Re: [ATM] More Mak-Cass questions

[Dominic-Luc Webb <dlwebb@canit.se>]

> 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!

I once lost a decent mirror from this kind of thinking. The cell
expanded slightly, the mirror was able to slip out, and amazingly,
it rolled down the length of the tube. It travelled further than I
realized and when I lifted the front of the tube up, hoping to stop
the mirror from rolling out, it instead was catapulted right over
my balcony, fell about 2 floors and hit concrete. About 10% of the
mirror area was lost from a single break at edge. The rest of the
mirror is fine, although, the figure was altered.

I still see other people pushing to preserve every last little
bit of the edge. I do not do this. If I want more aperture, I
simply still with a bigger piece of glass. My Mak-Cass corrector
is 12". The lip is about 5 mm, and I get the aperture I want plus
sense of security.

I could suggest in your case that you try screwing down several small
plastic "stops" around the circumference of the corrector that can
prevent the glass from ever falling out of the cell. They need not
actually have contact with the glass and if they only protrude 4-5
mm over the lip and are about 1 cm wide, their presence will not
be noticable.

Another trick, which I have considered, but not implemented, was to
drill a few small holes or a raceway along the side of the corrector
and then use stop screws. Again, actual contact with the corrector
is not necessary, since the function is to block any hope of the
mirror ever slipping out of its cell.

Why would a mirror slip.... Less than perfect tolerances, both in
the cell and the glass. Take a good look at your corrector. If it
is like mine, it is not perfectly circular, and it is slightly lopsided.
Similar issues apply to the cell. It can be that the exact position
you place the corrector in the cell was tight, but then, the mirror
rotates a couple centimeters in its cell, and ends up in a spot where
it is not tight. I think that is what happened with my flying saucer
mirror mentioned above. And yes, in case you're wondering, it was an
amazing thing to watch for the roughly 3 seconds of flight until the
crash. Some people here call the imnpact sight Roswell. You do not
want such fame, so make sure to learn from my experience.




> 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.

I think that these things all being fastened to a carbon fiber tube is a
big plus. Maybe it is enough with the secondary, although, amazingly, I
often end up wishing I could move/tilt the eyepiece slightly. The last
eyepiece bracket I made could do this. Somehow, there is sometimes a
"sweet spot" that is not in the middle of the plane no matter how
carefully you try to align the optics. I think part of the problem
is that we build scopes to our unique specifications and then try to
use common commercial eyepieces not really intended for that design.
Some commercial eyepieces (most?) have some kind of correction (coma,
etc) that was designed for a specific telescope from the same
manufacturer. I have a very nice 5 mm Orion eyepiece that performs
exceptionally well on one of the Orion telescopes, but is completely
useless on any of my homemade scopes thus far, although I found
other eyepieces that worked very well.


Dominic

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