Changes between Version 3 and Version 4 of GalaxyParameters

Timestamp:

Dec 3, 2014, 2:06:01 PM (12 years ago)

Author:

bills

Comment:

--

GalaxyParameters

@@ -1 +1 @@
 = psphot Galaxy Parameter Measurements =
+Bill Sweeney 2014-12-03
 
 For PV3 we plan to add some additional measurements of extended sources in psphotStack. This note describes these measurements
@@ -7 +8 @@
 
 In Section 5.6 of a paper published in 2002 [http://adsabs.harvard.edu/abs/2002ApJS..142....1S Luc Simard et al.] described
-two measurements for use in the morphological classification of galaxies (see section 5.6). 
+two measurements for use in the morphological classification of galaxies. See equations 11a and 11b in section 5.6. 
 
-The measurements are performed on residual images created by subtracting model fits of galaxies from images.
+The measurements are performed on residual images created by subtracting a galaxy model fits from the input images.
+The model used was the combination of bulge (de Vaucouleurs) + disk (exponential).
+These Asymmetry indices "provide an estimate of the overall smoothness of the galaxy image with respect to the fitting model".
+The program used to make these measurements is called GIM2D. It is performed on the results of processing by SExtractor.
+Since the indices consist of sums of absolute values of the residuals a positive signal will result even in the prescence of noise.
+So the algorithm subtracts off radomly selected "background pixels" which in the GIM2D program are chosen from the SExtractor
+segmentation image. 
 
 A later paper published in 2011 shows results of a study on [http://adsabs.harvard.edu/abs/2011ApJS..196...11S SDSS images].
+
+In the most recent version of psphotStack I have attempted to implement these measurements. 
+Since we do not fit a bulge + disk model we have chosen to use our sersic model fits. The measurements are preformed independently
+on each filter's sources as follows.
+
+1 At the beginning of the function the input image has all sources subtracted. The first step for each source is to add the
+subtracted model back in.
+
+2. Next if the source's model is not the Sersic model fit, the model is temporarily changed to make it so.
+The indices consist of sums over a specific radius. Following Simard 2011 the radius chosen is 2 times the half light radius for which
+the value that we have chosen is the major axes of the Sersic fit.
+
+3 The next step is to determine the background pixels. psphotStack doesn't have a segmentation image so the background pixels are 
+selected using the "footprints" measured during source detection. A pixel that is not part of any footprint is considered a background
+pixel. The source's pixels are examined and a vector of background and mirror background pixels are saved. During this loop the sum of
+the source's pixels is made as well.
+
+4. The sersic model is subtracted 
+
+5. The sums in the formulas for RT and RA are performed.
+
+The new code was used in psphotStack for SAS.37. 
+