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73238Re: aperture or focal length most relevant to good/bad seeing ?

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  • Stan
    Aug 23, 2013
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      --- "echesak@..." <echesak@...> wrote:
      > I see the macro effects of seeing, by looking at the guider
      > corrections on the scope...

      That is an easy and useful method of estimating conditions relative to particular equipment and practices. However, as you note it does not produce an accurate seeing FWHM that can be reliably compared with other systems and measurements.

      Guide exps are usually several seconds long in order to avoid having the mount chase the seeing. Longish guide exps blur most of the seeing effects to distill pointing errors. So bad seeing usually does not produce large guide errors since this method deliberately minimizes seeing effects. Guide errors can have a seeing component but are more often due to factors such as mount, vibrations, OTA response to wind, etc.

      AO guiding is another matter. AO guide exps are usually near 100ms. So most of the report error RMS is due to seeing wander. AO can remove a substantial amount of coherent seeing wander.

      Stan
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