[ATM] More Mak-Cass questions

[David Whysong <dwhysong@gmail.com>]

Hi everyone,

I'm about ready to start fabricating aluminum parts for my 10"
Mak-Cass. The optics (from Intes-Micro, ordered via APM) are supposed
to be finished by the end of the month.

First will be the cell for the corrector. I have one concern about
this. The diagram from APM (see:
http://www.physics.ucsb.edu/~dwhysong/mak/MK%2010f12.5.jpg ) shows a
2.0 +/- 0.2mm flat lip at the outer edge of the corrector lens. If the
lens cell has a 1.7mm lip, is that really enough to support the
corrector?

I think it should be less than the 2mm for two reasons: first, because
of the +/-0.2mm mechanical tolerance, and second because I don't want
the metal to be in direct contact with the glass; there should be a
little space to allow for differential thermal expansion. I am leaving
1mm of space around all the optical components (for example, the
meniscus lens is 266mm in diameter and 19mm thick at the edge; the
cell is designed to have an inner diameter of 268mm and an interior
depth of 21mm). The extra millimeter is to allow for a soft spacer to
hold the glass in place, such as adhesive-backed cork.

The only place where I don't have this space is the inner edge of the
supportng "lip" at the back of the corrector plate. It seems to me
that 2mm is not very much space to support the corrector, especially
if I can only use about 1.7mm of that due to mechanical tolerances and
to allow for thermal expansion. Is there any general wisdom for how
big that "lip" needs to be to safely support the lens? I don't want to
crack or chip the edge of the meniscus!

My second question regards aligning the optical elements. The main
tube will be carbon fiber. The lens cell will slip over the front of
the tube and be secured by bolts. In this design, the corrector has no
collimation adjustments. The secondary will have the usual 3-point
collimation adjustments.

The rear of the primary will be cemented to a metal annulus (OD=110mm,
ID=60mm) and a tube going through the perforation. I am reasonably
confident in this mounting assembly. However, I am unsure as to
whether I should have collimation adjustments for the primary. It
makes me nervous for several reasons. The primary needs to always be
collimated with respect to the focuser, which can't be adjusted. I
doubt that three collimation screws are sufficient to support the
primary and it's supporting structure, especially since they would
have to be placed at not more than ~ 50mm radius instead of at the
full radius. So I'm now planning to have the primary mounting directly
and securely fastened to the rear plate of the telescope, so the only
collimation adjustment is the secondary tilt.

In principle, I could fix the collimation of the primary and secondary
by adjusting how the front and rear cells are bolted to the tube. In
practice this may not be a good idea...

Does this all sound reasonable, or am I insane? Advice would be
greatly appreciated.

Thanks,

-- 
David Whysong
_______________________________________________
ATM mailing list http://www.atmlist.net/