RE: [ATM] More Mak-Cass questions

["Ellen Mackenzie" <mackenzieconsulting@sympatico.ca>]

Hi David:
	I tend to agree with you in regards to the lip. I would feel a lot
more comfortable with a 3mm lip with some kind of conforming material
between the cell and the lens such as a .02mm - .03mm poly or Mylar washer
to prevent metal to glass contact. Thermal expansion doesn't present near
the problem as contraction, you don't want the cell contracting on the lens
inducing stress. It can get very cool at night and aluminium has a very high
coefficient of thermal expansion/contraction.
Ellen


-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
David Whysong
Sent: Sunday, 8 August 2004 6:12 PM
To: atm@atmlist.net
Subject: [ATM] More Mak-Cass questions


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
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