RE: [ATM] More Mak-Cass questions

["Thomas" <tomjan@ozemail.com.au>]

Hi David,

Although I've never built a Mak-Cass I believe that the 1.7mm lip should
be plenty, remember your only supporting the weight of the meniscus and
the secondary, which when the load is spread out over the whole
circumferance of the corrector is actually quite small per unit area. 

I don't know how to calculate the exact force and force vectors but, I
think is should be along the lines of (total corrector assembly
weight)/(total area of support lip). You should be able to get the
mechanical properties of the corrector glass from the glass manufacture,
or look for a similar glass and you should be more or less right. From
that you can work out if you have a safty margin or not. Hope this
helps.

Clear skies,
Thomas Janstrom
http://www.tjanstrom.com
http://www.norsewines.com.au
"Your nobody untill you've been ignored by your seventh cranio-facial
nerve."


-----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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