eSASS task: ERMASK

Scope

This document provides a description of the algorithm and instructions for the correct usage of the ERMASK task, which is part of eSASS (the eROSITA Science Analysis Software System).

Summary:

Create detection masks for eSASS detection chain tasks defining valid image areas which are suitable for detection. The masks are based on exposure maps created by the task EXPMAP.

Algorithms:

The task ERMASK evaluates an exposure map and calculates a detection mask according to conditions specfied by parameters threshold1 and threshold2 . The detection mask is written into the primary extension FITS extension of the output image ( int8 ) , valid pixels are set =1, pixels to excluded from source detection are set =0.

Parameter threshold1 defines an exposure threshold, given as fraction of the maximum exposure value:

  detmask(x,y) = 0
  if (expimage(x,y) > maxval(expimage)*threshold1) detmask(x,y) = 1

With parameter threshold2 additionally a maximum local exposure gradient can be set:

  if (sqrt((expimage(x+1,y)-expimage(x-1,y))**2 + (expimage(x,y+1)-expimage(x,y-1))**2) > expimage(x,y)*threshold2 ) detmask(x,y) = 0

The detection pipeline requires only detection mask, if images from the same instrument in several energy bands are processed.

Parameters:

• expimage (string) REQUIRED [e.g "expmap.fits"]
  • Input exposure map
• detmask (string) REQUIRED [e.g "detmask.fits"]
  • Output detection mask
• threshold1 (real) OPTIONAL [e.g 0.2]
  • Minimum exposure threshold (fraction of max. value)
• threshold2 (real) OPTIONAL [e.g 1.0]
  • Maximum gradient threshold. Set to value >> 1 to disable
• regionfile_flag (logical) OPTIONAL [e.g "N"]
  • User defined region file for additional exclusion regions (currently not used)
• regionfile (string) OPTIONAL [e.g "mask.reg"]
  • Name of user defined region file (currently not used)

Input files:

• Exposure map

  FITS image ( real32) as created by task EXPMAP. The file name is defined by parameter expimage .

Output files:

• Detection mask

  FITS image ( int8) with file name defined by parameter detmask. Valid image pixels suitable for source detection are set to 1, masked out pixels are set to 0.

Examples:

• Create detection mask based on the exposure mask image_soft_exp.fits
• Valid pixels need to have at leas 10% of the maximum exposure
• Disable gradient condition by setting threshold>> 1

 ermask expimage="image_soft_exp.fits" \
        detmask="detmask.fits" \
        threshold1=0.1 \
        threshold2=100.