[ATM] Mak-Cass OTA mechanical design

[David Whysong <dwhysong@physics.ucsb.edu>]

Hi,

I'm an ATM novice, looking for some advice.

I want to construct a high quality OTA for CCD imaging, both wide-field
for SN and asteroid surveys, and high-resolution for photometry and
astrometry. If I had money I'd buy something like an R-C, but it isn't in
the budget, so... here is my current plan. I can buy a 10"  Maksutov
Cassegrain optical set from APM and build my own tube. The optics will be
quartz, made by Intes Micro; their model 1008 has fl=3000mm, but I was
told 256mm f/12.5 which gives fl=3200mm. I realize a Mak-Newt would
probably be better quality and far easier to baffle, but I believe the
illuminated field would be substantially smaller. Besides, the 10" MN
optical sets cost substantially more and the resulting 'scope would
require a bigger mount.

Some ray-trace output of the Intes-Micro 1008 design can be found here:
http://www.apm-telescopes.de/englisch/techspec/im.htm#ALTER_M_1008

The Mak-Cass spot diagrams look pretty good at 0.36 degrees off-axis, with
a little astigmatism and what looks like it might be a tiny bit of
spherochromatism in the deep blue. The corner of a 35mm format field
should be either 0.38 or 0.41 degrees off-axis, depending on the focal
length. So the correction is good at f/12, but undoubtably it will suffer
from more severe off-axis abberations and vignetting with the use of a
focal reducer.

For now, my difficulty is in devising a primary mirror mount. The primary
will be 266mm in diameter, with a 60mm core hole. The back of the primary
is flat out to 55mm radius, but then it is conical. The mirror will be 3
cm thick at the center but only about 15mm thick at the edge.

In order to simplify construction I'm avoiding a moving-mirror focus
mechanism. Unfortunately, I don't see a good way to mount this mirror
without using the baffle tube as a support.  My current thinking is to
have an annular metal support behind the mirror (60mm inner diameter,
110mm outer diameter -- the same dimensions as the rear flat surface). The
ring would not be attached to the baffle tube, which would be
approximately 56-58mm outer diameter. A thin layer of something soft will
be fit between the baffle tube and the mirror; maybe cork, which is used
between my Celestron's corrector plate and it's retaining ring.

The mirror would be bonded to some structure on the support ring. I'm not
sure exactly how to do this; perhaps some miniature triangular floatation
type structures? It's a little disturbing to read all the posts on the ATM
list about multi-point floatation cells, and yet Cassegrain mirrors are
typically centrally-supported, and I can't support the mirror evenly due
to the conical shape...

I'll have to put a flange on the baffle tube in front of the mirror in
case the mirror comes loose from it's mount.

The support ring will be attached to the back plate of the telescope with
a 3-point collimation adjustment. This would allow for very slight primary
adjustment -- but only slightly, as the tolerances are tight and the gap
will be filled with cork.

Thinking about it, I don't think this is a good design, but I don't know
how to do better. I plan to build a prototype OTA with plywood first. When
I've worked out the kinks in the design, a friend will help me with
machining the parts out of aluminum. I'll post more information as I make
progress on the design.

Any comments or suggestions would be greatly appreciated!

-- 
David Whysong                                       dwhysong@physics.ucsb.edu
Astrophysics graduate student         University of California, Santa Barbara

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