interesting to note that the method of evaluating pointing performance used here is purely geometric, i.e., it doesn't contain refraction. Maybe Meade came out smelling like a rose in forgetting to put refraction into the alignment algorithm because one doesn't want to use low-altitude stars anyway...

A rule-of-thumb for the azimuth choice is more difficult. In general, one wants to choose a 2nd star as far away, in azimuth, as possible from star 1. However, the available altitude choices interfere with this choice. For example, (Polaris, Hamal), with 145 deg azimuth separation, is a better choice than (Polaris, Diphda), 176 deg apart, apparently because Diphda is a lower altitude star. Other examples of this phenomenon can be dug out of the table.

"RMS" is the whole-sky average pointing accuracy as a factor times the centering accuracy, for example, 1 arc minute centering accuracy during a 2-star AltAz alignment using Polaris and Hamal results in .759 arc minute pointing accuracy. Please note that this calculation is only geometric and doesn't include esoteric errors such as tilt error misrepresentation, altitude axis nonorthogonality, worm wheel runout, etc.

Just for fun, here's the:

See "Software & Computer Issues, page 1" for the BestPair II freeware that will calculate the best alignment pair for you (Mac, PC, & Dos).

Subject: AltAz Re-Alignment by Sync Operation   

From: Rob Preston <rbrtprstnearthlink.net>, Date: Oct., 1998

The method under discussion here has two related but quite different features. First, it's an alignment trick (for both polar and AltAz modes, although it has, by far, its greatest usefulness with the known-site, non-leveled AltAz mode). Second (and perhaps less obvious at first sight), it makes it feasible to get into and out of the aligned mode in the blink of an eye. Thus, it is now a simple matter to **unlock the axes** of an LX200 and manually slew 180 degrees in less than one second, relock the axes, SYNC on any convenient star in that area of sky (e.g., one of the 250 brightest stars in the sky, listed in the database as star #1 to star #249, although any of the SAOs would work too), and be perfectly 2-star-aligned again. This could eliminate 90% of the slewing noise and motor wear associated with long-distance slews, for example.

One of the supposed advantages of the Celestron Ultima system (with encoders on the axes instead of the motors) is that the scope always "knows" where it's aimed, even if the clutches are released, so unlocked manual slewing is possible. The ease of the one-star realignment that Bill Arnett pointed out (which preserves the off-level tripod corrections that were achieved in the initial 2-star alignment) means that, essentially, the LX200 system also can be unlocked routinely. Since it's easy to remember the locations of the 250 brightest stars in the sky, a quick SYNC on any one of them can easily reestablish the accurate initial alignment, even if the tripod were grossly unlevel. It works! I tried it with a grossly unlevel tripod and got the alignment back near-perfectly. I assume it would have been perfect if I had used a reticle for the SYNC.

This method is mentioned in the LX200 user manual under the heading, "The permanently-mounted, polar-aligned LX200". For those of us who have no permanent polar alignment, it's easy to miss the significance of the method and suppose that it applies ONLY to the polar mode and a precise, permanent alignment. This is especially true since the manual doesn't even define the "sync" operation except by that casual mention in the polar alignment section (they forgot to put it in the manual!) As Bill Arnett discovered: in the AltAz mode, the bias corrections (made via a two-star alignment) for a non-level tripod are independent of the wormwheel position, and a SYNC (*without* GoTo) is possible and it restores the full 2-star accuracy after any unlocked axis motion.

Check it out, you AltAz people. Polar people, too, feel free to unlock those locks for super-speed slewing. A simple SYNC-without-GoTo on any object in the database restores the alignment.

P.S: SYNC means that, after selecting a star or M- or NGC- object from the database, so it's name is displayed on the controller paddle window, the object is then centered in the eyepiece, as during the initial alignment, and then the ENTER button is pushed and held down until the paddle beeps and displays "coordinates matched". (Most Mapuggers know that, but it isn't in the manual, at least, it's not in the manual that came with my own scope).

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

From: Ed Stewart <stargazerskymtn.com>

> 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: 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 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!

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-Astronomy Topical Archives.

Subject: Leveling LX200 and Entering Lat/Lon under Classic v3.34 Pointless?  

From: Paul Rodman <paulilanga.com> Date: Dec 2002

>From: Hari Seldon
>I came across this interesting point made by Meade in its official
>product guide for 1996, which put me into some thinking.
>Quote from page 20, titled "Version 3.34 software": "The very latest
>in telescope operating systems, Meade Version 3.34 software,
>included with all 8" and 10" LX200's, enables quick (less than 10
>seconds!) 2-star alignment of the telescope in the altazimuth mode,
>without requiring input of the observer's latitude or longitude,
>without even leveling the tripod. Just use 2 easy-to-find reference
>stars and the LX200 tracks and actuates GOTO with extreme precision."
>
>Now, the above statement clearly says that a) leveling the base, and
>b) entering the site geographic coordinates, is pointless with v3.34
>software for Alt-Az use. I have long believed (and empirical
>evidence supports this) that leveling the base as best as possible
>along with the input of site lat/long data improves pointing
>accuracy. [Still, the scope performs nicely even without being level
>or entering the site coordinates.]
>And if indeed leveling the tripod and entering lat/long is pointless
>under v3.34, why didn't Meade remove that little bubble level from
>the LX200's base after they started rolling out the 3.34 scopes?
>Could it be that a level base (along with the coordinates) is still
>needed for 1-star alignments in altaz?

If the computer knows how to point the telescope to two fixed (RA/Dec) positions in the sky (2-star alignment), then with a couple of 3D matrix transformations it can figure out the actual direction its physical axes are pointing with respect to the diurnal rotation axis (the invisible shaft through both celestial poles) and RA=0. From this it knows how to rotate its axes to point at various parts of the sky.

This doesn't require time, date, lat or long. However, not knowing the correct time, longitude, etc. means that it can't compute the current position of planets and other non-stationary solar system objects.

With a one-star alignment it can't do this bunch of 3D transforms. In order for the one-star alignment to work it has to make additional assumptions:

a. For AltAz: the scope base is exactly horizontal (i.e. RA axis pointing straight up) and the OTA is pointing exactly south and level (i.e. alt=0, az=0) when the computer is fired up, and the date/time/long are correct. This effectively means the computer can perform some math and figure out the RA/Dec of where the scope is initially pointing - giving it one "star". The one-star alignment gives it the other. In fact, you don't even have to do any alignment at all if the above initial conditions are exact...

b. For polar: the RA axis is assumed to be pointing exactly at the pole and the OTA is pointing at Dec=+/-90, RA=0 (?). The one-star alignment (or even just a GOTO+Sync) just calibrates the RA and Dec axes to a known RA/Dec. Since the RA axis is physically coincident with the diurnal rotation axis, two-star alignment is not necessary.

The GPS models allow a 2-star alignment in polar I believe - presumably to adjust for a bad physical polar alignment.

Subject: Solar Tracking URL  

From: Doug LePage

Check out this web page called LX200 tips and tricks by Anthony W. Haukap, He has some real useful information about solar tracking and alignment procedures down near the bottom of the long page.

<http://myweb.accessus.net/~090/lx200.html>
Note: should open a new browser window over this one.

Subject: Lunar Tracking Software URL  

From: James Burrows

Scott Rosenberg wrote:
>57.9 Hz Lunar rate; Best for tracking the Moon.

That's the average lunar rate in RA. The current rate depends quite a bit on the azimuth of the Moon because the observer is whizzing (almost supersonically, right?) around the Earth's spin axis. I reved up my "moon3" program and it said the current rate was 57.3 (with the Moon below the horizon) and declination rate -1.2% of sidereal. Changing the PC clock time to 23:00 with the Moon near upper transit, the lunar RA rate increased to 58.5. If anyone's interested in this niche of LX200 lore, the lunar pointing and tracking program is at:

<http://home.earthlink.net/~burrjaw/lx200/odyframe.htm>
Note: should open a new browser window over this one.

GoTo: Polar Aligning