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CalDB Release Notes
Effective Date: (UTC) 2006-11-06T21:00:00
Public Release Date: 07 November 2006
Version: 3.2.4
Release Type: SDP + PUBLIC
 

I. INTRODUCTION

CalDB 3.2.4 includes only the addition of six new ACIS time-dependent gain correction files (T_GAIN), upgrading T_GAIN Epochs 21 through 26 (i.e., Feb 2005 though April 2006). These upgrades are being introduced for two reasons:

(1) To address a discontinuity in the I0 and I2 gains versus time which occured at approximately 01 August 2005.
(2) To introduce new time-dependent gain corrections for Epochs 25 (Nov '05-Jan '06) and 26 (Feb '06-April '06).


II. SUMMARY OF CHANGES

A. ACIS T_GAIN updates (Epochs 21-26)
Location: $CALDB/data/chandra/acis/bcf/t_gain/
Filenames: acisD2005-02-01t_gainN0005_revA.fits    (EPOCHS 21 and 22)
acisD2005-05-01t_gainN0005_revA.fits    (EPOCHS 21 and 22 repeated)
acisD2005-08-02t_gainN0005.fits        (EPOCHS 23 and 24)
acisD2005-11-01t_gainN0005.fits        (EPOCHS 24 and 25)
acisD2006-02-01t_gainN0005.fits        (EPOCHS 25 and 26)
acisD2006-05-01t_gainN0002.fits (flat file, current in processing) (EPOCH 26 only)

Piplines/Tools affected:

DS 7.6.9 ACIS L1 pipeline and CIAO 3.3.0.1 tool acis_process_events,
parameter "tgainfile", L1 Event File Keyword "TGAINFIL".

Analysis threads affected:

"Create a New Level=2 Event File"
http://asc.harvard.edu/ciao/threads/createL2/index.html#acis

"Apply the Time-Dependent ACIS Gain Correction"
http://asc.harvard.edu/ciao/threads/acistimegain/

CIAO Information Pages:
Please refer to the web page "How CalDB 3.2.4 Affects Your Analysis"
http://asc.harvard.edu/ciao/releasenotes/ciao_3.3.0.1_release.html#HowCALDB3.2.4AffectsYourAnalysis

See also the ACIS Time-dependent Gain "why" page:
http://asc.harvard.edu/ciao/why/acistgain.html
 

III.  TECHNICAL DETAILS

A. ACIS T_GAIN updates (Epochs 21-26)

We have upgraded T_GAIN correction data for the Epoch 22, 23, and 24, principally due to the I0 and I2 gain discontinuity seen at approximately 01 August 2005, and the prevailing high particle background, which over time has reversed the trend of the time-dependent gains on other chips (notably ACIS-I3!). Furthermore we have reduced ECS data and calibrated the T_GAIN corrections for Epochs 25 (ECS data for 01 Feb 2006-30 April 2006) and Epoch 26 (01 May 2006-31 Jul 2006). This results in the production of five new T_GAIN files, as listed in table 1, along with the input files, EPOCH# keywords, and Calibration Validity Start Dates (CVSD* keyword set).

Table 1: New T_GAIN files and components, with the files being replaced. The  EPOCH1 and  EPOCH2 values given  are for the NEW files.

NEW FILENAME
(*.fits)

OLD FILENAME
(*.fits)
EPOCH1
(UTC date in sec)
DELTPHA1 INPUT
(*.fits)
EPOCH2
(UTC date in sec)
DELTPHA2 INPUT
(*.fits)
CVSD0001
acisD2005-02-01t_gainN0005_revA
acisD2005-02-01t_gainN0005
227448064.184
corrgain2005-02-01.v4
235310464.184
corrgain2005-05-01.v4
2005-02-01
acisD2005-05-01t_gainN0005_revA
acisD2005-05-01t_gainN0005
227448064.184
corrgain2005-02-01.v4
235310464.184
corrgain2005-05-01.v4
2005-05-01
acisD2005-08-02t_gainN0005
acisD2005-08-01t_gainN0005
243259264.184
corrgain2005-08-01.v4
251164864.684
corrgain2005-11-01.v4
2005-08-02
acisD2005-11-01t_gainN0005
acisD2005-11-01t_gainN0002
251164864.684
corrgain2005-11-01.v4
258984065.184
corrgain2006-02-01.v4
2005-11-01
acisD2006-02-01t_gainN0005
(new epoch)
258984065.184
corrgain2006-02-01.v4
266803265.184
corrgain2006-05-01.v4
2006-02-01
acisD2006-05-01t_gainN0002
(new epoch)
266803265.184
corrgain2006-05-01.v4
274752065.184
corrgain2006-05-01.v4
2006-05-01

The EPOCH1 value is the midpoint between the CVSD value of the given file and the CVSD of the NEXT file in the series. The EPOCH2 value is the midpoint (numerical average) of the CVSD values from the next TWO files in the sequence. For the last file in the table, EPOCH2 is the midpoint time between 2006-08-01T00:00:00 and 2006-11-01T00:00:00.

NOTE: The previous definition implies that a change in the CVSD of a given file will affect the EPOCH1 value of the previous T_GAIN file. Similarly, such a change also affects the EPOCH2 value of the file two epochs in advance of the given file. The CVSD of the August 2005 file was changed from 2005-08-01T00:00:00 to 2005-08-02T00:00:00. Hence there is a need to update EPOCH1 for the 2005-05-01 file, and the EPOCH2 value in the 2005-02-01 file.

The files tagged as version N0002 above have the same data in DELTPHA2 as in DELTPHA1, and hence the file interpolates over two identical data values, and so the result is an identical value. Hence such a file is known as a "flat file", preventing two-point trends from being extrapolated too far into the future. When EPOCH 27 (August-October 2006) is released, the N0002 file will be upgraded to an N0005 file, the permanent, interpolating T_GAIN file for Epoch 26.

GAIN DISCONTINUITY ON I0 and I2, ~01 Aug 2005:

The ACIS gain @ -120C page, at http://space.mit.edu/~cgrant/gain/ includes the following illustrations of the raw time-dependence in the gain of the several ACIS chips. Clearly illustrated are gain discontinuities located near 01-Aug-2005 which are problematic for the above CalDB T_GAIN file definitions. In short, the system should not attempt to interpolate the "corrgain" data across these discontinuities for these two chips.

./supporting/acis_gains.gif
Fig. 1: Average gain values for the ten ACIS chips while operating at -120C. These are measured from the ECS line source data, however they only include the bottom twenty rows of the chips, to avoid any CTI effects.

The adjustment to be made in the T_GAIN interpolation for I0 and I2 must not at the same time destroy the accuracy for the other eight chips across 01 Aug 2005. The file affected by this discontinuity is acisD2005-05-01t_gainN0005.fits, released in CalDB 3.2.2. The ACIS Calibration team has decided to use the previous datasets released in the file acisD2005-02-01t_gainN0005.fits to substitute for the data in the former file; however both files need to change the EPOCH2 data to adjust for the new end date of 02 Aug 2005. Hence the two files designated *N0005_revA.fits are included in this release.

Regarding Trends in I3 and S3:

The following plots have been produced by A. Vikhlinin of the ACIS M&A team:

./supporting/acis-i3_trend.gif
Fig. 2: I3 T_GAIN adjustments as derived from ECS data for the three X-ray lines illustrated. These data are taken MID-CHIP, and so include the radiation level effects on the electron traps which cause the CTI effects.
 
  ./supporting/acis-s3_trend.gif
Fig.3: S3 T_GAIN adjustments as derived from ECS data for the three X-ray lines illustrated. These are also near the mid-chip region.

The I0 and I2 discontinuities occur between Epochs 22 (May-July 2005) and 23 (Aug-Oct 2005). However, the  net T_GAIN correction for I3 in particular, including the particle background, as derived from the ECS data, shows that the correction for I3, while not discontinuous between Epochs 22-23, are also not monotonic in time. Hence there will be T_GAIN corrections versus Epoch that will reverse prevailing trends in previous data sets, such as for I3. S3 shows some similar behaviors over Epochs 15-17.

The radiation level changes the CTI gain correction by fully or partially filling the numerous e- traps between the selected detector row and the readout at the bottom of the chip.

ECS (External Calibration Source) data have been used as follows to verify improvement in the I0 T-GAIN corrections using the CIAO 3.3.0.1 software, currently released. Here is a summary J. DePasquale's results:

Expected energy ~ 5.895 keV

TABLE 2: Testing results for new T_GAIN files to be released.

OBSID
DATE-OBS
T_GAIN File
Description
Mn-K fitted peak
position (keV)
% Difference from
 5.895 keV
59536 2005-07-18 acisD2005-05-01t_gainN0005_revA NEW FILE
5.913 +0.31
59536 2005-07-18 acisD2005-05-01t_gainN0005 OLD FILE
5.860
-0.59
(out of spec)
59365 2005-10-25 acisD2005-08-02t_gainN0005 NEW FILE
5.909 +0.24
59243 2006-01-31 acisD2005-11-01t_gainN0005 NEW FILE
5.900 +0.08

Interpretation:

[1] The behavior is as <we> would expect for OBSID 59536 with the N0005 and N0005_revA Tgain files.  The 2005-05-01 N0005_revA (new) Tgain file produces a peak which is justbarely out of our spec (but very close) and the N0005 (old) file produces a peak which is significantly lower and out of spec.  This is consistent with <our> expectation that the N0005 file interpolates over the gain discontinuity and the N0005E file does not interpolate over the gain discontinuity.

[2] The results for OBSID 59365 and 59243 are within spec.

[3] We get the same results for OBSIDs 59365 and 59243 when we used the new gain files.



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