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Re: more flatfields
> Chris wrote:
> >I think the true soluution is to take the above "sub-region method"
> >to the limit of where the "number of sub-regions approaches infinity"
> >In other words just accept that our images have continously varying
> >PSFs and gradients and process accordingly. I have been arguing for
> >using 2D curve fitting with a varying PSF model for some
> >time now. I appears to me from recent posts that the assumption of a
> >constant PSF made by the current software cost us about mag. 0.02+
> If one uses an analytic form for the psf, then you could have that
> form depend on declination, and match what you are saying. You only
> need a 1-D variation due to the drift-scan process on the mark III,
> though a true 2-D model would be required for the mark IV. DAOPHOT does
> not assume an analytic model, however, and if you look at typical
> seeing-limited profiles, like King profiles, they are definitely difficult
> to model analytically.
It looks like there is conciderable flexibility in the PSF model DAOPHOT
can use. It can be purely analytic, analytic with a table of residuals
or if the analytic portion is define as a constant zero, purely table
driven. The analytic component can have x,y as parameters or not.
Yep, there are about a billion parameters to set. It may be slow but
still runs faster then real-time. That is DAOPHOT will reduce a frame
faster then the exposure time so it could keep up. For the MkIII I'd
try varorder = 1 or maybe 2.
A description of how the PSF model is stored from the HELP file:
The output PSF image psfimage is normally a 2D image containing
the image header parameters, "XPSF", "YPSF", "PSFMAG" and "PSFRAD"
which defines the centroid, magnitude and size of the PSF, the
parameters "FUNCTION", "PSFHEIGH", "NPARS", and "PAR#" which define
the analytic component of the PSF, and a single look-up table of
residuals from the analytic fit, which has been subsampled by a
factor of 2.
If the DAOPARS parameter varorder = -1, the PSF is fit by the
analytic function only and psfimage has no pixel file.
If the DAOPARS parameter varorder = 1 or 2, then two or five
additional lookup tables are computed and psfimage is a 3D image
with 3 or 6 planes respectively. The first two additional look-up
tables contain the first derivatives of the PSF wrt the x and y
positions in the image (varorder = 1), and the next three contains
the second derivatives with respect to x ** 2, xy, and y ** 2
(varorder = 2).
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