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Main
Date: 30 Nov 2006 09:45:00
From: BClay
Subject: Guide Scope
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I just purchased an 8" LX200GPS. I have a DSIC and DSIC2 cameras and want to
use one for guiding. What is a good scope to use for guiding without
spending a fortune? I have spent enough already for the LX200. :-) I do have
an EXT90 sitting around. Would that work? Thanks.
Barry
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Date: 30 Nov 2006 15:25:43
From: Chris L Peterson
Subject: Re: Guide Scope
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On Thu, 30 Nov 2006 09:45:00 -0500, "BClay" <bclay@comcast.net > wrote:
>I just purchased an 8" LX200GPS. I have a DSIC and DSIC2 cameras and want to
>use one for guiding. What is a good scope to use for guiding without
>spending a fortune? I have spent enough already for the LX200. :-) I do have
>an EXT90 sitting around. Would that work? Thanks.
Choosing a guidescope is a matter of balancing conflicting demands. You
want it to be short focal length, but not so short as to cause guiding
errors. You want as large an aperture as possible, but at the same time
you want as low weight as possible. Above all, you need the scope and
its mounting system to be extremely rigid.
Start with the focal length. For a reasonably low noise camera and good
guiding software, you should be able to comfortably guide with a guider
pixel scale about ten times your imager pixel scale. If the two cameras
have the same size pixels, that means your guidescope can be one tenth
the focal length of your imaging scope.
Next, you choose the largest scope with this focal length that you can
afford, or that your system can carry. I wouldn't go with an aperture
less than 40 mm. You do NOT need good optics. In fact, guiding works
better with poor optics, or with a slightly defocused system if the
optics are very good. A simple achromat is what you want.
Now you need to consider your mounting system. For that, go with a
Losmandy dovetail plate system. I've tried others, and believe me, most
are not rigid enough.
Where does all this leave you? If you want to get everything off the
shelf, an ST80 clone (80 mm aperture, 400 mm FL) is a good choice. The
focal length is a little long, but it's hard to find scopes much shorter
than this. You'll probably have to do some modifications, however,
adding second locking screws to both the focuser draw tube and the
eyepiece holder. It's a good idea to drill holes in the OTA to act as
capture points for the mounting ring screws, too. (Some people epoxy
washers around the OTA instead).
Alternatively, you can build a guidescope. This will cost you a lot less
and work better. An example is here:
http://www.cloudbait.com/observatory/guider.jpg . The system is
completely rigid and requires neither aiming nor focusing. Cheap
achromats are readily available at http://www.anchoroptics.com/ .
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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Date: 30 Nov 2006 22:25:33
From: starburst
Subject: Re: Guide Scope
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>
> Alternatively, you can build a guidescope. This will cost you a lot less
> and work better. An example is here:
> http://www.cloudbait.com/observatory/guider.jpg . The system is
> completely rigid and requires neither aiming nor focusing. Cheap
> achromats are readily available at http://www.anchoroptics.com/ .
Brilliant, elegant design. Well done, indeed! - C
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Date: 01 Dec 2006 03:57:15
From: Chris L Peterson
Subject: Re: Guide Scope
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On Thu, 30 Nov 2006 22:25:33 -0500, starburst <nope@nospam.net > wrote:
>Brilliant, elegant design. Well done, indeed! - C
Thanks. I should have mentioned that in normal operation there is a tube
(made from a paper towel roll) between the objective and the camera. It
isn't structural; it just blocks stray light.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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