[APML] Just when you think you have it all sorted out ....(a bit long)

["William R. Mattil" <wrmattil@ix.netcom.com>]

Hi,

I thought that I would share this with the group so that everyone could
have a bit of a chuckle and if there is anyone who hadn't run across
this (doubtful) then it might save them some trouble. I have been
moderately successful imaging with a pair of Schmidt Cameras (8in and
5.5in) using a Losmandy GM100 with the high precision RA gear. Set up
was using a DSBS and 90mm guidescope ALA Kent Kirkley (Thanks Kent).
Since I had a 10" f/4 Parks OTA laying around I thought that I would run
some tests and see if I could get it mounted as well. I located the
appropriate aluminium plate and mounted the radius blocks to it and then
mounted a DC14 to it and then worked out the balance so that everything
was peachy. The last trip up to Atoka OK I took 4 images (M33, M45 and
B33). All exposures were 1 hour in length on Supra 400 ASA film @ Prime
focus. Guiding was handled by the ubiquitious ST-4 using a 90mm
guidescope that was mounted on Parks Rings above the OTA. I was (I
think) pleasantly surprised to find that ST-4 seemed to be quite happy,
running A1's and some A2's. After development of the negatives I was
.... well, less than pleased. These were probably the worst I had ever
taken. Stars were badly trailed but (the good news) was that they didn't
appear to suffer from field rotation. So at Kents suggestion I ventured
outside to the driveway and ran some tests with a pair of guiding EP's,
one in each OTA. Hoo Boy ..... this was interesting. Any movement of the
system (less than 5 degrees) resulted in a significant shift between the
two. After some head scratching and advice from those having been down
this road (are you guys all laughing yet ?) I came to the conclusion
that the original rotating rings and the guidescope rings were
contributing to the problem. So a call to Parallax and two weeks later I
basically gutted the optical tube and remounted it as follows:

Parallax rings sandwiched by two DC14's .DR125's mount to the top DC14
and the bottom one mounts to the saddle on the GM100.

During the disassembly of the previous ring assembly I came across the
following....

1) Mirror was a bit loose in the cell (what a doofus eh?)

2) Inner race of the rotating ring assembly was *very* loose. In fact
most of the screws could be removed by hand. This was really unexpected
actually given that this was a new OTA.

Of course its raining now and the prognosis isn't good for the weekend
either but time will tell. Everything certainly seems to be more rigid
at this point and I remain hopeful that things will be improved.

Astrophotography is a cruel mistress and its almost always a humbling
experience.

Any comments or further suggestions on refining this setup would be most
welcome. 

Regards

Bill (A lot more humble now)
-- 
William R. Mattil	| Fred Astaire wasn't so great.
wrmattil@ix.netcom.com	| Ginger had to do it all backwards
(972) 399-4106		| and... in high heels.

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