Description of the algorithms used by IPP full force stage

This page gives a brief discussion of the full force pipeline. More details may be found by examining the source code.

Inputs

The Sky Calibration stage of stack photometry takes the cmf files created by psphotStack and calibrates measures the zero point by comparing the sources to the reference catalog.

In the current version of the IPP skycal produces a new fits table called a cff file. This file contains the parameters to be used for forced photometry for each object. The skycal cmf files are not used directly because they are vastly larger and would cause a much higher I/O load.

This table has one row for each source with the following columns extracted from the cmf file.

Column	Value for Point Sources	Value for Extended Sources
ID	IPP_IDET	IPP_IDET
X	X_PSF	X_EXT
Y	Y_PSF	Y_EXT
FLUX	PSF_INST_FLUX/EXPTIME	10**(-0.4*EXT_INST_MAG)
AP_RADIUS	AP_MAG_RADIUS	AP_MAG_RADIUS
KRON_RADIUS	MOMENTS_R1*2.5	MOMENTS_R1*2.5
PETRO_RADIUS	PETRO_RADIUS	PETRO_RADIUS
FIT_GALAXY	0	1
PSF_STAR	PM_SOURCE_MODE_PSFSTAR	PM_SOURCE_MODE_PSFSTAR
R_MAJOR	0	EXT_WIDTH_MAJ
R_MINOR	0	EXT_WIDTH_MIN
THETA	0	EXT_THETA
MODEL_TYPE	-1	model type for best fit
INDEX	0	0.5 / EXT_PAR_07

For sources with extended fits, the FIT_GALAXY column is set and parameters from the model with the lowest EXT_CHISQ is chosen from the EXP, DEV, and SERSIC model types. Note that the INDEX parameter is nonzero if the model type is sersic.

Full Force Run

A full force run consists of a cff file from a skycal run and the single frame warped images for the input stack's skycell filter combination. First the warps are processed by psphotFullForce and once they have all been processed the program psphotFullForceSummary "summarizes" the results.

psphotFullForce

psphotFullForce is run separately for each input warp. Source detection is not performed, the source positions are given by the input cff file. The following psphot modules are invoked

• Model Background
• Subtract Background
• Load External Sources (read cff file)
• Full Force Stats - Construct sources and measurement moments and other basic stats on the supplied sources.
• Choose PSF
• Guess Models (only generates psf models)
• Fit Sources Linear - linear PSF fit to source peaks, subtract the models from the image (in PSF mask)
• Kron Flux - using supplied kron radius
• Petro Flux - using supplied petrosian radius
• Radial Apertures
• Galaxy Shape
• Magnitudes
• Sky Replace

The output of this process is a standard IPP cmf file with an additional extension called 'XGAL'. This table contains the results of the galaxy shape measurements

Galaxy Shape

The galaxy shape function takes the extended fit model parameters and creates a set of models with a range of major and model radii. A range of values centred around the input values are tried. For each trial a extended model with the specified type is created and the chi-squared of the model versus the warp pixels is calculated. The results of for each trial are saved in the XGAL extension.

psphotFullForceSummary

The new program psphotFullForceSummary reads the individual full force cmfs and combines the galaxy shape measurements from the individual warps to estimage the major and minor axis pair that has the best fit to the source.

In order to contaminating the measurement by sources contained in chip gaps or thatare otherwise masked, measurements are excluded if the PSF_QF_PERFECT value is >= 0.9.

These results are saved in a cmf file.

Note that in the current version, only the extended model fit parameters (XFIT) in the summary cmf are meaningful. The parameters in the other extensions (PSF, XSRC, and XRAD) are placeholders copied from the first warp containing the source.