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68357Re: [ccd-newastro] Guide Scope Question

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  • Ron Wodaski
    Nov 2, 2010
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      Most likely, you have run into the primary issue with an external guider: differential flexure.

      All optical systems flex as you slew the scope. And an SCT can be especially problematic, since the primary mirror floats on its support, and is therefore not fixed.

      The problem is that each optical system has it's own reaction to the changing direction of gravity, and will flex in different ways, at different rates.

      You can secure each optical system better to get improvement. However, if a scope has internal flexure - such as most SCTs do - you may not be able to get rid of the differential flexure easily, and sometimes not at all.

      Ron Wodaski



      On Nov 2, 2010, at 12:06 PM, Mike wrote:

      > Hi Group,
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      > I have a 12" SCT with a ST10XME camera at F/6.7 and, mounted piggyback on that scope, a 102mm F/6.9 refractor with an ST2000XM camera attached to it. I want to use the refractor as a guide scope. According to my calculations (from Ron's CCD Calculator), the pixel scales for each optical configuration are:
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      > 1.38 arcsecs/pixel for the SCT (with 2x binning)
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      > 4.33 arcsecs/pixel for the refractor (with 2x binning)
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      > I have been having at best mediocre results guiding with the refractor. I'm using MaximDL for imaging/guiding.
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      > For me to achieve the best guider corrections, should I make any changes to the above configuration? Should I not use 2x binning with the guide scope (and thus have about 2.16 arcsecs per pixel? What is the ideal configuration given the pixel scale of the main scope?
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      > Thanks,
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      > -Mike Renzi
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