ACIS Pixel Randomization

The CXC Standard Data Processing (SDP) randomizes the positions of events detected within a given pixel. The event coordinates are randomized over the pixel size. That is, a uniform random deviate in the range [-0.5,+0.5] pixel (i.e. about -0.25 to +0.25 arcsec) is added to the integer values of CHIPX and CHIPY when the real-valued sky coordinates X and Y are computed. This randomization is done to remove the instrumental "gridded" appearance of the data and to avoid any possible aliasing affects associated with this spatial grid.

Removing the randomization

The default randomization is adequate for most users. Disabling pixel randomization can somewhat improve the resolution of on-axis Chandra sources. This procedure may be useful for users interested in closely-spaced point sources in a crowded field or closely-spaced features in an extended source.

Removing the PHA randomization should only be done by advanced users who know exactly what they are doing. Be aware that there are no calibration products appropriate for data processed with the randomization disabled.

To remove the PHA randomization, run acis_process_events with rand_pix_size=0.0.

Using a level=2 event file as input

Since removing the pixel randomization does not affect any of the status, grade, or time filters, it may be run on level=2 imaging data. If you are analyzing grating data, you must run this thread on the level=1 event file, then complete the appropriate "Obtain Grating Spectra" thread to create an new level=2 event file.

In this case, several additional parameter are set so that acis_process_events does not try to randomize PHA (rand_pha=no), calculate the PI channels (calculate_pi=no), or compute event grading (doevtgrade=no). A custom "level 2" eventdef is specified. The reason for this is explained in the "Why is my level 2 event file larger than the one from the pipeline?" FAQ.

unix% punlearn acis_process_events
unix% acis_process_events infile=acis_evt2.fits outfile=acis_new_evt2.fits \
      acaofffile=pcad_asol1.fits rand_pha=no doevtgrade=no gradefile=none \
      calculate_pi=no rand_pix_size=0.0 \
      eventdef='{d:time,s:ccd_id,s:node_id,i:expno,s:chip,s:tdet,f:det,f:sky,l:pha,f:energy,l:pi,s:fltgrade,s:grade,x:status}'

# acis_process_events (CIAO 4.0): WARNING: The time-dependent gain adjustment to PHA will not be (re)computed or removed because doevtgrade=no. 
# acis_process_events (CIAO 4.0): WARNING: The values of PHA, ENERGY, PI, FLTGRADE and GRADE may be inaccurate because doevtgrade=no.
# acis_process_events (CIAO 4.0): WARNING: The values of ENERGY and PI may be inaccurate because calculate_pi=no.

The warnings are expected; they trigged as a result of the doevtgrade parameter setting.