CALDB 4.2.1 PUBLIC RELEASE NOTES
Public Release Date: 05 APR 2010
CalDB 4.2.1 is a patch upgrade to CalDB 4.2.0 for use in CIAO 4.2 and later versions of the Chandra analysis software. This upgrade includes the following items:
- ACIS T_GAIN Epoch 40 (Nov 09 through Jan 10)
- ACIS "Blank Sky" BKGRND files group "E".
- LETG GREFF (Grating Efficiencies) version N0006.
II. SUMMARY OF CHANGES
Time-dependent gain corrections, updated for the period Nov 2009 through Jan 2010, have been released as ACIS T_GAIN Epoch 40. The updates are nominal, with no striking new trends. The data affected by this upgrade are any taken since 2009-08-01T00:00:00 to varying degrees based on the interpolation time; however all changes are within 1% of the energy value, versus the previously applied temporary corrections. Note also that data taken since 2009-11-01T00:00:00 (UTC) will still have a temporary T_GAIN correction applied (the version N0002 files above) when reprocessed, until the Epoch 41 data become available. See the Technical Details section for more information about Epoch 40 corrections.
For more information on the ACIS Time-dependent Gain correction, consult the T_GAIN why page.
CIAO/SDP reprocessing tool acis_process_events.
ACIS-I in focal plane:
ACIS-S in focal plane, no CTI corr:
ACIS-S in focal plane, CTI corr ON:
A new group of ACIS BKGRND files, applicable to all extended source observations taken since (UTC) 2005-09-01T00:00:00, has been released. Once installed in your local CalDB area, these files will be selected automatically using the CIAO contrib tool "acis_bkgrnd_lookup." See the Technical Details section for details on the new BKGRND events files.
CIAO contributed tool acis_bkgrnd_lookup.
The LETGS Calibration Team has found that the predicted higher-order grating effective areas are too large by up to 10%, and have provided a correction for orders +/-2 through +/-7. The full calibration for all higher orders will require several more months of work; Hence another more complete upgrade file is anticipated by the end of calendar year 2010. See the Technical Details section for details.
III. TECHNICAL DETAILS
The ACIS T_GAIN Epoch 40 completed on 31 January 2010, and new T_GAIN corrections have been derived. As with each T_GAIN upgrade, the previous epoch's interpolation files are finalized with the following one, while the new epoch's files are generated in a "flat," non-interpolating form in anticipation of the current epoch. The effects of the new upgrade files, relative to the previous sets, are illustrated below in Figs. 1-3. Fig. 1 gives the corrections in eV for the aimpoint of ACIS-I3, CTI corrected. Fig. 2 gives the same for ACIS-S3 with the BI chip CTI-corrector applied. Fig. 3 gives the same data for ACIS-S3 with no CTI correction. This latter is relevant to GRADED MODE datasets for ACIS-S3.
Newly-analyzed background datasets dating from late 2005 through 2009 have been compiled into the several new blank-sky files that form the fifth group (Group "E") of the ACIS BKGRND calibration products. The calibration products are listed in Table 1 below, including their filenames, LIVETIME values, chips included, and ACIS positions.
|FILENAME||ACIS POSITION||CHIPS||CTI CORR APPLIED||LIVETIME|
Recent discussions of the LETG Higher-Order efficiency extraction regions have identified an order of 10% error in the derivation of the existing 2004 Grating Efficiency (GREFF) calibrations for the LETG, currently stored in version N0005 of the file in CalDB.
Specifically, the 2004 higher order efficiency calibration of orders 2 through 7 used sector regions (3.302-degrees full angle), centered on 0th order and opening along the dispersion axis. These regions were designed to maintain a constant extraction efficiency for 1st order by excluding fine-support-structure cross-dispersion orders, which contain ~10% of the total flux outside the primary order. Instead, a rectangular region should have been used in order to maintain a constant extraction efficiency for photons of a given energy regardless of order. As a result of the progressively wider extraction region used for higher-order photons, more of the cross-dispersed photons were collected for higher orders than for 1st order, with an increasingly large excess (asymptotically approaching 10%) with larger orders.
The Figure shows the relative excess, using data from the LETG/ACIS-S observation of Mkn 421, ObsID 4148, and incorporates small corrections to account for ACIS pileup. Straight lines are drawn to approximate the correction factors (which level out at 1.0 at long wavelengths) and were used to derive revised GREFFs.
The modified grating efficiencies relative to 1st order are estimated to be accurate to +/-10% for m=2 and 3, but efficiencies in 4th order may be too large by 10%, 10-20% for m=5 and 6, and 10-30% in 7th order. Uncertainties for several orders beyond m=7are probably of order +/- 30%. An improved calibration for all orders is expected in roughly one year. However, we believe the current corrections warrant an immediate interim upgrade to the existing LETG GREFF file.