Pointing Accuracy & GoTo Issues--Page 1

       

Subject: LX200 Classic Test for Pointing Accuracy --part 1 of 2  

From: Ed Fitzgerald <fitzgeraldmsn.com> March, 1999

Michael--I had the same problem at one time, and it turned out that the RA motor was bad. It had to be returned to Meade for repair. Then ditto on the Dec motor. This time, Meade sent back an engineering change that added a couple of power resistors to the circuit board.

The way to tell if you have a problem like this is as follows:

1. Set up the scope in LAND mode and focus on a fixed reference point (e.g., a neighbor's roof corner) that you can return to, preferably with a reticle sight.
2. Press the MODE key on the keypad until you get the ALT/AZ coordinate display. Write down the Altitude and Azimuth values.
3. Slew the scope in the azimuth direction back and forth. I stood behind the control panel and first slewed the scope CCW so it pointed at me. I then slewed it in the CW direction until it again pointed at me. Finally, I slewed the scope back to get the reference point centered in the reticle sight. Again write down the altitude and azimuth readings. The altitude should be unchanged and the azimuth should be within 1 minute of your starting azimuth.
4. Repeat this procedure only this time slew in the altitude-only direction. Again return to the starting reference point and write down the altitude/azimuth readings. This time, the azimuth should be unchanged and the altitude should be within 1 minute of the starting value.

Be sure you lock down the Alt and Az knobs before starting this to make sure there is no slippage. Let me know how things work out.

Subject: LX200 Classic Pointing Accuracy Test --part 2 of 2  

From: Michael Sparks sparksmdzoomnet.net Date: Sept., 1999

Herb Rehire wrote:

> Still having trouble with my 10" LX200's slewing to Polaris being off by 5-10
> degrees, despite multiple iterations and corrections. Level, date, time,
> lat/longitude all double-checked (on Astropier with Meade Superwedge).
> Declination circle reads 84-85 degrees (instead of 90) when slewing to
> Polaris. Slewing to objects (e.g., Jupiter) also off by similar amount in same
> direction (west). Could the declination gear/drive be off? Is there a way
> to correct this without shipping back to Meade? If this is the case,
> pointing will be off even if I do a precise drift alignment, correct?

Herb, I have fought this same monster from day one when I received my 12" LX200 in July. I asked the same question of this group and got a lot of excellent advice and suggestions. I tried them all. Unfortunately I still have significant problems with pointing accuracy. After speaking to Mead several times the only response I got from them is send it back. Here is a step by step test that I have developed to tell me once and for all if my problem is mechanical of electrical. I am open to suggestions where my analysis might be misguided.

1. With the scope mounted on the SuperWedge perform a precise drift alignment. 10 or 15 min. with no drift.
2. Choose five or six bright alignment stars that Meade uses for scope alignment procedure. I chose Fomalhaut, Vega, Altair, Deneb, Cappella and Aldeberan. These stars cover a good spread of the sky.
3. Start with one star, say Fomalhaut, and synch on it. Write down the DEC and RA displayed by the hand paddle.
4. Select one of the other test stars, say Vega, and use the scope's GoTo function to go to it. Of course if your scope is like mine Vega will not even be in the eyepiece. Use the N-E-W-S buttons on the hand paddle to center Vega in the reticle. Read and write down the DEC and RA from the hand paddle.
5. Now with Vega centered in the reticle synch on Vega. Write down the DEC and RA. They will change once you synch. The difference between the before synch and after synch DEC and RA is how much the scope stopped short of or overshot Vega using Fomalhaut's synch.
6. Now use the scope's GoTo function to go back to Fomalhaut. Of course Fomalhaut is nowhere to be found in the eyepiece. Again use the N-E-W-S buttons to center Fomalhaut in the reticle. Write down the DEC and RA. Synch on Fomalhaut and write down the new DEC and RA. Again the difference is how much your scope stopped short or overshot Fomalhaut using Vega's synch.

I did this test with all six stars, going from each star to the other five, synching on the starting star going to the GoTo star, recording offset, resynching and going back to the starting star and checking offset. This gave me 30 sets of data.

Next I went back and did the same 30 test runs again but this time when I got to the GoTo star I did not synch on it. I simply centered it with the N-E-W-S buttons and the used the GoTo function to return to the starting star. Here is what I discovered and deduced from the test.

First, I noticed that the amount of undershoot in DEC and RA when going from Fomalhaut to Vega with a synch on Fomalhaut was within 3 arc minutes of the amount of undershoot recorded when going from Vega to Fomalhaut with a synch on Vega. This observation applied to every set of reciprocating data. Deneb to Altair was off by the same amount as Altair to Deneb, and so on.

I also found that when going from the start star to the GoTo star and going back to the start star without synching on the GoTo star that the start star ended up very close to the center of the reticle. From these results I believe that my electronics, encoders and drive motors are working perfectly. I believe this because no matter where I slew to in the sky I can always use the GoTo function to go back to the star that the drive is synched to with reasonable accuracy. I also believe that the OTA is misalign on the forks or the forks themselves are misaligned. (Nice QC Meade!). In fact if I wanted to take the time I could probably use the test data to calculate precisely how much the OTA is misaligned and in what direction.

My next decision is whether I should attempt to correct the misalignment myself or return the scope to the same group of Bozos who couldn't get it right in the first place. I haven't made up my mind yet. Any advise from this group is welcome. Sorry for the long post. But, I have a lot of sympathy for others who are going through the same frustrations I have gone through in the last 2 months. Spending $4500 on a piece of precision equipment that arrives in the box from a so-called reputable company not working properly is an absolute shame. Hopefully this will become one more valid test for the archives to help other Meade users in the their battle to make their scope work the way it was advertised to work.

Subject: LX200 Classic Pointing Accuracy --Part 1 of 2  

From: Doc G, Date: Jan., 2000

I have read commentary on MAPUG about the pointing accuracy of LX telescopes for over three years now. To say that experiences differ would be a gross understatement. Many have said that the LXs meet the Meade claims and as many, or more, have pointed out that they usually, but not always get the object into the eyepiece (26 mm) with larger slews. (large slew is say 90 degrees) The former means slew accuracy of less than 5 arc minutes and the latter about 30 arc minutes. There are a few who claim that they do not get even 30 arc minutes and some (rare cases) where they get 2 arc minutes or so.

These reports are factual and I believe real experiences. Why are there such great discrepancies? I believe that the mechanics of the LX are such (and they vary by great amounts) that doing the GOTO with larger slews (say 90 degrees or so) should be expected to vary by large amounts. The factors that effect these broad variances have several sources. One is that the LXs have rather poor mechanical systems (compared to mounts that cost many thousands of dollars) and that the mechanical details vary greatly from scope to scope. Thus one might have a "mechanically challenged" instrument. Or one might have a mechanically superior instrument. A second factor is the condition of the mechanical system. This can be improved greatly by the user after a study of the many "fixes and tweaks" that are in the MAPUG archives and my own web site. Many, probably hundreds, of users have reported to me over the past three years that they have effected significant improvements in the mechanical performance of the LXs by applying these tactics.

Another factor is the mechanical acumen of the user. The user must be able to recognize the mechanical problems and the general area of the problem so that a fix can be implemented. This requires a natural intuition into mechanical systems and careful attention to what is going on while the scope functions. It is clear from the posts on MAPUG that these abilities vary greatly among users.

Still another element is the mode of use and actual use pattern for the telescope. It is quite one thing to use the telescope visually, be at its side, use HP pointing, have a well adjusted and balanced scope and so forth, or not do one or more of these things. Great attention has to be paid to every detail to get the most out of the LX scopes. Regular users have often developed techniques to compensate for any mechanical defects or idiosyncrasies of their instrument. Some of these may even be subliminal. They greatly affect the perceived and real precision attained.

Another factor, and there may be more, is the actual propose to which the telescope is put. Visual use, that is fully attended operation of the telescope, is much more forgiving than remote use. In close use any pointing misses can be quickly fixed by a few key controller actions or by evoking an HP cycle or simply a re-synchronization to the new region of the sky. These

actions are almost automatic. But for remote operation the situation is quite different and immensely more difficult. No errors at all are allowed. The slew must put the object on the chip every time. If it fails, pointing reference is lost. A search about the desired point is difficult and time consuming. The GOTO operation must work perfectly every time. So for remote operation or sequential searching or long scripting operations, the mechanics must be much, much better than for visual use.

These factors must be evaluated by the user and applied to his/her particular situation. My impression is that an LX telescope can be made to work for totally remote operation, but it is not easy. Rarely will the scope come out of the box and be ready for the most demanding use. There have been enough reports of good GOTO operation that one must believe it is possible to get GOTO accuracy of a few arc minutes over long slews. I personally have an LX 12" and LX 10" which will regularly do this. The 12" is much more stable since it is permanently mounted. The 10" is rarely used for other than more casual viewing.

I would estimate from many reports and many personal correspondences I have had, mostly as a result of my web site information, that a lot of users are getting good results from their instruments. But they have had to go through the "fix and tweak" process in more or less detail. It can be done!!

However, for remote operation, I know of no instances where the LX system can be made perfect enough and reliable enough to get by without some occasional attendance over the imaging session. This is with the exception of several persons who have permanently mounted telescopes and have done a thorough TPoint mapping of their mounts. The last bit of perfection in slewing/pointing accuracy is hard to attain. But, it can be done. See more here.

With a few other mounts, it is likely easier to do. These mounts usually cost twice the cost of the entire LX telescope or more. Even a well known $10K mount will not slew to better than a few arc minutes without application of TPoint. In fact, even gigantic professional telescopes will not point with sub arc minute accuracy without some software that resembles TPoint. These mapping programs were after all developed first for professional telescopes.

So what is the point of this long discussion? (I apologize for its length.) But the point is that better results can be had with careful attention to the problems (fixing and tweaking). Some persons are very good at this and some are not. At their best, the LX mechanics (as long as it works at all) can be made to do very acceptable GOTOs for remote CCD imaging, but TPoint may well be required.

I strongly suggest all who are interested read Mr. Frank Loch's excellent article on T-point. It is on my website.

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Subject: LX200 Classic Pointing Accuracy --Part 2 of 2    

From: Jim Sgt. <JimSargeaol.com> Date: Jan., 2000

I have not used TPoint, but instead a different approach to overcoming pointing problems with my LX200.

I've mounted a PC23C video camera behind a 200mm f/3.5 telephoto lens and use the combination as my finder. A coax takes the signal from the camera indoors (about 50') to a small monitor. I GOTO a bright star (at least mag 5) near the object I'm looking for. The LX200 always points well enough for the bright star to show up in the approximately 60h x 80w arcmin view on the monitor. Using either the hand controller, ACP, or the telescope motion controls in TheSky or Guide, I steer the telescope to center the bright star in the CCD capture box I've drawn on the monitor. At this point the operation becomes a typical center, synch, and GOTO the dim object I'm looking for. Even at f/20 with my 12", I can put whatever I'm looking for on my ST7 chip with a minimum of (or no) searching.

All of this is very like a LX200 HiRes pointing operation, except that I never have trouble finding the bright star with that big field of view. This approach has proven very reliable and rapid to use.

Subject: Pointing Accuracy & Fork Flexing --part 1 of 2 

From: Doc G, Date: May 2001

I absolutely agree that the LX mount can hold much larger loads in the alt/azm mode. I agree that when in equatorial mode it looks precarious. And it certainly has a load limit in the polar mode.

Still the measurements indicate that the bearings are stiffer with the pre load. This is to be expected since common machine design practice uses prelude to accomplish this very factor. (stiffness)

I have been trying to make some additional measurements on an LX base and now am starting to believe that the springiness I found in the RA axis may be do to deformation of the base plate of the mount to which the RA bearings are mounted with a conical part. This may be the weak spot in the design. (not sure yet)

I would be 100% in favor of alt/azm mounting of all telescopes. Then a de-rotator could be used for imaging. This is commonly done for all new BIG scopes. (I mean REALLY BIG) But for small scopes the perfection you can get with a wedge for modest cost is very enticing. De-rotators exist for small scopes, notably the LX scopes. I know of no others. But, there are many disadvantages with de-rotators. I will not go into these here since they have been discussed at length on mapug.

So in the final analysis you have one choice to correct field rotation. That is to mount it in equatorial mode. If a scope is to be used for visual work only there is no point whatever to equatorially mount it.

To your point about fork tine flexure, there is so little that I could never measure it. The same for the wedge. I determined that the super wedge contributed no more that 10% of the flexure of the RA axis structure.

I do not know your latitude, but I had an interesting post from someone who was at about 10 degrees, as I recall. That puts the RA axis darn near flat. He is going to convert the LX fork to a true double fork. This is the classical English mount. The English mount is very stable.

Paul Luckas wrote:
> I know you've done all of the mechanical analysis - and I don't want to
> start anything here (because I actually agree with you) but, even though the
> mount's bearings perform better preloaded - what about fork tine flexure ?
> It is possible that this is more of an issue - particularly with a full
> load of equipment - in polar mode than it is in Al/Atz mode where the moment
> / force travels straight down thought the forks and base regardless of
> where the scope is pointed ? In other words, is it still possible that
> LX200s are better at "pointing" in Al/Atz mode, than they are in polar mode

> ----- Original Message -----
> From: Doc G
> > Some years ago I measured the spring constant of the first bending
> > mode of the LX RA axis. As I remember, the amount of bending was
> > about 100 arc seconds per kilogram.

> > Thus I have always recommended that the scope be polar aligned with
> > the full load of equipment that is attached when the scope is used.
> > Those who want to know much more about the mechanical properties of
> > the LX scope mounts can find this information on my website under the section
> > on LX200 Mechanical Analysis.

> > It is interesting, that the RA stiffness gets higher in polar mode
> > because the bearings are pre-loaded.

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

Subject: Pointing Accuracy & Fork Flexing --part 2 of 2

From: Jim Slay <JimSlayNTSaol.com>

Fork flexure does happen, and appears as retrograde motion!

What happens is; the DEC correction produces a very small torquing action and twist the fork arms. This appears as movement in RA axis, and then settles as long as movement is done in same direction. Working with Bruce Johnston of Johnston Computing we have measured approximately 0.005" or 15 arc seconds of what appears as retro grade movement induced by DEC corrections with RA locked in place. This varies from LX to LX and even after DEC bearing rebuild this condition still exist!

I have found that by closing in the U channel of fork arms with 3/16 thick aluminum and securing with stainless steel fasteners from base of fork arm to just under DEC axis and bracing underside of fork arm with 1/4" aluminum that this removes most of this flexure.

At this time we have not tested the corrected unit by measurement but the condition is no longer apparent in braced LX scope. I hope to post this fix along with many more when time and job will allow. I am looking into semi-retirement, and in process of purchasing small CNC mill, lath, and other equipment and may offer ultra precision fixes for those that would want this type of work done for them.

Subject: Centering Test Results  

From: Paul Markov

I have more slewing accuracy & centering results, but this time using the SuperWedge. All this is with a 10" f/10 Classic (about 5 years old, no modifications of any kind). I am posting this data for your reference so you can compare your results to my "average" LX200 and also so that the experts on MAPUG can offer their comments.

First, I did a very accurate measurements of the actual fields of view (FOV) of my eyepieces (more on this under separate email). The Meade 12 mm MA illuminated eyepiece gave me a FOV of 10.5 arc-min (therefore, center-to-edge is 5.25 arc-min), while the Meade 26mm Super Plossl is 27.25 arc-min (centre-to-edge is 13.6 arc-min).

Standard polar alignment as per Meade's manual, synched on Antares:
  Antares to Arcturus (56 deg. separation) - 6 arc min
  Antares to Vega (72 deg sep) - 15 arc min.
  Antares to Denebola (79 deg sep) - 8 arc min
  Denebola to Arcturus (35 deg sep) - 9 arc min
  Arcturus to Antares (56 deg sep) - 1.5 arc min
  Antares to Alphekka (54 deg sep) - 9 arc min

Then I synched on Alphekka:
  Alphekka to Arcturus (19 deg sep) - 4 arc min
  Arcturus to Vega (59 deg sep) - 10 arc min
  Vega to Deneb (23 deg sep) - 17 arc min
  Deneb to Altair (38 deg sep) - 14 arc min

Then I refined the polar alignment using the iteration method (as per MAPUG Archives). Four iteration Polaris to Altair. After the third iteration the error to either Polaris or Altair was just 1 arc min. This iteration method took just 12 minutes to do. Here are the errors now after synching on Altair:
  Altair to Vega (34 deg sep) - 8 arc min
  Vega to Deneb (23 deg sep) - 7 arc min
  Deneb to Altair (38 deg sep) - 1 arc min
  Altair to Arcturus (81 deg sep) - 14 arc min
  Arcturus to Deneb (80 deg sep) - 9 arc min

Then I resynched on Deneb:
  Deneb to Vega (23 deg sep) - 5 arc min
  Vega to Altair (34 deg sep) - 9 arc min
  Altair to Arcturus (81 deg sep) - 16 arc min

Looks like improving the polar alignment via the iteration method did not improve the centering accuracy (perhaps my initial standard polar alignment was already good). Comparing my results of a few nights ago (where I set up in alt-az mode), looks like centering was better in alt-az mode for short slews, while polar mode is better for longer slews. But I am not entirely sure about this conclusion - more testing required.

I did not remember to note in which direction the error occurred, however I seem to recall the error was all over the place; east, west, north, and south of center depending on the target. Next time I do testing I will make sure to record this information. I am not too impressed with the above results since very few where within the 12 mm eyepiece field of view, however most targets did end up within the 26 mm eyepiece.

Subject: Excellent Pointing Accuracy Detailed  

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

Well I decided to do a little test last night to determine how good the pointing accuracy is on my polar mounted 12" LX200. I knew it was pretty good but I hadn't really measured the errors yet. I also wanted to do this since I figured this was an alternative way to determine how accurate my polar alignment is. I must say I'm rather impressed and I think I may have an exceptional unit. Maybe not but it sure is a lot better than the 8" that I had before. Then again I never spent as much time trying to perfect everything with it (i.e. alignment, balance, backlash, etc). Having it permanently mounted sure does motivate you to perfect everything.

Anyway, I basically did a GOTO on 10 stars in various parts of the sky. I did NOT do a resync on any of these stars. I measured the error using my CCD to measure the distance from star centroid to CCD center. The results are:

• 1'24"
• 1'47"
• 0'47"
• 2'09"
• 2'06"
• 2'42"
• 1'08"
• 1'13"
• 1'01"
• 3'14"

This gives me an average of 1.75' which I think is great! I can't imagine how good I can get if I get TPoint. Somehow I suspect that this accuracy will get worse though with wear and tear since its pretty new.

Subject: LX200 Classic 'GoTo' Adjustment (gear sweetspot)  

From: Bruce Johnson

I came up with a pointing fix for my LX200, where I was having *bad* results with my 'GoTo' operation. I posted the technique, and if anyone is having this problem, they might want to stop by and see it. It just might help. It can be seen at:

<http://www.mapug-astronomy.net/ccdastro/goto.htm> Note: should open a new browser window over this one.

Subject: LX200 classic Unique Slew Problem?--Bet No One Has Had This One  

From: New Mexico Skies, Inc. <nmskiespvtnetworks.net>

Wayne Watson <mtn_viewsirius.com> wrote:
Either from the keypad or from a software package (MegaStar), I cannot slew N ,E, S, W. However, I can direct the scope to move to a specific location with either the keypad or software.

Response:

Sounds to me like the 74LS14 chip on the main board that interfaces the autoguider port. I've had this happen to me from hot-plugging an autoguider.

When the autoguider is sending a move command, (e.g.) one of the gates on the LS14 chip is held low by the autoguider the slew buttons are locked out by the software. Everything else works but not the slew buttons. A shorted input on one of those gates (from static or 'hot' plugging) will do it every time. A variant is where the scope won't track (East button stops the scope from tracking) and everything else works but the slew buttons. I believe the above statements are correct. Its happened here several times on our 12" and once on our 16". We put in sockets for that chip and keep a few on hand.

Subject: Worm Wheel Lapping 

From: Chris Heapy <Chrisheasynet.co.uk>

This is a Really Bad Idea. A lap works by having the abrasive embedded in it, and it's the harder material that will be worn down. Further, carborundum abrasive will be virtually impossible to remove once embedded in the softer worm gear and it will continue to grind away. A method of making laps involves rolling a copper cylinder over diamond dust on a hard steel plate, the diamond gets embedded in the copper (just to illustrate).
I*have* tried 'lapping' the precision worm gears I made myself, but this was using metal polish which was subsequently removed with solvent. The 'lapping' effect is very modest indeed, and it probably does no more than put a surface polish on the the teeth, it was not intended to correct any tooth irregularities. If you could mount the worm gear and worm in a hobbing machine you could use pressure to 'cold-form' the teeth - unlikely the average amateur will have the equipment though.
For an overview on making precision worm gears in a home workshop I have written an article which is listed below:
    <http://easyweb.easynet.co.uk/~chrish/worms.htm> Note: should open a new browser window over this one.

Editor's Note: also see Doc G's discussion of worm gears and what they really are here.

Subject: TPoint and LX200 

From: Patrick Wallace

In view of the recent postings about TPoint it might help if I listed a few FAQs. Also see the personal experience of Frank Loch & Doc G's experience in Doc G's website hosted here at MAPUG-Astronomy.net.

*What are TPOINT and SLALIB?

TPOINT is a computer package that analyses telescope pointing observations and tells you about the misalignments, flexures, runouts and so on in your telescope.

SLALIB is a library of (mainly) positional-astronomy routines. TPOINT does all its precession etc. by calling SLALIB routines. (SLALIB covers much the same ground as Jan Meeus's "Astronomical Algorithms". The latter is a better starting point for hobbyist programmers.)

*Are TPOINT and SLALIB academic or proprietary software?

There is a Fortran-based version of SLALIB supported (mainly for professional use) by the UK Starlink Project. Libraries for various

UNIX platforms are available. There used to be a Fortran-based version of TPOINT as well, developments of algorithms originally developed for the AAT project in the 1970s. This is no longer supported. There are also proprietary versions of SLALIB and TPOINT, both written in C, and marketed by TPoint Software under arrangements agreed some years ago with CCLRC/RAL.

Although astronomy research institutes use SLALIB for free (not TPOINT though), note that this concession does not extend to for-profit use. Potential users should refer to the copyright and disclaimer notices.

*How do you use TPOINT?

Whenever you want to check out your pointing, you observe maybe 30 stars all over the sky and feed the data into TPOINT. The package has rich model-building, fitting and graphical display tools that allow various deficiencies to be exposed and measured. The numbers can be fed into your telescope control system to improve the pointing and tracking. Or you can use the information to tell you what needs adjusting mechanically, for example the polar axis.

Once you've got a good model for your telescope, you can use it as the basis for a brief start-of-night recalibration, correcting just the zero-points, plus collimation errors perhaps, and polar axis misalignment if the telescope is non-permanent. This may only take two or three stars at a pinch.

Repeated pointing tests over time will monitor changes in your telescope and give early warning of mechanical problems. You will also build up a large data set from which additional pointing terms can be gleaned.

*How does TPOINT work?

TPOINT reads files of pointing data and does all the necessary positional-astronomy transformations to predict where in the sky the star should have been seen. The raw telescope readings will not, in general, agree with these positions; models describing how the disagreement varies around the sky are constructed interactively from a repertoire of likely terms. SVD-based least-squares fitting techniques estimate the coefficient values for the model, and tell you whether a given term is statistically significant. Through a number of graphical tools, remaining uncorrected errors can be viewed.

*How well does TPOINT work?

TPOINT squeezes out of any given telescope all the available pointing performance: some large telescopes deliver pointing at the 1 arcsecond level using TPOINT models. An LX200 should achieve about 1 arcminute RMS, all-sky, and drop sources straight into the center of a CCD. You should also be able to measure polar alignment on an LX200 to better than 30 arcsec using TPOINT.

*What's the difference between the two TPOINTs?

The Software Bisque version and the TPoint Software version both use not just the same algorithms but in fact the same core of C code. The differences are principally to do with the user interface. The SB version is integrated with TheSky and has a windows interface. The native, or "pro", version is command-line driven.

Subject: Best Stars for Polar Iteration --part 1 of 2 

From: Ralph Pass <rppassrppass.com> Date: Aug 2001

<desantis.rmpg.com> wrote:
> Somewhere I remember reading that if one was to do polar alignment for the
> iteration method that there were some positions of stars (RA, that is) that are
> better than others. I can't find the references so I was hoping someone could
> point me in the right direction.

Avoid stars near RA 2h 30m at all costs .

Be careful with stars near RA 14h 30m . If you use a star close to 14h 30m, use the 'half the apparent correction method'.

To determine the best stars easily, use my freeware program BestPair II for Mac, PC, & Dos.
See Software & Computer Issues, page 1 topic.

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

Subject: Best Stars for Polar iteration --part 2 of 2  

From: Greg Hartke <ghartkeclark.net> 

Choose stars that are between 4 and 8 hours of right ascension away from Polaris. Ideally, choose a star 6 hours away in either direction. This will work excellently.