ACIS Dead Area Correction
The dead area affect on the ACIS FI chips has been calibrated from an I3 measurement of a cluster. This calibration accounts for the effect of cosmic rays which strike the detector and render a portion of it dead for an entire frame. The correction is dependent on CHIPY and ranges from approximately -2.2% at the readout to -4% at the top of the chip.
The correction is non-zero for the 8 front-illuminated (FI) ACIS chips; the effect is not detectable for the back-illuminated (BI) chips, so there is no correction applied to them.
The calibration file was released in CALDB 3.3.0 on 15 December 2006. As of CIAO 4.0, the dead area correction is turned on by default.
The cosmic ray flux incident on ACIS reduces the usable area in a probabilistic fashion dependent upon the time a pixel is exposed to cosmic rays during readout. Since that time depends on the clocking parameters and upon parallel read row, it cannot be treated as a scalar factor on the QE or on the effective exposure time. In effect, it is properly a position-dependent exposure time correction factor.
The ACIS "DEAD_AREA" file is implemented in the Chandra CalDB to store for each CCD the fractional area loss per unit time due to cosmic rays. These values are used in the response tools (mkarf, mkinstmap, mkgarf, and mkwarf) in conjunction with the observation-dependent parameter block to compute the average loss factor as a function of position to modify the resultant effective area or "FRACEXPO" (used in pileup modeling, where the exposure time cannot be absorbed into the QE).
A calibration is being introduced as a single coefficient for each chip. A non-zero value - 0.006508 - is being included for the eight FI chips in version N0001 for this file. The effect is not detectable for the BI chips, so the nominal calibration value is 0.0. The resulting chipy-dependent reduction in the EA will be approximately 2.2% at the readout, and 4.0% at the top of the chip.
For further technical details, refer to the memo "Factoring the ACIS QE".
Applying the Correction
The ACIS dead area correction is "on" by default as of CIAO 4.0.
The following CIAO response tools can apply the dead area correction:
The dafile parameter has a default value of "CALDB". The pbkfile parameter must be set to the observation-specific parameter block file (pbk0.fits). The parameter block file contains ACIS clocking parameters required to scale the coefficients in the dead area correction.
unix% pset mkgarf pbkfile=acisf063875928N002_pbk0.fits
Using the CIAO Analysis Scripts
The following CIAO scripts also have pbkfile and dafile parameters included in their own parameter files:
Turning Off the Correction
It is possible to "turn off" the correction, e.g. if you would like to compare results with and without it applied. To do so, set the dafile parameter to "NONE" and leave the pbkfile parameter blank. For example:
unix% pset mkgarf pbkfile="" dafile=NONE