Quote:
Originally Posted by Shiraz
Hi Rolf. The DIMM certainly does measure image motion at high speed, but only to get a full bandwidth measure of the effect of the turbulence. The long term seeing FWHM applicable to DSO imaging is then obtained by plugging the DIMM output into the standard seeing model - nicely summarised in the reference.
Measured image FWHM is a fair estimate of seeing, but it also includes mount tracking and guide wobble effects that are automatically removed by a DIMM - these effects are convolved with the atmospheric seeing when imaging, so image FWHM is worse than that from seeing alone. DIMM seems to be the gold standard at present.
ref: http://www.ctio.noao.edu/~atokovin/papers/pasp2002.pdf |
Hi Ray, thanks for the info re DIMM, I was not aware of the details.
Yes you are absolutely right that measured FWHM includes contributions from other sources. I was just thinking that in order to compare results we must measure the same thing, and nearly all replies on this thread has referenced FWHM as measured directly on images.
The actual seeing is certainly better than what we can record on long exposures, but not many have DIMM or other such equipment to measure that.