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71348Re: [ccd-newastro] Seeing

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  • Ron Wodaski
    Jul 1, 2012
      In simplest terms, seeing will scatter the incoming photons. Bad seeing is more 'violent' and will do more scattering. Good seeing is gentler, and will do less scattering.

      So if, using your equipment at your site, the star sizes are larger, then seeing is worse than average. If stars are tighter, then seeing is better than average.

      Seeing is not simple, however - yet the above holds generally. Ways in which seeing is not simple:

      * There are multiple causes of poor seeing. If your mirror is warmer than the ambient air, for example, this creates convection currents that will introduce 'mirror seeing'. If there is an upper-air distrubance being carried quickly past your area by the jet stream, that will cause seeing problem. And there's plenty in between those two extremes.

      * The size and speed of seeing effects varies. On some nights, you might see a slow wobbling effect, but with a fairly tight image doing that wobbling. On other nights, the seeing may change very rapidly on small scales; this will break up the image entirely. The former can be overcome with very short exposures (e.g., planetary imaging), and the latter cannot be recovered from in any way.

      * Measurement of seeing is not accomplished just by measuring your FWHM. The FWHM is an integration - the result, over time, of seeing effects (and your optical capabilities). To measure seeing accurately, you need to make very fast measurements (on the order of 100/second or faster), you need to measure differences in star motion from image to image, and then you must apply some fancy math to those results (e.g., the DIMM technique). Generally, then actually measuring seeing requires specialized hardware and software. Image FWHM is a decent proxy for seeing, but it can never give you the full information about seeing conditions.

      Ron Wodaski

      On Jul 1, 2012, at 9:24 AM, cfogarty1964 wrote:

      > I am not sure I fully understand how you tell whether seeing is good or not. More importantly the relationship og fwhm to seeing.
      > ------------------------------------
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