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71RE: [ccd-newastro] guiding techniques

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  • Ron Wodaski
    Aug 1, 2000
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      Because 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
      unguided exposures.

      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

      Ron Wodaski
      The New Astronomy Book Site - http://www.newastro.com

      -----Original Message-----
      From: Kevin Dixon [mailto:ksbtk@...]
      Sent: Tuesday, August 01, 2000 1:55 AM
      To: ccd-newastro@egroups.com
      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?

      Clear skies,

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