MOPEX Online Manual
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Basic Concepts: Point Response Function
Point source detection and extraction in APEX relies upon a good measurement of the point response function (PRF). The term PRF is not just an alternative way of saying point-spread function (PSF). PSF-fitting is a commonly used term. However, PRF and PSF are two different objects. A PSF is an image of a point-source, and it is often oversampled; i.e., the pixel size of the PSF image is a fraction of the pixel size of the detector array or the mosaic image for which the the PSF is applicable. A PRF, however, is not an image of a point source. It is a table of values of responses of the detector array (or mosaic) pixels to a point source. The positions of the pixel for which the response is calculated are on a grid. Normally, the PRF is oversampled, which means that the spacing of the grid is a fraction of the detector pixel size. The two --PRF and PSF-- are one and the same if they are not oversampled; i.e., the pixel size of the PSF is equal to the pixel size of the detector array (mosaic) and equal to the grid spacing of the PRF. The advantage of using a PRF versus PSF has to do with the way they are derived from the data. A PRF is closer to what is being observed by the detector array pixels. Every processing step inevitably introduces errors in the product being calculated. The only error introduced in estimating a PRF is the one produced by the shift of the observed pixels to the fixed grid. The estimation of the PSF involves an additional step of interpolating from the bigger detector array pixels to the smaller PSF pixels. This step introduces an additional error. However, for the purposes of fitting, one has to reverse this and integrate the small pixels back into the bigger detector array pixels.NOTE: Standard PRFs may be used in APEX, or they may be measured from the mosaic with which they are to be used. Standard PRFs are available from the SSC website (follow the links on the Post-BCD pages). Users may produce their own PRFs using the command-line script prf_estimate.pl, which is part of the MOPEX distribution. At this time, PRF estimation is not part of the MOPEX GUI. Documentation for the command-line scripts is also available on the SSC website's Post-BCD pages.
IMPORTANT: APEX normalizes the PRF before fitting. If Normalization_Radius is specified in the Source Estimate module block, the normalization is peformed over the pixels within that radius (IN PRF PIXEL UNITS) from the center of the PRF. If Normalization_Radius is not specified, APEX will normalize over the available range that the central PRF covers in the PRF FITS file. Getting correct PRF-fitted absolute fluxes requires having the correct PRF normalization for your data!
Variable PRF
A provision is made to use a variable in the field of view PRF using the so-called PRF maps. It is assumed that several areas in the detector field of view will be identified, such that PRF variation within each area will be negligible and a constant PRF can be used for each area. A PRF map identifies the areas of constant PRF.
A PRF map maps an image with respect to the PRF image used for any pixel of the image. In the PRF map header, the keyword Number_PRF gives the number of different PRF's that should be used for an image. The names of the files containing the PRF's are given by the keywords PRF_Filename_?, where "?" runs from 1 to Number_PRF. The PRF's can be in the form of a fits file (.fits) or an IPAC table (.tbl). The keyword HaveSigma is used to indicate whether PRF Sigma images are available. If they are, HaveSigma = 1, HaveSigma = 0 otherwise. The names of the images with the Sigmas are given by the keywords PRFSigma_Filename_?. The keywords ImageX and ImageY give the sizes of the images for which the PRF's are to be used. Here is a sample:
\char comment = PRF Map
\int Number_PRF = 5
\int HaveSigma = 1
\int ImageX = 256
\int ImageY = 256
\int PRF_Filename_1 = /ssc/pipe/davidm/pipe/Test/IRAC.3.8um.PRF.12.fits
\int PRF_Filename_2 = /ssc/pipe/davidm/pipe/Test/IRAC.3.8um.PRF.12.fits
\int PRF_Filename_3 = /ssc/pipe/davidm/pipe/Test/IRAC.3.8um.PRF.12.fits
\int PRFSigma_Filename_1 = /ssc/pipe/davidm/pipe/Test/IRAC.3.8um.PRF.12.Sigma.fits
\int PRFSigma_Filename_2 = /ssc/pipe/davidm/pipe/Test/IRAC.3.8um.PRF.12.Sigma.fits
\int PRFSigma_Filename_3 = /ssc/pipe/davidm/pipe/Test/IRAC.3.8um.PRF.12.Sigma.fits
|PRFNum| NAXIS1| NAXIS2| PRFPos1| PRFPos2|
|i |i |i |i |i |
1 100 200 1 1
2 256 56 1 201
3 50 50 101 1
2 156 150 101 51
1 106 50 151 1
The geometry for the above table file:
------------------------------- | PRF #2 | | | |-----------------------------| | | | | | | | | PRF #2 | | | | | | | | | | | | | | PRF #1|---------------------| | | | | | | PRF #3 | PRF #1 | -------------------------------
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