73235Re: aperture or focal length most relevant to good/bad seeing ?
- Aug 21, 2013I'd like to try this out, Stan. But I have two questions:
1) Your second equation seems to mix units of seconds (time) with arc-seconds. The usual "sidereal" conversion factor doesn't seem to make sense here - what's the deal?
2) How long is a "short" exposure - something like 1 second?
3) Would it make sense to take, say, twenty 1 second exposures and use an average?
--- In email@example.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)
> 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)
> 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)
> 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.
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