Donald Nichols wrote:

What I would do to examine the rack is to check it on an optical
comparator (after a through cleaning, of course).

I looked in my optics books and didn't find "optical comparator", so I
did a google search and found some information about it. Based on that,
I'm wondering whether an old microfilm reading machine I fished out of
a dumpster could be adapted for this purpose. It's housing is made of wood,
the optics are in metal and it is intended for strips of microfilm,
but I did manage to look at some crystals with it. It's a piece of junk
but I've been reluctant to give up on it. The point is moot at the moment
since it is in storage where I can't get at it, but I expect that situation
to change. Even so, I'll need to acquire more practical knowledge about
optics than I have at the moment before tackling it. This would be a lot
cheaper than buying a comparator, I think, which would probably cost more
than a new scope. But I'm looking forward to trying out this comparator
idea for checking out the rack.
--
Ignorantly,
Allan Adler
* Disclaimer: I am a guest and *not* a member of the MIT CSAIL. My actions and
* comments do not reflect in any way on MIT. Also, I am nowhere near Boston.