Drift Test for Mirror Slop
By Chris Vedeler
Copyright 1997
1- Introduction
I found it odd and frustrating that while most of my astrophoto's were guided very well using my autoguider and guide scope, sometimes a shot would show oblong and even trailed stars. After several months of informal observations it seemed that the trailed pictures tended to be taken above 20 degrees North and always West of the meridian. The further North up to about 60 degrees, and the further the shot was started to the West, the worse the trailing. Most of the time, my other shots showed only the slightest hint of oblong stars or where guided where no detectable error was made (i.e. the smallest stars being perfectly round).
You can clearly see the oblong stars in the 1 hour exposure of M33 above where there was about 30 to 60 arc seconds of drift. I have pretty much ruled out differential flexure between my guide scope and the LX200 as the cause of this drift and so it must be the result of mirror slop. I didn't expect 60 arc seconds of mirror slop and so I didn't believe it.
On the weekend of November 15th and 16th 1997 I designed the following test to help determine if the problems were due to the primary mirror shifting as the telescope tracked. This hypothesis seemed the most likely and was the topic of discussion on APML about the same time. I also wanted to see the real results of using my mirror stabilizer and determining objectively if it solved the mirror slop problems well enough to be used with astrophotography.
1- The Test
Scope and conditions:
10" f/10 LX200 setup in polar mode with the Meade Superwedge and my Meade Superwedge Mount Stabilizer installed. 80mm f/11.4 guide scope was attached to the LX200 using my home made mount and rings. The Meade Pictor 208XT autoguider was used with the guide scope.
The tests consisted of centering a guide star in the guide scope and centering a star (not necessarily the same star) in the cross hairs of my Meade 9mm reticle eyepiece in the LX200 at f/10 (277x). Then I would install the Pictor autoguider on the guide scope and let it guide as I would normally do while doing astrophotography. Then I would periodically check the tracking by observing any apparent movement of the star in the 9mm reticle with the LX200.
I used the following values to estimate the amount of guiding error during each test:
Meade 9mm reticle eyepiece on 10" LX200 f/10.
Diameter of outside ring: 52.95 arc seconds
Diameter of inside ring: 23.5 arc seconds
Space between the lines: 5.23 arc seconds
These figures where determined by timing the rate in which the point of glare from a distant object would move in daylight with the telescope tracking in RA (i.e. 15 arc seconds per second).
Tests where performed with and without the mirror stabilizer as well as locking down the mirror completely. I locked the mirror down completely by stacking 3 one inch nylon washers without the spring on the 6" bolt (see for a picture of the mirror stabilizer with the spring ). I would leave the wingnut loose while I get the star in focus. Then I would gently tighten down the wing nut and watch which way it pulls the star out of focus. Then I loosen the wing nut and put the star slightly out of focus the other direction. To do the final focus I would tighten the wing nut until the star is perfectly in focus. It takes some practice to get just the right tightness on the wing nut (you don't want to over tighten) and have the star perfectly in focus, but it is not hard.
Also I had noticed that the cables hanging off the autoguider added a slight amount of weight and the coiled cable acted as a spring pulling down on the autoguider as the telescope tracked in the Western sky. In some of the tests I secured both the power cable and the coiled CCD cable around one of the wingnuts on the guide scope mount to test if this weight and spring action may have some adverse effects when tracking in the Western sky.
November 15th 1997.
Conditions: Moon full, seeing 5 out of 10, in central Tucson AZ.
Test 1:
1 hour tracking near M31 (Northeastern sky) with the mirror stabilizer installed with spring, autoguider cables dangling free.
Results: the star had drifted about 3 arc seconds in 1 hour.
Test 2:
1 hour tracking Deneb (Northwestern sky) with the mirror stabilizer installed with spring, autoguider cables dangling free.
Results: 20 minutes into the tracking the star had drifted 15 arc seconds. Test aborted.
Test 3:
1 hour tracking Deneb (Northwestern sky) with the mirror locked down tight autoguider cables dangling free.
Results: tracking with 7 arc seconds of drift in 1 hour.
Test 4:
1 hour tracking near M42 (Southeastern sky) with the mirror locked down tight autoguider cables dangling free.
Results: tracking with 2 arc seconds of drift.
Test 5:
1 hour tracking near M42 (Southeastern sky) with the mirror locked down tight autoguider cables dangling free.
Results: tracking with 4 arc seconds of drift.
November 16th, 1997.
Conditions: Moon 1 day past full, seeing 6 out of 10, in central Tucson AZ.
Test 1:
1 hour tracking of star near M31 (Northeastern sky) no mirror stabilizer installed at all, autoguider cables dangling free.
Results: tracking with 6 arc seconds of drift.
Test 2:
1 hour tracking of star near Deneb (Northwestern sky) no mirror stabilizer installed at all, autoguider cables dangling free.
Results: 20 minutes into the tracking the star had drifted 25 arc seconds. Test aborted.
Test 3:
1 hour tracking of star near Deneb (Northwestern sky) mirror locked down tight autoguider cables dangling free.
Results: tracking with 4 arc seconds of drift.
Test 4:
90 minute tracking of star near Deneb (Northwestern sky) mirror locked down tight autoguider cables secured.
Results: tracking with no detectable drift.
Test 5:
90 minute tracking near M42 (Southeastern sky) mirror locked down tight, autoguider cables dangling free.
1 hour tracking with 2 arc seconds of drift.
3- Conclusion
It appears from these tests that the trailing I experienced was indeed due to mirror slop. There was a 60% improvement of guiding accuracy with the mirror stabilizer with the spring vs. no stabilizer at all. Yet even with this dramatic affect, it is still not within tolerance of astrophotography. The only way to reduce the mirror slop enough to be acceptable for astrophotography is to lock the mirror down completely. There was a 107% increase in guiding accuracy with the mirror locked down tight vs. no stabilizer at all. With the autoguider cables secured, the accuracy improved even more and in one test the scope tracked perfectly for 90 minutes even in the Northwestern sky where I had historically seen the most problems.
Although the mirror stabilizer helps a lot for reducing mirror slop and helps in the feel and movement of the focuser, it is not good enough for the demanding needs of astrophotography. The only option is to lock the mirror down completely. While photographing anything West of the meridian it clearly is advantageous to secure the power and CCD cable of the autoguider so that as the autoguider moves up away from the ground it does not have to be subjected to the slight forces of lifting the cables against gravity and the slight spring forces of the coiled cable.
Comments: ckvedeler@access4less.net
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