RE: [APML] 8" 1200mm Dob using Heavy Duty Equatorial Mount.Is it possible?

[Jon Kolb <jkolb@mindport.com>]

Tommy,

> May i ask you one question? is it possible if i put my 8" 
> 1200mm Dobsonion to 
> the Heavy Duty Equatorial Mount and plus a guidescope on the 
> top of the 
> Dob........

There are other people on the list who know a lot more about using
Newtonians for astrophotography than I do, but I'll try to answer your
question anyway.

I'm not familiar with that particular mount, but in theory it should be
possible to do what you suggest.  You will need to use mounting rings to
hold the scope on the mount, and then smaller rings attached on top to hold
your guidescope.  You should check the specifications for your mount to see
if it can hold the weight, and if so, add counterweights as needed to
balance the load.

Now the bad news:

1) Your Dob may not have enough "backfocus" to allow a 35mm camera to come
completely into focus.  You can check this before you buy the mounting rings
while the scope is still in its Dobsonian mount.  Simply attach your camera
to the scope with a T-ring and adapter for your focuser and aim at Polaris,
then see if you can focus your camera.  Did you say you were in Singapore?
Polaris is pretty low in the sky, but I've seen it from the airport there.

2) If the focuser on your scope is a 1.25 inch type, then you will not be
able to completely illuminate a 35mm negative.  If it is a 2 inch focuser,
you have a chance, but the T-ring will still vignette the corners of the
frame somewhat.

3) If all of the above works out, and you've got the scope and guidescope on
your mount, you may still have problems with parts of the Newtonian moving
during an exposure.  This is called "flexure" and is constant problem in
astrophotography.  Chances are that your mirror mount is not rigid enough to
keep the mirror from shifting slightly during a long exposure.  If the tube
on your scope is made of compressed paper (Sonotube or cardboard-like
material), then you will probably have a lot of flexure in the tube.

4) If you have a rigid metal tube, you can cure the mirror flexure problem
by using an off-axis-guider (OAG) and eliminating the guidescope.  The OAG
will reduce the weight of your setup and will use light from the mirror to
let you guide on a star.  This means that when the mirror shifts slightly,
you can correct for it since the star you are guiding on is also in the
frame of your exposure.  The problem with this solution is that you will now
require even more backfocus, since the OAG sits in between the focuser and
the camera.

In general, scopes designed strictly for visual use are usually not suitable
for astrophotography.  Many scopes are designed to be used for both visual
and photographic purposes, but the average Dob is not. If you try this with
your scope, you risk spending money on the project that may be better saved
for a truly photo-capable instrument.  Probably the best investment you can
make right now is to purchase "Wide Field Astrophotography" by Robert
Reeves, or "Astrophotography for the Amateur" by Michael Covington.  These
books will help you greatly in getting started with astrophotography.

Good luck, I hope this answers your question,

Sincerely,
Jon Kolb
Adventures in Astrophotography
http://home.datawest.net/jkolb/
jkolb@mindport.com

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