MOPEX Online Manual

APEX Modules: Point Source Probability

PURPOSE

This module filters the input co-added image to estimate the probability at each pixel of having a point source above the noise.

PARAMETER BLOCK

&POINTSOURCEPROB
 PRF_ResampleX_Factor = 4,
 PRF_ResampleY_Factor = 4,
 PRF_Xsize = 3,
 PRF_Ysize = 3,
 Noise_Type = 'external_noise',
 Apriori_Probability = 0.1,
&END

INPUTS

PRF_Resample_X(Y)_Factor: (int) the ratio of the PRF pixel size to the PRF sampling interval in the x- and y-direction.

PRF_X(Y)size: (int) the size of the portion of the PRF image, in input pixels, used to convolve with the input image

Noise_Type: (char) options are 'external_noise' and 'internal_noise'. The first option requires an uncertainty image. For the second option the noise is estimated from the input image.

Apriori_Probability: (float) the lowest probability of a source. It is recommended that it be left to its default of 0.1.

OUTPUTS

Generated FITS (*_PSP.fits or *_Filtered.fits): The output is the PSP image(s), which shows the point source probability at each pixel.

DISCUSSION

Here s is the input background subtracted image, σ is the input uncertainty image, which may either the unc or std image. Note that the choice of input uncertainty image is not set from within this module, but from the Global Parameters section of the namelist, using the keywords use_std_to_detect and use_unc_to_detect.

The output of this filter P(j) at pixel j can be interpreted as a probablility of having a point source above the noise at this pixel. The summation is for all pixels i within the area defined by the PRF_X,Y_SIZE parameters. The values of the probability should use the full dynamic range from Apriori_Probability to 1. If the uncertainty is not well estimated it may lead to the output of the filter either not using the whole range or having too many pixels saturated very close to 1. To prevent this from happening, the argument of the exp function is rescaled to utilize the full dynamic range.

There are several possible data input images to this module, but the difference only becomes significant in the later module Detect. The choice of input is specified in the Global Parameters keyword use_psp_to_detect where a value of 1 uses the output of this module to carry out source detection and a value of 2 uses the original background-subtracted image to carry out source detection (in both cases the output from this module is identical, and as described above). Optionally, a third choice is available, by setting use_psp_to_detect = 0. In this case, a "Filtered" image is used, defined as the product of the PSF image times the background subtracted image: F=P*s. If the Filtered image is used, the name of the output file from this module will be *_Filtered.fits instead of *_PSP.fits.