MOPEX Online Manual

Modules: Med Filter

PURPOSE

This module performs the background subtraction of the individual input images. It needs to run only if you use single frame or dual outlier rejection. The output of this module is not used for the final mosaic, only for outlier rejection.

PARAMETER BLOCK

&MEDFILTER
 Window_X = 45,
 Window_Y = 45,
 N_Outliers_Per_Window = 500,
&END

INPUTS

Window X, Y: (int) the X, Y size in pixels of the window used to compute the median of the background. The default value is 45.

N_Outliers_Per_Window: (int) the number of outlier pixels (N) being rejected from the X*Y window when computing the median background. A rule of thumb to remeber when setting these parameters is that if you have a very crowded field, the fraction of rejected pixel would be higher than a less crowded field. A fraction of 1-2% is an acceptable number for most dataset. If N is set too high, the background will be under-estimated.

OUTPUTS

Generated Fits Files (*_minback.fits): The background subtracted, individual images.

DISCUSSION

This module is required to run if you plan to use single frame or dual outlier rejection (see Outlier detection). The idea is to subtract out the background before running outlier detection, either through single frame (detect_radhit) or multiframe dual (spatial & temporal) outlier rejection. The background subtraction is only for the purpose of outlier rejection, and the background subtracted images are not used for later mosaicking.

The program computes an asymmetrically skewed median for each pixel in the input image using a rectangular window of Window_X by Window_Y pixels. It is achieved by omitting the N_Outliers_Per_Window highest pixels from each median window.