CalDB 4.4.0 Public Release Notes

I. Introduction

CalDB 4.4.0 is a significant decimal upgrade to CalDB 4.3.1, which includes the following items:

• ACIS Grade Image (GRDIMG) CalDB File.
• ACIS temperature-dependent CTI correction files.
• PIXLIB GEOM file upgrade, with correction to the ACIS-S/LETG tilt angle.

CalDB 4.4.0 was not released to the public until CIAO 4.3 was released, because the software changes to allow the new ACIS Graded-mode CTI correction, the ACIS temperature-dependent CTI correction, and the new PIXLIB GEOM file to be used properly were not included in CIAO before 4.3 was release. For the CIAO 4.3/CAlDB 4.4 release notes, and How CalDB 4.4.0 Affects Your Analysis, see this link.

II. Summary of Changes

A. ACIS Grade Image (GRDIMG) CalDB file

Location:	$CALDB/data/chandra/acis/grdimg
Filename:	acisD2000-01-29grdimgN0001.fits

DS 8.3.3 and CIAO 4.3 both introduced the ACIS GRADED-mode CTI calibration algorithm, which requires a single additional calibration file, namely the GRDIMG file. The GRDIMG file consists of 256 3x3 images, one for each flight grade, that are used to map a FLTGRADE to an effective 3x3 pha island, which is required for CTI correction.

Pipelines/Tools affected:

CIAO/SDP ACIS L1 (re)processing tool acis_process_events.

Threads affected:

Reprocessing Data to Create a New Level 2 Event File

B. ACIS Temperature-dependent CTI correction files

Location:	$CALDB/data/chandra/acis/cti
Filenames:	acisD2000-01-29ctiN0007.fits acisD2000-01-29ctiN0005B.fits acisD2002-08-01ctiN0007.fits acisD2002-08-01ctiN0005B.fits

The temperature-dependent CTI correction (T-CTI) algorithm adjusts the CTI correction of each event based on the point-by-point variation of the focal plane temperature from the standard 153.45K (-119.7C) for which the basic CTI correction is calibrated. The point-by-point FP_TEMP values are stored in the Mission Time Line file, *mtl1.fits, included with the secondary products of each Obs_ID. The T-CTI was implemented in DS 8.3.3 of standard data processing (SDP) on 02 November 2010. It was released in CIAO 3.4 as of 15 Dec 2010. The ACIS reprocessing tool acis_process_events implements the algorithm, and requires one of the above new CTI files (parameter ctifile) and the corresponding mtl1.fits file (parameter mtlfile) before it will run. While in SDP the T-CTI runs by default, in CIAO 4.3 the default is not to run the correction unless the user sets the mtlfile parameter manually. Otherwise, the basic CTI correction, without the temperature correction, is run.

For more information on the ACIS T-CTI correction algorithm see either the ACIS CTI Why? page, or the demonstration of the T-CTI contributed software.

Pipelines/Tools affected:

CIAO/SDP ACIS L1 (re)processing tool acis_process_events.

Threads affected:

Reprocessing Data to Create a New Level 2 Event File

C. PIXLIB GEOM and SKY file upgrades

Location:	$CALDB/data/chandra/default/geom/
Filename:	telD1999-07-23geomN0006.fits
Location:	$CALDB/data/chandra/default/sky/
Filename:	telD1999-07-22skyN0002.fits

In order to resolve a long-standing problem with the geometry of the LETG/ACIS-S configuration relating to the grating dispersion angle, a new PIXLIB GEOM file has been developed, which specifies all grating angle/detector configurations separately and independently. This solution is not a physical correction of the configuration, but an adaptation of the current setup to allow a more accurate derivation of LETG/ACIS-S wavelengths in the dispersion plane. The new grating tilt angle value assigned to ACIS-S/LETG is -0.01670 degrees.

There is an incumbent change in the PIXLIB "SKY" file that defines a new Grating Dispersion Plane coordinate system, which corresponds to the new independent-angles definitions in the GEOM file. This new configuration is called "GDP-1.3", which is the designation that appears in the keyword "ACSYS" of any data file that was derived with the new coordinate system.

An adjustment in the PIXLIB software is required to implement the new coordinate system, both for CIAO and for SDP. These new GEOM and SKY files are required for the new software implementation, which has been released into SDP in DS 8.3.3 (02 Nov 2010). It has been be released to the public in CIAO 4.3 (15 December 2010).

SDP Pipelines / CIAO Tools affected:

CIAO/SDP Grating L1.5 (re)processing tool tg_resolve_events for the ACIS-S/LETG configuration only.

Threads affected:

LETG/ACIS-S Grating Spectra

III. Technical Details

A. ACIS GRDIMG CalDB File

The ACIS graded-mode CTI correction algorithm, and the associated CalDB file, are described in the memorandum "Approximate correction for parallel CTI in GRADED mode data" by Alexey Vikhlinin, which is available in PDF form at http://hea-www.harvard.edu/~alexey/acis/memos/gradecor.pdf

From that memorandum:
"We show the ACIS trap maps developed for the CTI correction algorithm can be applied to the GRADED mode data. The technique uses the fltgrade (flight grade) information available in the GRADED mode telemetry, and average 3 X 3 PHAS images for each fltgrade pre-calibrated using the FAINT mode data. The results (recovery of photon energies and energy resolution) are nearly identical to those from the CXC CTI corrector implemented in acis_process_events."

Thus the GRADED mode CTI correction is for parallel CTI only, and only on the FI chips.

A single additional calibration file is required to implement the changes in acis_process_events that are indicated above, namely the subject GRDIMG file. The GRDIMG file consists of 256 3x3 images, one for each flight grade, that are used to map a FLTGRADE to an effective 3x3 pha island, which is required for CTI correction. The Fig. 4 in the above memoranum lists all of 256 of the flight grade images contained in the file. NOTE: These are not stored as literal FITS images in the file, but instead in BINTABLE format. There is separate, but identical, BINTABLE included for each chip, so there are ten extensions. Also, only the GOOD flight grade distributions are described in any numerical detail; the bad ones are listed trivially. Also, since the corrections do not apply to the BI chips, ALL the ACIS-5 and ACIS-7 grade images are trivial.

B. ACIS Temperature-dependent CTI correction files

From the CXC contributed software web page ACIS T-CTI contributed software:
"The spectral resolution of the ACIS CCDs is substantially improved by a charge transfer inefficiency (CTI) correction algorithm included in acis_process_events. However, the behavior of the charge traps that cause CTI is temperature dependent: warmer-than-nominal focal plane temperatures reduce the effectiveness of the correction algorithm. As the insulation on the exterior of the SIM and the ACIS radiator shade have aged, the surfaces around the ACIS focal plane and radiator have become warmer, leading to FP temperatures which are sometimes a few degrees warmer than desired, particularly for observations done at spacecraft pitch angles greater than ~140 degrees. The ACIS team successfully reduced average focal plane temperatures by shutting off the detector housing heater in April 2008, and a heater on the SIM was turned off in August 2009 providing additional margin, but many warm observations exist in the archive, and observations done at 'tail-Sun' attitudes still often have warm focal plane temperatures.

"Catherine Grant developed an algorithm to correct for the temperature dependence of the ACIS CTI (see her Chandra Calibration Workshop presentation 'Correcting for the temperature dependence of ACIS charge transfer inefficiency')."

The newly modified acis_process_events uses the FP_TEMP data from the "*mtl1.fits" file from the ACIS Level 1 products, and new tabulated trap-density correction factors added to Block 2 of the ACIS CalDB CTI file, to correct events on each applicable chip for the temperature dependence of the CTI effect, by modifying the PHAS column values that go into the level 1 events file.

There are actually four different CTI files applicable to the several different cases of datamode and mission timeline. First the CTI trap maps were intially effective with the start of -120C operations, nominally on 2000-01-29T20:00:00. However, they were later modified for the period after 2002-08-01T00:00:00. Secondly, there are the CTI trap maps for the BI chips, both serial and parallel, which are not applicable to GRADED mode datasets, and hence are not included in two of the CTI files (N0005B versions) designated for GRADED modes. This is because no mapping has been done for BI chips in the GRDIMG file for GRADED MODE CTI correction (ECR-2010-025). Historically, this is because there is already a refined RMF for the BI chips w/o CTI correction in CalDB, and the ACIS team has elected to maintain this for GRADED mode BI chips.

C. PIXLIB GEOM and SKY file upgrades

All the changes to version N0006 GEOM file for PIXLIB are in the [GRATINGS] extension of the file, where each of the nine possible detector/grating-arm configurations are represented separately, allowing ACIS/LETG/LEG to be assigned a separate tilt angle "ALPHA". The ALPHA values for the other eight configurations are assigned the same as before; only ACIS/LETG/LEG is assigned ALPHA=-0.01670 degrees.