ACIS CTI Correction
The tool acis_process_events includes a charge transfer inefficiency (CTI) adjustment algorithm that can be used to compensate for most of the effects of the CTI. This adjustment can significantly improve the spectral resolution of the data.
All available CTI corrections are applied to ACIS datasets when acis_process_events is run in standard data processing. The thread Create a New Level=2 Event File explains how to determine if your data should be reprocessed in CIAO.
Summary of the Correction
The available calibration is based on the data mode that was used for the observation; refer to the DATAMODE header keyword.
The focal-plane temperature must be -120 C. There is no CTI adjustment for data taken at temperatures from -110 to -90 C.
FAINT mode datasets:
Parallel CTI correction is available for all ACIS data taken in FAINT mode (i.e. CC33_FAINT, FAINT, and VFAINT).
Serial CTI correction is available for the back-illuminated CCDs (S1 and S3). The effects of the serial CTI are negligible on the eight front-illuminated CCDs.
GRADED mode datasets:
Parallel CTI correction is available for the front-illuminated CCDs in GRADED mode (i.e. CC33_GRADED and GRADED). This correction was added in CIAO 4.3 (15 Dec 2010) and DS 8.4 (28 June 2011).
No adjustments are performed for GRADED mode events on back-illuminated CCDs.
A temperature-dependence was added to the ACIS CTI correction in CIAO 4.3 (15 Dec 2010) and DS 8.3.3 (02 Nov 2010).
How the Data are Affected
When the tool acis_process_events is used to reprocess FAINT mode event data to include the latest temperature-dependent CTI adjustments, the process includes the following steps.
The unadjusted pulse heights in each pixel of the central 3x3 event island (i.e. in PHAS) are used to compute the adjusted pulse heights (i.e. PHAS_ADJ).
The adjusted pulse heights PHAS_ADJ are used to compute the FLTGRADE.
The "ASCA" GRADE is computed from the FLTGRADE.
The total (or summed) pulse height PHA of an event is computed from the central 3x3 event island of the adjusted pulse-height distribution PHAS_ADJ instead of from the unadjusted pulse-height distribution PHAS.
A time-dependent gain adjustment is applied to compensate for gradual changes in the CTI.
The ENERGY is computed from the PHA.
The pulse invariant PI is computed from the ENERGY.
If the CTI adjustment algorithm did not converge after the specified maximum number of iterations, then one of the bits in the column STATUS is set to one.
ACIS CTI_APP Keyword Required
When acis_process_events is executed, the output file includes the CTI-related keywords:
As of CIAO 4.1, CALDB 4.1, and DS 7.6.10, the keyword CTI_APP must be included in the header of an event file in order to reprocess the data. The keyword is a ten character string with one character for each CCD. The value for each character is
- N: no CTI adjustment applied
- P: only a parallel CTI adjustment has been applied (current setting for front-illuminated CCDs)
- B: both parallel and serial CTI adjustments have been applied (current setting for back-illuminated CCDs)
The latest CTI adjustments are applied
unix% dmkeypar acis_evt2.fits CTI_APP echo+ PPPPPBPBPP
Only the front-illuminated CCDs have been adjusted (using an older version of the calibration data)
unix% dmkeypar acis_evt2.fits CTI_APP echo+ PPPPPNPNPP
No CTI adjustments have been performed
unix% dmkeypar acis_evt2.fits CTI_APP echo+ NNNNNNNNNN
Note that it is not possible to apply only a serial CTI adjustment to the back-illuminated CCDs. The length of the string CTI_APP is always ten characters even though no more than six CCDs can be used for an observation.
In addition to the tool acis_process_events, the following tools and scripts now require CTI_APP:
- acis_bkgrnd_lookup (script)
- acis_fef_lookup (script)
- mkarf (via ardlib)
- mkgarf (via ardlib)
- mkgrmf (via ardlib)
- mkinstmap (via ardlib)
- psf_project_ray (via ardlib)
These tools will either fail or return incorrect results if the header keyword CTI_APP is missing.
The keyword CTI_CORR is a boolean. It has a value of "TRUE" (or "1") if a CTI adjustment has been performed. It has a value of "FALSE" (or "0") if a CTI adjustment has not been performed. For example,
unix% dmkeypar acis_evt2.fits CTI_CORR echo+ 1 unix% dmkeypar acis_evt2.fits CTI_CORR echo+ 0
Although use of the keyword CTI_CORR has been superseded by use of the keyword CTI_APP, CTI_CORR is retained for back compatibility.
The keyword CTIFILE is the name of the calibration file that was used to apply the CTI adjustments:
unix% dmkeypar acis_evt2.fits CTIFILE echo+ acisD2000-01-29ctiN0006.fits
The keyword MTLFILE is the name of the observation-specific file that was used to obtain the focal-plane temperature as a function of time:
unix% dmkeypar acis_evt2.fits MTLFILE echo+ acisf07366_000N001_mtl1.fits
Adding the CTI_APP keyword
If you choose not to reprocess your data, then you can run the script check_ctiapp.sh to update the header:
unix% check_ctiapp.sh acis_evt2.fits Setting CTI_APP value to PPPPPNPNPP
The script check_ctiapp.sh is equivalent to the following.
If CTI_CORR=yes, then set CTI_APP=PPPPPBPBPP:
unix% dmkeypar acis_evt2.fits CTI_CORR echo+ 1 unix% dmhedit acis_evt2.fits file= op=add key=CTI_APP value=PPPPPNPNPP
If CTI_CORR=no or does not exist, then set CTI_APP=NNNNNNNNNN:
unix% dmkeypar acis_evt2.fits CTI_CORR echo+ # dmkeypar (CIAO 4.4): ERROR: Keyword 'CTI_CORR' was not found in file 'acis_evt2.fits'. unix% dmhedit acis_evt2.fits file= op=add key=CTI_APP value=NNNNNNNNNN
Note that the script check_ctiapp.sh or the instructions in this section should not be used if the keyword CTI_APP exists in your event file.
Reapplying the CTI Adjustment
If you reprocess an event data file that has already been CTI adjusted and choose to reapply the adjustment (i.e. use apply_cti=yes) with an appropriate mtlfile, then the adjustment will be recomputed from the unadjusted pulse heights PHAS. It is not possible to "double count" the adjustment. The values of FLTGRADE, GRADE, PHA, ENERGY, and PI are recomputed after the new adjustment is applied.
Removing the CTI Adjustment
If you reprocess an event data file that has already been CTI adjusted and choose to remove the adjustment (i.e. use apply_cti=no), then the values of FLTGRADE, GRADE, PHA, ENERGY, and PI are recomputed after the adjustment is removed.
Caveats: Temperature, Frame times, and CC mode
- The effects of the CTI have been calibrated using observations at or near a focal-plane temperature of -120 C. The effects of the CTI are known to be different for observations at warmer temperatures. Although, the CTI adjustment algorithm includes a temperature dependence as of CIAO 4.3 (15 December 2010), the adjustments cannot be applied to data taken at temperatures from -110 to -90 C.
- The effects of the CTI have been calibrated using observations with frame times of about 3.2 s. The effects of the CTI are known to be slightly different for longer or shorter frame times (e.g. for observations using subarrays or the interleaved mode). The difference is due to the fact that cosmic rays can partially fill charge traps. Although it is suggested that the standard CTI adjustment be applied to data with frame times that differ from 3.2 s, users should be cautious when interpreting the results of spectral analyses of such data. Please consult the CXC Helpdesk if you have specific concerns about your data.
- The effects of the CTI have been calibrated using (V)FAINT mode observations. Although the effects of the CTI are thought to be more or less the same for continuous-clocking mode observations, users should be cautious when interpreting the results of spectral analyses of continuous-clocking mode data. Please consult the CXC Helpdesk if you have specific concerns about your data.
When X-rays (and cosmic rays) deposit charge in an ACIS CCD, the charge is read out using one of four sets of read-out electronics. Each read out is used for a specific 256 pixel x 1024 pixel subset (node) of the CCD. Since charge is read out at only one location on a node, the charge at all other locations must be moved to the read out. Charge is moved both vertically (i.e. in the negative CHIPY or "parallel" direction) and horizontally (i.e. in the positive or negative CHIPX or "serial" direction). The total number of pixels through which charge must be moved depends on the location at which charge is deposited on the CCD.
As charge is moved, some may be lost to charge traps that are distributed across the detector. The mean fractional amount of charge lost per pixel transferred is called the charge transfer inefficiency. At launch, the values of the CTI on front-illuminated CCDs < 1 x 10-6 and < 3 x 10-6 for parallel and serial motion, respectively. The values for back-illuminated CCDs were 1-3 x 10-5 and = 8-16 x 10-5, respectively. These values are representative of 5.9 keV photons at a focal-plane temperature of -120 C. Due to the accumulated effects of cosmic radiation damage, the number of charge traps (and, hence, the CTI) on the CCDs is increasing with time. As of December 1, 2010, the values of the CTI = 1-2 x 10-4 and < 4 x 10-6 for parallel and serial motion, respectively, on a front-illuminated CCD and = 2-3 x 10-5 and = 6-14 x 10-5 for parallel and serial motion, respectively, on a back-illuminated CCD (at 5.9 keV and -120 C).
As of CALDB 3.1.0 (23 June 2005), parallel CTI calibration products were available for the front-illuminated CCDs ACIS-I0, I1, I2, I3, S0, S2, S4, and S5. Parallel and serial calibration products were released in CALDB 3.3.0 (18 December 2006) for the back-illuminated CCDs ACIS-S1 and S3.
Gain Shift and Spectral Resolution
The CTI affects the measured spectral distribution of astrophysical sources in two ways.
- Since some of the charge is trapped, the amount of charge read out is less than the amount of charge deposited. This effect causes the measured pulse-height distribution for a source to be shifted to lower pulse heights (i.e. results in an apparent gain shift).
- For a variety of reasons, the CTI causes a degradation in the energy resolution of a CCD. The measured pulse-height distribution of a monoenergetic source (or a line feature) is broadened.
These effects are functions of the location where an X-ray interacts in a CCD because they depend on the number of traps through which charge is moved. Therefore, it is necessary to calibrate the gain and spectral response of several separate regions on each CCD.
An algorithm was developed to estimate the amount of charge deposited on a CCD for an event from the amount of charge read out, the location of the event on the detector, and the focal-plane temperature. This algorithm is implemented in the tool acis_process_events. Use of the CTI adjustment eliminates nearly all of the apparent gain shift and can significantly improve the energy resolution of a detector. The energy resolution is not fully restored because charge trapping is a stochastic process and the unique charge trap history of each event is unknown.
Grade Migration and Detection Efficiency
Charge captured by a trap is typically released on a short time scale. A significant amount of the trapped charge is released into the pixel immediately following the pixel from which it was trapped. As a result, the distribution of charge in a 3x3 pixel event island is "smeared out" in the read-out direction.
The GRADE (and FLTGRADE) of an event is a numerical representation of the distribution of charge in the event island. If enough charge is added to a pixel to yield a pulse height that is greater than or equal to the split threshold (e.g. 13 adu), then the GRADE associated with the measured distribution of charge at the read out may differ from the GRADE associated with the distribution of charge produced at the location where the event interacted with the detector. This effect, called "grade migration," depends on the amount of charge deposited (i.e. the energy of an X-ray), the location of the event on the CCD, and the focal-plane temperature.
Since events whose GRADEs are changed from a "good" value (0, 2, 3, 4, or 6) to a "bad" value (1, 5, or 7) are excluded from Level 2 event files, grade migration results in an apparent reduction in the detection efficiency of a CCD.
When the CTI adjustment is applied to events on the back-illuminated CCDs, a number of low-energy events, which would otherwise have been lost due to grade migration, are recovered. This effect was calibrated for data obtained at -120 C and is contained in the QEU file. For the front-illuminated CCDs, the effect of grade migration is negligible.