Changeset 9840

Timestamp:

Nov 3, 2006, 5:11:38 AM (20 years ago)

Author:

eugene

Message:

added notes

File:

• trunk/psphot/doc/notes.txt (modified) (1 diff)

trunk/psphot/doc/notes.txt

@@ +1 @@
+
+2006.11.02
+
+  Looking into the growth curve problem.  RHL sent his recipe for
+  correcting to true circular apertures.  I've done a test fix of
+  interpolating the stellar image to the desired center.  it looks
+  like bilinear interpolation is insufficient (about 8% error for 2
+  pixel radius aperture).  Unless I've got the formula wrong, the
+  effective smoothing of the gaussian is moving flux from the center
+  out and making small apertures in error that depends on the offset.
+  Bicubic could be enough to clean this up?  
+
+  I think the current code may need to be careful modified to include
+  the interpolation for every aperture measurement.  there are paired
+  'pmModelAdd' and pmModelSub steps to add and remove the model flux.
+  I need to be careful about how the mask affects these steps.
+
+  I also need to be careful in interpreting the test results: since
+  the simulated images create objects by filling in a pixel and then
+  smoothing, all objects have centroids which are exactly 0.5 pixel
+  offsets.  These all then have a fixed growth correction error, which
+  fortunately happens to be substantial (because of a 0.5 pixel offset
+  in the growth curve reference object!)
+
+  I need to double check the 0.5 pixel error issues.  There are
+  problems with the pslib version of psImageShift and
+  psImageInterpolate (or at least inconsistencies).  the pmModelAdd
+  and Sub functions and the centroid functions are probably also
+  inconsistent.
+
+2006.11.01
+
+  Trying to understand low-level errors in psphot using simulated
+  data.  the main issue I am concerned with is the bias RHL is seeing
+  in SDSS-psphot comparisons.  But there are other issues that are
+  creeping in and probably point at logical errors in the code.
+
+  * in pmPSFtry, I measure the first pass on the aperture residual (to
+    choose between PSF model options, if that is being tested).  This
+    analysis currently uses a polynomial fit (1st order) for the offset
+    and the sky bias (using r^2/flux as an independent variable).
+
+    Notes:
+
+    - is the psf-fit error distribution reasonable?  perhaps a little
+      on the high side: at 10000 DN, noise is 0.0138, should be
+      0.0107.
+
+    - scatter of ap-fit is surprisingly high.  M_input - M_fit has a
+      median chisq of 2 (mean of 5), while M_input - M_ap has a median
+      chisq of 125 (mean of 380).  This is using a radius of 15.  It
+      cleans up hugely when I go to a radius of 7 and even more so
+      with radius = 4. (In this case, the PSF sigma is 1.5, so 4 is
+      well matched and 7 is only slightly large).
+
+      * make the initial PSF fitting radius and aperture radius
+          different?  
+      * make the initial PSF fitting radius a function of the measured
+          PSF sigma?
+
+    - I have been fitting ap-fit mag without weights; the resulting
+      ap-fit errors are highly over-estimated since the ap-fit scatter
+      is too high.  I have added code to carry the weights (error from
+      the fit only at the moment, ie, not the ap error as well).  This
+      is good, but not as important if the radius is chosen well above.
+
+  * a bias in the psphotEnsemble results depending on the sky level
+    but not in the PSF model fits?  turning on/off CONST_PHOT_WT has
+    an equivalent effect, but is quite small.  why is one sensitive to
+    the sky and the other is not?  
+
+  * the error measured by psphotEnsemble was wrong (and inconsistent
+    for a given weighting situation): fixed the def of the error.
+
+  * still trying to understand the corrections being made.  running a
+    few tests:
+
+    keeping PSF_FIT_RADIUS at 4.0
+
+    t1 : no growth correction,    constant weights
+    t2 :    growth correction,    constant weights
+    t3 : no growth correction, no constant weights
+    t4 :    growth correction, no constant weights
+    t5 :    growth correction,    constant weights, constant ap (4)
+    t6 :    growth correction,    constant weights, integer radius
+    t7 :    growth correction,    constant weights, integer radius
+    (for t7, I've fixed the bug that the growth correction include the
+    min radius).
+
+  * psphotGrowthCurve is messed up : the measurement does not correct
+    for the actual object center, and at small radii this is an
+    important source of noise.
 
 2006.06.28