Re: ATM Beginners questions

[Stephen Tonkin <sft@aegis1.demon.co.uk>]

Ross_Dawson@county.com.au wrote:
>3.  I have read on the web that motorised Dobson mounts are not suitable
>for use with a CCD, but then I have seen pictures captured using just such
>a setup (ala Mel Bartels).  I haven't been able to acertain the reason why
>the mount is not suitable but I suspect that it is because it has to rotate
>around 2 axis to track the object where an equatorial mount only has to
>rotate around 1 axis.  Can anyone help clarify this for me?


The one-axis/two axis thing is one problem. Trying to follow a star
whilst manually regulating the motor speeds on two different axes is not
a simple task. (As far as I'm concerned it is impossible, but there is
no doubt someone somewhere with the evidence to show that s/he has
managed it <g>). The obvious solution is computer control - let the
'pooter take the strain.

One other problem is image rotation. The image in an altaz mount will
rotate. Think of an imaginary constellation shaped like an arrow rising
in the east, point up. When it sets, it will be point down. It has
rotated. An equatorially mounted telescope will rotate with it (which is
one reason Newtonian eyepieces get into such awkward places); an altaz-
mounted one won't.

One solution to this is to rotate the focuser to compensate for this.
Again, this is an appropriate task for a computer.

So, to truly emulate equatorial tracking, you need to drive in not two
but three axes (alt, az, focuser). 


>5.  Are there any good sources/descriptions on Mirror construction and
>testing on the Web.

It's not an organised resource, but the archives to this mailing list
must be one of the best. If you have the patience to search them, I
think you will more comprehensive and more up-to-date information than
you will in most (any ?) of the books. They will also give indicators to
web sites which cover different aspects of this hobby.  May I suggest
that a possible route would be to do a trawl of the archives then ask
questions of clarification.

-- 
Stephen Tonkin