Index: /trunk/psphot/doc/notes.txt
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--- /trunk/psphot/doc/notes.txt	(revision 9927)
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+
+2006.11.08
+
+  I have been testing the pmSourceMagnitudes interpolation of the
+  source position when measuing the aperture magnitudes.  I built a
+  test suite for pmSourceMagnitudes and pmGrowthCurve.  even with
+  bicubic interpolation, there are noticable errors for small
+  apertures (ie, small compared to the PSF sigma).  I used a gaussian
+  PSF to make the tests.  it seems that, if the radius is less than 2
+  sigma or so, the interpolated aperture mag is in error by as much as
+  12mmag.  The size of this effect must depend on the smoothing
+  algorithm, the shape of the PSF, the size of the PSF relative to the
+  pixels, and the size of the subpixel interpolation.  I believe the
+  effect is a result of the smoothing introduced by the interpolation:
+  the smoothing pushes the flux in the inner portions of the PSF down
+  and enhances the flux in the outer portions.  thus, there is
+  actually a turn over in the error: at very large values, the error
+  is very small because the aperture contains all of the flux, and the
+  interpolation has little effect.  as the radius shrinks, the error
+  grows (in the sense of M_raw - M_interpolated; ie, the flux in the
+  interpolated source is too large).  around 2-3sigma, the error turns
+  over and becomes negative (too little flux in the interpolated
+  source). 
+
+  The sigmas I used for these tests (1.0, 1.5, 2.0 pix) are
+  appropriate for the range we expect to see from PS1, or any
+  well-sampled detector.  Thus, even though the effect is probably
+  less important for poorer seeing, we have to be careful to avoid it
+  in our Pan-STARRS analysis.
+
+  Currently, the minimum aperture is defined only by the config
+  variable PSF_FIT_PAD, which really implies a padding.  My proposal
+  is to require the minimum aperture to be a number of sigma (an
+  additional parameter).  Then, the formula for the radius would be
+  something like:
+
+  MAX (modelRadiusPSF (PSF_FIT_NSIGMA), PSF_FIT_RADIUS_NSIG) +
+  PSF_FIT_PAD
+
+  with PSF_FIT_RADIUS_NSIG (choose a better name!) set to something
+  like 2 or 3.
 
 2006.11.06