ATM It's the Daigonal, Stupid! .... again

[Jack Schmidling <arf@mc.net>]

One of my big disappointments with the 16" scope on the AP mount was
that I could not use the 90mm Astrograph piggyback on it and had to
abandon.  I made two smaller astrographs in the hope that the shorter
focal lengths would mask the problem.  They did but I really miss the
great photos I could take with the 90mm.

I had presumed that the big paper tube was just too flexible to deal
with this application.  It's not quite as stringent as guide scope
guiding, in fact it is the reverse but that only minimizes the error
which is still visible.  

I never liked the monster Novak mirror cell and was pondering doing a
lighter design.  I then recalled my experience with the 10" and guiding
and started fiddling with the diagonal it became obvious that the flimzy 
diagonal spider could not possible be relied upon as the telescope
moves.

The ubiquitous sheet metal spider vanes may block minimum light but
just do not have stability in all axis.  I spent a day in the shop
making a 1.5" diameter hub and a set of vanes out of 1/8" x 2" stock.
These are supported at the tube ends by 1/2" x 3/4" bars.  I of course
made 4 vanes but by the time I had 3 installed, it became obvious
that the job was done.  The fourth could provide no useful additional
support.  The 3 at 90 degrees already prevented any movement whatsoever 
in the 3" diagonal.

Last night was the third and most stringent successful test of the
setup and I am delighted with the results.  A 90 minute color exposure
of the Pleiades shows perfect star points with zero tracking error.

I am happy as a pig again.  There is a discussion of this astrograph
on my web page if anyone is interested.

     http://user/mc.net/astrog.htm

js

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