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73236Re: aperture or focal length most relevant to good/bad seeing ?

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  • Stan
    Aug 22, 2013
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      --- "waddington50" <bw_msg01@...> wrote:
      > 1) Your second equation seems to mix units of seconds (time)

      in the eq, "exp" is meant to refer to the FWHM of the exp

      > 2) How long is a "short" exposure - something like 1 second?

      The full answer would be a long discourse because seeing is a time based dynamic. "Seeing wander" is often a significant component of image FWHM but it is not measured by instruments like DIMM because such devices normalize wander and only measure relative wavefront distortions. Also, wander is a function of aperture (larger apertures produce less wander) so including it in the measurement produces an aperture dependent seeing value.

      To derive the seeing independently of wander via a normal scope/exp requires very short exps, such as 10-20ms. But such short exps produce varying, irregular non-Gaussian PSF that yield unreliable FWHM. A stream of a few hundred 20ms exps can be aligned and stacked to produce a virtually wanderless FWHM and I do that frequently with intensified video. But that's not practical for slow CCD and not even possible for charge transfer CCDs (shortest exp time is >100ms for many such cameras).

      So 1-5 seconds is usually enough time to get a near Gaussian PSF and short enough to escape most mount issues (except vibrations), especially if you take a dozen or so exps. But it does include wander and as such may over-estimate the core seeing. However, it should be fairly consistent for that scope and can be a useful measurement.

      > 3) Would it make sense to take, say, twenty 1 second
      > exposures and use an average?

      Yes.

      Stan
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