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73233Re: aperture or focal length most relevant to good/bad seeing ?

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  • CurtisC
    Aug 20, 2013
      I often see people in astro forums confidently stating that their seeing is a certain value. But, having observed what they go through at Palomar to derive seeing, I often wonder how the average amateur astronomer can be so positive of what they're getting. I often check FWHM values on my downloaded images, mainly to verify good focus, but I never see FWHM values anywhere close to 1 arc sec. As to what the Night Assistant reports at the 200-inch Hale Telescope, the 1.2 average figure I cited for the night of Aug 18-19 is pretty typical for this time of year. Sometimes it's a bit better (down to, say, 0.9), sometimes quite a bit worse, especially in spring. My site here in Anza, CA, is similar enough to Palomar that my experience of "good" vs "poor" seeing usually parallels what they get, but I can only see it after the fact ("Oh, they had a lousy night too!")

      --- In ccd-newastro@yahoogroups.com, "Stan" <stan_ccd@...> wrote:
      > --- "CurtisC" <calypte@> wrote:
      > > as a practical matter, how do you measure seeing...
      > Setting aside serious measuring machines (scintillation monitors, DIMM and such) a "practical" method is to really know your instrument and site thru experience and measurements.
      > A key measurement is the FWHM for the better short exps on the best night. This value can be adjusted and used as the instrument's resolution limit. The instruments resolution limit can then be used to calculate the seeing on any other night by quadratically subtracting it from the FWHM of short exps.
      > short exp FWHM = sqrt(instrument^2 + seeing^2)
      > thus
      > instrument = sqrt(exp^2 - seeing^2)
      > In the absence of impendent measurements, it is necessary to make some reasonable assumptions. Seeing (measured in any way) is very rarely below 0.5" and most non-professional sites rarely experience less than 1". So it is not unreasonable to assume that a very good night has base seeing <=1"; be warmed that there are some chronically bad sites that never approach this. So use 1" as a first approximation:
      > Instrument limit = sqrt(exp^2 - 1)
      > Examples:
      > if best night FWHM = 1.5" then the instrument limit approx = 1.1"
      > if best night FWHM = 2.0" then the instrument limit approx = 1.7"
      > if best night FWHM <= 1" then the seeing was less than 1" and the instrument is very good.
      > Once you have some experience and confidence with the instrument then lesser seeing can be easily estimated via short exps:
      > seeing = sqrt(fwhm^2 – instrument^2)
      > examples:
      > if instrument = 1" and short exp FWHM = 2.0 then seeing = 1.7"
      > if instrument = 1" and short exp FWHM = 2.5 then seeing = 2.3"
      > if instrument = 1" and short exp FWHM = 3.0 then seeing = 2.8"
      > if instrument = 2" and short exp FWHM = 2.5 then seeing = 1.5"
      > if instrument = 2" and short exp FWHM = 3.0 then seeing = 2.3"
      > if instrument = 2" and short exp FWHM = 3.5 then seeing = 2.9"
      > Estimation error increases as (exp - instrument) decreases.
      > Stan
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