Image health seeing and obstac » History » Version 6
Image Health seeing and Obstac¶
The lore is that the image health seeing values are sometimes unrelated to the real seeing.
What do we know?
DIMM free air seeing values are in red, Quick Reduce produced Sextractor FWHM measures of the
delivered image quality (free air seeing + dome seeing + optics + tracking) is in green,
and the Image Health FWHM measures are in blue.
(This image is from Douglas Tucker, October 26, 2012 )
Note that IH and QR track each other well at small seeing values, but that IH
has a cutoff at about 1.5".
If one finds a image with bad seeing, say 2", reported by QR and asks the SISPI
database for the harmonic_mean_seeing values for all 62 ccds, often there will
be only a single ccd reported in the database.
It is likely that Image Health has a threshold above which its seeing measurements
are thrown away: not used in the fp_harm_mean_seeing calculation and not
sent to the SISPI database.
Next, Jiangang (amongst others) points out there is a scale difference between
IH and QR. Jiangang's report is here
but I want to point out the following plot: