On Sun, 10 Dec 2006 16:36:53 GMT, Jim Klein
wrote:

In visual and film photography through telescopes on equatorial
drives, the old RA drive system might use commercial gears (Boston
probably) and a synchronous drive motor like a BODINE or a HURST. This
would be controlled using a variable frequency controller.

This was used in conjunction with a DEC drive with another synchronous
motor and gears.

A contol box, held in the hand with 4 buttons and a reostate type
device for setting the system to a siderial rate would be used.

The user kept the target or a near by star in the cross hairs of a
guide scope or off-axis guiding device.

I have been told that this configuration is insufficiently accurate
for CCD photography and a periodic correction via a computer driven RA
drive is needed to compensate for periodic errors in the RA worm gear
and other associated gears.

Is this correct ?

A system such as you describe would probably be adequate for CCD
imaging, but you would neither want to use one, nor be able to find one
outside an antique shop. For more than a decade, nearly all imagers
(film or CCD) have used mounts with somewhat more sophisticated drive
systems.


If so, when I buy my next telescope with which I plan to do CCD
photography, what do I need to look for and ask for in the product I
buy.

I will be getting something in the 11 to 14 inch aperture range. It
will be on a permanent mount on a concrete pier and isolated from the
concrete pad upon which I will be walking.

Above all, you want a mechanically solid mount. I prefer fork mounts,
but this largely limits you to using the mass produced products of Meade
or Celestron. While these certainly make fine imaging systems, you can
get much better mounts from other manufacturers. These will be GEMs,
however, not forks. You will want a goto controller; these days,
whatever controller the mount comes with will include periodic error
correction and autoguider inputs (you will want to avoid manually
guiding unless you are a masochist).

On a side note, consider avoiding completely a concrete pad around your
scope. Concrete has a very large thermal mass, and can easily contribute
to deteriorated local seeing. A raised wooden platform is much better in
this regard, as well as being easier on your feet and back (and on
anything you accidentally drop).

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com