Alternate 2 OTA & Fork Alignment Procedures
and Other On-Line Resources

       

Subject: Pointing Accuracy/OTA Alignment Procedure --part 1 of 3

From: George Nikolidakis

I had a bad pointing with my telescope a 12" LX200. I tried several methods trying to improve it including the second part of the OTA Alignment method with the reflection on the corrector but I had no luck. I recently developed a procedure based in real situations in order to bring the OTA axis and RA axis in parallel. It is a very simple procedure and it is worked very well in my scope. Note: only use this method if you have already adjusted the fork arms to the same height-- see OTA Alignment.

You can find the procedure in the follow address:
   <http://www.geocities.com/ginik_gr/ota_alignment.htm>

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Subject: Pointing Accuracy/OTA Alignment Procedure --part 2 of 2  

From: Bill Wood <wrwoodqwest.net>

As to OTA alignment I would suggest that everybody interested in this issue look at Mark Simpson's and George Nikolidakis' procedures. (Editor's Note: if the main RA drive gear is out-of-round or mounted off-center, it can cause pointing errors which will affect Mark's method.) Both methods look a lot more likely to produce excellent alignment accuracy than the laser beam idea to me. Mark's procedure is at:
   <http://www.aurorachasers.com/Astro/Lx200AlignmentProc>

and see George's procedure above. These fellows are using Polaris or a distant land based object viewed directly and aligned in a reticle EP or the finder scope for accuracy. Mark separates OTA alignment and fork alignment into separate tests. I think he has the right idea. His is the definitive method of aligning your OTA to the forks and aligning the forks to each other because it uses the real life view through your eyepiece to make and verify the adjustments. It definitely works and the OTA alignment can be done in daylight. In my case only the OTA alignment was necessary. By luck my fork alignment was OK from the factory and I did not have to touch it but my OTA to fork alignment was way, way off from the factory. Note that I do not have a wedge and know zero about polar mounting and pointing accuracy in that mode may be a completely different situation dependent mostly on the quality of your polar alignment.

I have found that the principle factors in AltAz GoTo accuracy are:

1. OTA to fork alignment and fork to fork alignment must be near perfect.
2. Use reticle EP to center alignment stars
3. Use BestPair2 to pick alignment stars but filter out low Alt stars, I don't use anything below 30 deg.
   Stars near zenith work fine for me.
4. Use backlash aware centering (see elsewhere in Archives) of stars in your reticle EP. Let the star sit dead center for at least 10 seconds before you press enter.
5. If you want to hit the planets on the nose make sure your Lat/Lon/Time are correct. My scope does not hold accurate time for long. I sync with atomic clock to the second every week, probably overkill.
6. Tripod must be dead solid, I also make it level just because I am obsessive. If setup on questionable ground, you must be sure the legs don't sink in.

Meade guarantees 2 arc minute pointing accuracy and I was not getting anywhere near that with my new scope as received from the factory this May. Targets were all within the FOV of the Meade 26mm Plossl but that is a far cry from 2 arc min. It took me 3 months of fiddling and reading Mapug posts and the invaluable help of experienced folks to work thru the above list. But my pointing accuracy is now well under 2 arc min consistently. Verified again this morning at 3:45am using Bogardus and Markab. Hit 6 random stars and Jupiter and Saturn with 1 arc min accuracy.

Subject: OTA Alignment Suggestion

From: John Mahony <jmmahonyhotmail.com>

I worked out the issues of axis orthogonality a few years back. I check fork height (polar/dec axis perpendicularity) by slewing along an RA line from +30* to -30* dec (preferably along the meridian, to eliminate east/west fork flexure), but then the rest is simple, based on the method mentioned above: move to 90* dec and rotate the scope in RA. If the optical axis is not perpendicular to the dec axis, then the image won't rotate around the center of the FOV, because the optical axis isn't parallel to the polar axis. Tweaking the dec to get closer to true 90* dec may help, but that will move the apparent center of rotation of the field along a straight line in the EP, and this line will miss the center if the optical axis is not perpendicular to the dec axis, because the optical axis cannot be made parallel to the polar axis, regardless of dec reading, in this case.

I have a web page for this at: <geocities.com/jmmahony/LX10/forkalign.html>. This was written back when I was using an LX10, so a few of the details are different for other scopes, but the basic concepts are identical for all fork-mounted scopes.

Subject: LX200 (classic) Fork Mis-Alignment --part 1 of 5    

From: Gene Chimahusky <lynol1000yahoo.com>

Thanks for your reply, please see response inline.
>From: Doc G
>Gene Chimahusky wrote:
>> no amount of checking/tweaking fork/OTA alignment and
>> still nasty point errors.
>> Last night I mounted a laser pointer on top of the DEC
>> fork arm (not on OTA, on top of arm) and did slews N-S
>> and S-N, well the laser dot would move to the left at
>> 0 deg. and right at 90 deg. The amount of movement was
>> about 0.08" at 49" throw, or about 6 arcminutes.
>> Mounted it on other fork arm, same thing, same amount.
>> put laser on steel plates that mount to pier, slew, no
>> movement (all four corners), placed laser on the cover
>> between where fork arms attach to base, slew, no laser
>> dot movement.
>> So it seems the runout in the Dec. axis shafts is
>> enough to cause a twisting of the top of the forks.
>> Anyone else ever try this?
>Doc G: First let me say that you have devised an excellent way to measure
>this mechanical problem. Very clever and perceptive of you. I have
>not seen this problem directly, but I can see how it could happen. I
>think my reassembly method fixes the problem for the most part which
>is why I do not see it as a final artifact.

Thanks. If you really want to see the mechanical response of your mount or forks, mount the laser various places to project a beam about 100-200 feet and have a helper kick the pier or rap the tube.

>I still use the old method of aligning the OTA described by Michael
>Hart on my web site. There are other ways to get it right, but this
>one works for me.
>Doc G: The problem you describe could come about in at least three obvious
>ways. One would be that the Dec. shafts are not perpendicular to the
>OTA. This would be caused by a badly machined shaft and saddle part
>on either or both sides. I would hope this is not the problem since
>it is difficult to fix. I suspect that these parts are made with
>fixed jigs and should be quite good.
>A second way would be that the forks are not aligned perfectly back to
>front. This misalignment is fixed by using the Hart method using a
>precision steel bar as described on my web site.
>A third way would be that the Dec shafts are perpendicular to the OTA,
>but they are not exactly opposite one another. There is an adjustment
>for this build into the saddle where it connects to the OTA. The
>alignment technique on my web site describes how to make the final
>adjustment of the OTA Dec shafts. Following this method should
>eliminate the problem you are having. I have always used the Hart
>method to do alignment and while there are other clever methods, this
>one I know works.

I fully understand how the non-perpendicular shafts can cause the problem, but am at a loss why rotating a perfect cube (the OTA for arguments sake) around an axis (the DEC axis) can cause a change in the support of the axis, no matter what initial conditions the axis is supported at, high or low, fore or back. This is the top of the fork arms that are torquing, not the path described by the OTA, unless balance of the OTA comes into play. I did try the slews with the OTA cover on/off and no counterbalancing whatever with the same results. I even tried with the tension adjustment of the worm to wheel way loose and held it away manually so as to not bind, same result.

I have used Mark Simpson's OTA Alignment Method of sight on object, rotate 180 RA, rotate 180 DEC, re-site on object, tweak OTA till I get 1 arcmin error, always approaching from same direction.

For the fork alignment I have done a manual tpoint type calculations in Al/Atz mode based upon sync followed by measuring the positional inaccuracies of distributed stars (GoTo star, jot down RA/DEC, then center star and jot down RA/DEC) then plotting the errors based upon Al/Atz, mix some trig in and out pops a 'tweak' figure in millimeters or fractions thereof to be applied to a fork. Measurements are made with a dial caliper to base of both sides of fork arm mounted to front of square extension on the base. Latest iteration had sync on stars at 32 degrees alt (Hamal) and Vega at 54 degrees alt. Deneb at 77 degrees is within 2 arcmin, yet Fomo and Diphda have the OTA pointing 9 arcmin directly south of actual, and Altair/Enif had the OTA both pointing about 6 west and 5 south total .

I could not reconcile the geometry for further tweaks with physical movement of the forks, so I went looking elsewhere, like mount flexure, etc., till I found the torqued fork arms.

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Subject: LX200 Fork Mis-Alignment --part 2   

From: Bruce Johnston

Gene, I went through the same thing, and chased the problem for TWO YEARS!! After getting the scope back from Meade after a 'repair', I had this. I even sent it back to Meade, but according to them, it passed all of their alignment tests just fine. The fact is, what they actually did during the first 'repair' was to bang one of the forks and misalign it.

<<Seems to be my experience that it can be aligned at the factory but not everything is accounted for.>>

Very true, as can be seen by my experience. However, when the failure is solid, and the symptom is documented in detail, as well as a very simple method for recreating the failure, even with no power to the scope, one would expect that, for $500, the manufacturer should be able to locate the cause and correct it, or so I feel.

I tested using a dial indicator against the OTA, instead of using a laser but I still had your kind of results. The fork assembly would shift E-W when I moved the scope only N-S. This, even after having replaced the RA bearings.

<< How was the dial indicator positioned against the OTA when the OTA was moving N-S,
or was it against the top of the forks?>>

Actually, both. It was first set up against the side of the OTA. Along the flat portion of the block that holds the OTA to the forks. Then, of course, it was a simple matter to just set it against either the North or South side of one of the forks, which I did. Both forks ..... the entire fork assembly .... rotated E-W when there should have been no E-W movement. The mount for the indicator was resting on the pier plate. The basic measurement actually was the same as what you made. That is, whether or not the forks rotated E-W at all.

But long after I knew this, it still took going to the site that Doc G. is referring to, to use a method that could determine for certain, that the forks were not parallel, and what to do to correct the situation. The important thing here is, at least you have a pretty good idea as to what may be the problem, and that's a lot, in and of itself. See my procedure at:
   <http://www.mapug-astronomy.net/ccdastro/goto.htm>

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Subject: LX200 Fork Mis-Alignment --part 3    

From: Bob Klein

Hi Gene, I have an LX200 12" with newly installed roller bearings per Doc G and others. I found that when I reassembled the fork arms after shimming, aligning etc. and tightening those two .5" bolts to 65 inch-pounds they wouldn't stay put. There was a tendency for the arms to sag and twist the bolts. (I didn't want to go higher in torque than 65 inch-pounds into an aluminum casting) Solution was to align and torque with the fork arms in a vertical position and flow some Eastman 910 or similar super glue into the interface between the arm and the spacer and the spacer and base. With TPoint I get less than 1 arc min pointing error. My Dec to RA axis orthogonality is about 17 arc sec according to TPoint.

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Subject: LX200 Fork Mis-Alignment --part 4    

From: Gene Chimahusky <lynol1000yahoo.com>

Even though I use my scope in Alt-Az mode, the twist I found does seem to be weight induced sag related. I found that the forks were offset with respect to each other about 0.25 deg. from vertical (1/16" off at top of fork). In that situation I was seeing a twist of about 8 arcmin during the sweep. I set them up as close as I could and now have a twist of about 1 arcmin as sweeping N-S/S-N. What I also found is there seems to be a fair amount of angle from vertical required for each fork to get orthogonality and the top bearings in position relative to each other. The right fork needs to be tilted N and the left fork needs to be tilted S (from Alt-Az home position) to bring the DEC axis of the forks in alignment. The amount for my scope is about is about 1/8" each fork as measured at top of fork (right back 1/8, right forward 1/8) which is a FAR cry from what it originally shipped with and also was subsequently returned with from Meade after an alignment.

My error now seems to be about 4.5 arcmin max, so I still have more alignment to do.

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Subject: LX200 Fork Mis-Alignment, End of Saga --part 5 of 5   

From: Gene Chimahusky:
> My error now seems to be about 4.5 arc min max, so I
> still have more alignment to do.

Seems that I have finally reached the 2 arcmin pointing, the last tweaks were caused by DEC runout, the symptoms and solution are described perfectly by Bruce Johnston at:
   <http://www.mapug-astronomy.net/ccdastro/goto.htm>

This was in AltAz mode using two stars at roughly same RA. Assuming scope is level to start with, turn scope on, slew to low star, sync, GoTo high star and check how much under or overshoot. Then I slew N or S 10 degrees, unlock DEC clutch, manually re-center high star, re-lock DEC, sync on high star to provide a reference, then GoTo low star. I based everything off the low star, so I re-center it, sync, then GoTo high again. I kept repeating till I saw offsets start to increase again and backed off by the last 10 degrees.

My RA has not needed the procedure, but if yours does, just pick two stars at roughly the same DEC and slew back/forth in RA.

BTW: I can affect the amount of runout on my mount by applying pressure to the top of the fork and front/back of the OTA. I discovered this while performing the Mark Simpson's OTA alignment method. Seems I can cause about a 0.002/0.003" change, enough for my 'extra' 2.5 arcmin of N/S under/over shoot.