Light Bucket

I am in the middle of building a large lidar and I am wondering about
cassegranian telescope alignment. The telescope that I am working on
consists of four 50" primaries and their secondaries mounted in a single
az/el mount. The telescope is basically a large light bucket and isn't
required to make pretty images as all I need to do is count photons. The
other limitation is that it has a narrow field of view...a few mili-radians
so that I don't get too much light from the sky getting into my
measurements. I was wondering if anybody had any bright ideas on alignment
and focusing.

Thanks

That is a nice little scope!
First off, I assume that you are trying to get the 4 primaries to focuse at
the same point. This can be the biggest problem! The Multi-Mirror
Telescope is being redone as a single telescope for this particular problem
although your requirements are going to be a lot easier to do.
If you have the scope mounted where you can see a hilltop or other high spot
a few miles away, the probem becomes a lot easier as you can setup an
artifical star on that hilltop and use that for alignment. Start with one
primary and do all of the alignment with that master mirror and then make
the other 3 match to it for the final image. Since you are counting
photons, you won't have to worry about exact alignment to within the Airy
Disc although that will be good for the amount of light you actually obtain.
Preliminary alignments would probably be best done with a laser at the
distant hill and following the path of the beam as it goes though the
system. Make sure that you do a decent job of setting the mirrors in the
approximate right place so that the final alignments are going to be
correct.
I might note that if you provide for a CCD camera at the final focus, you
can also do some interesting planetary photography as well with the systm.
Good luck.

--
Bob May
Losing weight is easy! If you ever want to lose weight, eat and drink less.
Works evevery time it is tried!

"Bob May" wrote

That is a nice little scope!
First off, I assume that you are trying to get the 4 primaries to focuse
at
the same point. This can be the biggest problem! The Multi-Mirror
Telescope is being redone as a single telescope for this particular
problem

Ummm.. the 6.5 meter MMT been in operation for over 3 years. And, as I have
said many times here, the MMT was not coverted into a single mirror because
of any problems with a multiple telescope system. It was done because there
was a window of opportunity at the Steward Mirror Lab to get a large mirror
made. It was done to compete with the newest telescopes being built that
are getting bigger and bigger. We saw it was possible to retain the same
mount and building, thereby keeping costs relatively low. And, it was an
opportunity to create it with wide field capability: 1 degree at f/5. The
Multiple Mirror Telescope was a highly successful system. Ask any
professional astronomer who used it.

Howard Lester
MMT Observatory




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