Re: ATM R-C questions

[Michael Peck <mpeck1@ix.netcom.com>]

Asaf wrote:

>Hi,
>
>
>                                             Another question I have is
about Mirror quality for
>                                             Astrometry. Does the Mirror
quality define the quality
>                                             of my Astrometric work?

I assume you mean that you plan to do imaging of some sort. If so then
image quality in time exposures will usually be limited by tracking errors
(mostly) and seeing. So, mirror quality is a little less important than it
would be in a 'scope intended for visual use. And a good thing too, since
the conventional wisdom is that cassegrain secondaries are hard to do well.

Another thing to consider if you're doing astrophotography with film is
that you'll probably want a comparatively fast system and also want a
reasonable size area of the focal plane to be fully shielded from stray
light. You'll probably find that a properly designed set of shields for
that purpose will leave you with a 40-50% central obstruction, which in
turn will make for a poor telescope for visual use.

>
>                                             Finnaly- the reason of this
e-mail- mounts for
>                                             cessegrains. I need the R-C
to be a star-tracker, so
>                                             can I just make it a
Dobsonian with setting circles?
>                                             Why is it that every
cessgerain is NEVER a dobsonian?

Mainly because in a conventional cassegrain with focal plane behind the
primary mirror you'll end up scraping your face on the ground trying to
observe, if you can reach the eyepiece at all! The obvious solution to that
is a Nasmyth modification of the OTA, with the optical axis made to
coincide with the mount's altitude axis. Or, if you're only going to use
the telescope as a camera lens you could maybe get away with a short back
focus distance and mount the camera hardware immediately behind the primary.

Again assuming you're planning to do time exposures you're going to need to
track, and that means you need either an equatorial mount or an alt-az
system driven on three axes (an alt-az-fp system). Mel's been giving away
the software for the latter type system for years but you'd have to do most
of the engineering work on the mount yourself (or hire it out). I would
think any field rotation at all would be fatal to a serious astrometry
program.

Mike Peck



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Michael Peck
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