71RE: [ccd-newastro] guiding techniques
- Aug 1, 2000Because the mirror of an SCT shifts for a variety of reasons, and can shift
even during an exposure, it's not possible to maintain alignment between a
guidescope and an SCT. In other words, you either need to use a setup that
puts the guider in the same light path. That could be a camera with an
integrated guider (SBIG ST-7/8/9), or an off-axis guider. The SBIG camera's
are the simplest solution.
You can also stick with unguided exposures, and throw out any that show
mirror shift problems.
The longer your focal length, the more critical good guiding is. In the case
of the Fastar, you are shooting at a very short focal length (400mm), and it
is quite possible to take 1-2 minute unguided exposures successfully. I have
gotten good results this way, including color images, by stacking the
The alternative is to lock the mirror in place, and use some other device
for focusing (e.g., a Crayford focuser on the back; the NGF-S motorized,
The most common application for using a guidescope successfully is when two
refractors are involved; it is relatively easy to make that kind of assembly
rigid and reliable. However, even something like a cheap focuser can throw
off a guidescope combination, and modifications to increase rigidity are
The New Astronomy Book Site - http://www.newastro.com
From: Kevin Dixon [mailto:ksbtk@...]
Sent: Tuesday, August 01, 2000 1:55 AM
Subject: [ccd-newastro] guiding techniques
Good Morning Ron:
I have a few questions regarding guiding while using a CCD camera.
If the camera is being used with an 8" SCT at a focal ratio of f/10
or f/6.3, can a guidescope be used or is it better to use an off-axis
guider (or similar device).
Also, if the ST-237 is used with the Fastar lens assembly, it seems
that guiding would be done via a guidescope. Is this correct?
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