Scope

The fifth reprocessing campaign started on 9/22/20 and will include all observations (~24,000 observations with level 2 event files) taken from 30 January 2000 up to current data (July 2020). All observations will be reprocessed from the raw telemetry data.

There are three data sets, based on the observation date, the previous reprocessing status and the applied improvements in REPRO-V.

• Period A: Early observations that were taken before Jan. 29, 2000.

  These early data will be reprocessed in Repro Vb, scheduled to begin after completion of the processing of the Chandra Source Catalog 2.1.

• Period B: Data reprocessed in Repro IV (2000-2011).

  While these observations have been processed with Repro IV, the improved calibration files for ACIS, HRC, and ASPECT will be applied.

• Period C: Recent observations that were taken in 2012-2020.

  These observations have not been processed with Repro IV. In addition to the improved calibration files, the interpolated spacecraft clock correlation files, definitive ephemeris, and the proper time-dependent ACIS and HRC gains will be applied.

We plan to process roughly in order of reverse observation time, i.e., C and B, so that the new (recent 3 years) data are reprocessed first and the early 1999 data at the end.

Number of Observations per Repro V period

Instrument	Period A	Period B	Period C	Total
	1999-07-20 to 2000-01-29	2000-01-30 to 2011-12-31	2011-12-31 to 2020-07-20	1999-07-20 to 2020-07-20
ACIS-S	240	6081	4331	10652
ACIS-I	124	3397	2482	6003
HRC-S	78	866	442	1386
HRC-I	227	802	434	1463
CTI*	228	3009	2307	5544
TOTAL	897	14155	9996	25048

* CTI (Charge transfer inefficiency) ER (engineering request): ACIS data taken for a calibration purpose with no astronomical target.

Highlights

• ACIS: ACIS detector gain improvement
• ACIS: ACIS time-dependent gain application
• ACIS: A full upgrade to the ACIS T_GAIN library
• ACIS: ACIS bad pixel library upgrade
• ACIS: ACIS header keyword updates
• HRC: HRC-I gain map application
• HRC: HRC-S Degap improvement
• HRC: HRC-S time-dependent gain map
• HRC: HRC event files with new columns
• Grating: Grating response files
• Aspect: Aspect Camera Assembly (ACS) plate scale correction
• General: Off-axis angle correction
• General: Improved timing accuracy
• General: Applying definitive ephemeris
• General: DOI (Digital Object Identifier)
• General: Replacement of bad target names
• General: Improved sky fov (field-of-view)
• General: Updated Header keyword

Details

ACIS

• ACIS detector gain improvement

  A new Phase2 Response (DET-GAIN) was implemented in CalDB 4.9.1 (March 24 2020) to fix the small gain depression (1-3%) at mid-chip (around chipx=512) of the front-illuminated (FI) CCDs. This improves most ACIS-I observations (except graded-mode observations). Note that the back-illuminated (BI) CCDs (CCDID = 5 and 7) are not affected by this gain droop. More information can be found in the CalDB 4.9.1 release notes and the Chandra calibration page

• ACIS time-dependent gain application

  ACIS T_GAIN is regularly updated every three months from February 2012 to November 2015, upgraded every six months thereafter. All ACIS observations (2012-2020) since the last repro will be improved. The background information can be found in the 'CIAO why' page of ACIS time-dependent gain.

• A full upgrade to the ACIS T_GAIN library.

  In addition to the incremental upgrade, the entire T_GAIN files for all -120C epochs (from January 29 2000) were upgraded with an improved statistical averaging technique for data with decreasing counting statistics. Also, T_GAIN files were recompiled with (CHIPX x CHIPY) of (32 x 128), which reduced the file size significantly without compromising T_GAIN correction detailing . More information can be found in the CalDB 4.9.2 release notes.

• ACIS bad pixel library upgrade

  The Framestore shadow regions at the bottom of each chip are added to ACIS bad pixel (BADPIX) CalDB library. It was implemented in CalDB 4.9.0 (December 17 2019) and the corresponding s/w was fixed in DS 10.8.3 (March 24 2020). This applies to all ACIS chips except in CC-mode observations for the data taken from the beginning of the mission. More information can be found in the CalDB 4.9.0 release notes.

• ACIS header keyword updates

  Unnecessary ACIS header keywords (CCD_ID and FEP_ID) were removed from the ACIS event file. It was implemented in DS 10.8 (June 18, 2019).

HRC

• HRC-I gain map application

  For the time-dependent HRC-I gain correction, HRC-I GMAP (Gain Map) is regularly updated every year to compensate for the decline in HRC-I gain, typically a few % per year. This affects the last quarter of each year where there is a 3-month lag between the calibration data and the actual release into CalDB. The background information can be found in the CIAO why page of HRC-I Gain Maps.

• HRC-S Degap improvement

  The HRC calibration team has uncovered a small error in the determination of the plate edge positions in the V-direction (along the length of the array). The fix (in HRC-S GAPLOOKUP table) was Implemented in CalDB 4.8.3 (May 24, 2019) and applied in DS 10.9 (July 22, 2020). This improves the energy scale for all LETG observations by correcting HRC-S plate corner positions. Further details can be found in the CalDB 4.8.3 release notes.

• HRC-S T_GMAP (Time-dependent Gain Map)

  The HRC-S gain degradation has taken place since the beginning of the mission and more rapidly since 2015. The proper correction for the time-dependent gain degradation was implemented in CalDB 4.9.2 (July 06, 2020) and the corresponding s/w fix in DS 10.8.3 (March 24, 2020). More information can be found in the CIAO ‘why page’ of HRC-S Time-dependent Gain and the CALDB 4.9.2 release note.

• HRC event files with new columns

  HRC event files retain more columns which were previously excluded. They include amp_sf (amplifier scale factor), sumamps (scaled sum of three nearest amp signals) and raw coordinates (crsu/v, au/v and rawx/y). It was implemented in DS10.8 (June 18, 2019).

GRATING (HETG and LETG)

• Grating response files

  The new GARF (grating Auxiliary Response File) and GRMF (grating response matrix file) for multiple orders are generated at the default position. It was implemented in DS10.8 (June 18, 2019) and affects all grating observations for both LETG and HETG. Further details can be found in the CIAO 4.12 mktgresp description.

Aspect

• Aspect Camera Assembly (ACS) plate scale correction

  Improvements in the aspect camera plate scale calibration and other algorithmic changes, implemented with CALDB 4.9.1 (March 24, 2020), can improve astrometric accuracy by up to 0.10 arcsec. This is mostly relevant for data taken over the last five years. Further details can be found here.

General

• Off-axis angle correction

  A new tool (axiscorrect) was implemented in DS 10.8.3 (March 24, 2020) to improve the boresight correction. The OAA (off-axis angle) correction is up to 30” in recent data but much less for early phases of the mission. Further details of the off-axis angle algorithm can be found in summary and the full documents.

• Improved timing accuracy

  A new set of interpolated clock correlation files will be applied. This update will improve all data since the last REPRO-IV (Period C). Further details can be found in a memo on Absolute time calibration for the Chandra X-ray Observatory.

• Applying definitive ephemeris

  The definitive orbit ephemeris data are not available until about a week after the observation. Instead, a predictive orbit ephemeris is used in the pipeline processing. This is important in the high precision timing analysis e.g., applying barycenter corrections to fast pulsars, and affects all data since the last REPRO-IV (Period C). See also Absolute time calibration for the Chandra X-ray Observatory.

• DOIs (Digital Object Identifier)

  From DS 10.8.3 (March 24, 2020), Digital Object Identifiers (DOIs) replace the existing dataset identifiers (DS_IDENT keyword) to provide a versatile and powerful mechanism for fast, efficient, and durable access, citation, and cross-linking of Chandra observations in the scientific literature. The Chandra DOIs follow the DataCite standard and are available in the headers of all FITS data products that can be unequivocally associated with a single Chandra Observation Identifier (ObsID). For example, the DOI of ObsID 23336 is defined as "10.25574/23336" where the prefix "10.25574" identifies the Chandra Data Archive. A full description can be found at this link.

• Replacement of bad target names

  The target names of observations taken before January 1st, 2018 that did not resolve into correct sky coordinates, contained a typo, were uninformative or too generic, have been replaced to increase the discoverability of these observations through the archival interfaces. The new target names have been selected to reflect as closely as possible the original target name and/or the scientific motivation of the observation. No other metadata that can be used to identify an archival Chandra observation (ObsID, pointing coordinates, etc) has been modified. More details can be found at this link.

• Improved sky fov (field-of-view)

  An improved skyfov algorithm was implemented in DS10.8.1 (October 2, 2019) to increase the precision of the (dithered) fov region by estimating the extent of the dither pattern with a convex hull solution (method=convexhull), upgrading from the previous 8 point min and max solution (method=minmax). More details can be found in the CIAO 4.12 skyfov description.

• Updated Header keyword

  The old header keyword MJD_OBS was replaced with a new header keyword MJD-OBS. It applies to all Chandra data products to bring into compliance with the FITS-3.0 standard. It was implemented in DS 10.8.1 (October 2, 2019).

Previous reprocessing notes

Repro-IV Repro-III

Pipeline s/w release dates

DS 10.9 2020-07-22
DS 10.8.3 2020-03-24
DS 10.8.1 2019-10-02
DS 10.8 2019-06-18

CalDB release notes and dates

4.9.2 2020-07-07
4.9.1 2020-03-24
4.9.0 2019-12-17
4.8.3 2019-05-24