|Chandra Electronic Bulletin No. 5|
******************************************************************************** | | | CCC XX XX OOO | Chandra | CC XX XX OO OO | CXC Electronic | CC XXX OO OO | Number 5 Bulletin | CC XX XX OO OO | June | CCC XX XX OOO | 2001 | | ******************************************************************************** Welcome to the Chandra X-ray Center's Electronic News Bulletin Number 5. If you would like to unsubscribe from this alias, please let us know by filling out a Helpdesk ticket (cxc.harvard.edu and click on "Helpdesk"). Contents: 1. HRC-S and LETG Processing 2. Aspect Data Caveat 3. Contact Scientist Update ------------------------------------------------------------------------------- 1. HRC-S and LETG Processing Announcement taken from: http://asc.harvard.edu/cal/Links/Letg/User/Problems/index.html See that website for the url links mentioned in the text below. LETG and HRC-S data processing problems B. Wargelin, 2001 May 14 Introduction One major and two minor problems have recently been discovered in the Automated Processing (AP) of HRC-S and/or LETG data, and another minor problem in one of the optional offline pulse-height filters. Solutions to these problems have been or will soon be implemented, and data reprocessing has started. Observers are referred to the LETG/HRC-S CIAO analysis thread for instructions on how to reprocess data themselves. Summary of problems 1) The HRC "PHA ratio test", which is applied just before Level 2 in the AP pipeline, removes a large fraction of valid x-ray events in the HRC-S (up to 30%) on an 8-tap (13 mm, 15 Angstrom) period (see Filtering Errors). This test, which is part of the Status Bit filtering (bit 21 in the event file status field), compares the pha and sumamps values for each event. As described in a memo by M. Juda, the pha/sumamps ratio is not well-behaved in the HRC-S; the data processing problem arose when HRC-I parameters were used for the test. Note that all HRC-S data are affected, not just grating spectra, since even in the "good" areas, roughly 5% of valid x-ray events are removed. The degree of loss may also vary across the dither pattern, particularly at some off-axis locations, introducing spurious periodicity into source lightcurves. 2) The background regions that go with the LETG "bowtie" spectral extraction are chopped off (in the cross-dispersion direction) beyond about 120 Angstroms, because of an incorrectly set parameter in tg_create_mask. This error occurs between level 1.0 and 1.5, and results in incomplete background subtraction at long wavelengths (see Background Region Error). LETG/ACIS-S data are not affected because those spectra extend to only about 60 Angstroms. 3) The "heavy" (nominal 5% x-ray event loss) HRC-S PI filter, which is NOT part of the AP pipeline and is applied between level 1.5 and level 2 at the discretion of the observer/analyst, removes more than the originally advertised 5% of valid x-ray events at long wavelengths, and should not be used to create publishable spectra (see Filtering Errors). Note that "light" and "medium" PI filtering are still valid. 4) The "degap map" for some areas of the HRC-S leads to spatial distortions in the degapped image. An illustration is shown from an off-axis calibration observation of Ar Lac. This problem does not affect data in the standard LETG spectroscopy region which uses the central 6 taps of the HRC-S, but may affect observations which use the full detector width, such as "timing mode" observations, if the source is extended or off-axis. Fixes All problems (except #3, which does not arise in pipeline processing and for which warnings have been posted) have been corrected in the AP pipeline as of early May 2001. Reprocessing of all existing HRC-S data has begun, and is expected to be completed by July 2001. For observers who wish to reprocess data themselves, the corrections are as follows: 1.HRC-S Status bit #21 -- Do not filter on this bit. See analysis thread for new status-bit filter command. 2.LETG background region -- Change the value of width_factor_arms from 20 to 40 in tg_create_mask. See analysis thread for proper CIAO commands. 3.Heavy PI filter -- Users are warned on the PI filter page and in the analysis thread that heavily filtered spectra will not be properly calibrated. In the longer term (est. summer 2001), a finer-scale gain map and improved filter will be developed. 4.Degap map -- The new map is available on the HRC Degap Coefficients page, but the necessary processing steps can only be carried out within the CXC until June 1, when an updated CALDB will be publically released, along with an updated analysis thread. Observers who have used the full width of the HRC-S to study off-axis or very extended sources and can't wait until June 1 should contact the Help Desk. ------------------------------------------------------------------------------- 2. Aspect Data Caveat Aspect Data Caveat: ACIS-I data includes 1" aspect offset Announcement taken from: http://asc.harvard.edu/udocs/arcsec_error.html We have discovered a small offset (~1") in the aspect solution for ACIS-I data which is in the processed and re-processed data since approximately Feb 2000. Details of the current aspect quality which demonstrate this offset (in DY for ACIS-I alone) are posted at http://asc.harvard.edu/mta/ASPECT/celmon/. We also provide a web page (http://asc.harvard.edu/mta/ASPECT/fix_offset.cgi) which will give information on aspect offsets for any specific dataset and will generate commands to remove the offset. For complete information on the Chandra aspect please refer to the Aspect webpage (http://asc.harvard.edu/mta/ASPECT/) and the Aspect Caveats Page (http://asc.harvard.edu/mta/ASPECT/aspect_caveats.html). ------------------------------------------------------------------------------- 3. Contact Scientists Contact Scientists - Update With the availability of substantial software analysis capability in CIAO 2 and with the various threads and documentation which have been provided, the CXC intends to modify the current approach of assigning contact scientists to help individual observers with data analysis problems. We will rely on the Help Desk as the primary source of assistance to Chandra observers. We also expect that the regular scheduling of workshops to familiarize users with CIAO and the existence of the Chandra-Users email group will provide additional assistance for observers. To provide for a reasonable transition, we also are e-mailing Cycle 2 observers (via the electronic bulletin) and plan to e-mail Cycle 3 Chandra observers. We will assign contact scientists to any observers who indicate that they require extra assistance beyond the support specified above. For Cycle 2 observers who wish to have a contact scientist and who have not yet received their data, the previously assigned contact scientist may be changed in order to more equitably distribute the work-load. Cycle 2 observers who desire continued (or future) assistance from a contact scientist will need to so indicate by responding to this announcement. To request a contact scientist please go to the sign-up page at: http://cxc.harvard.edu/udocs/request_cs.html Our preferred approach is to encourage all observers to utilize the Help Desk for primary assistance, and to request contact scientists only if they experience unanticipated difficulties in using the CIAO software, the threads, the Help Desk, etc. When required, we also will provide instrument expertise to assist observers. Plans beyond Cycle 3 will be developed based on the experience over the next year, incorporating the advice of the CUC and feedback from observers. We will consider implementing a check-box system concerning the possible need for a contact scientist as part of the Cycle 4 proposal form. Assignments of contact scientists when requested will be co-ordinated by a senior member of the CXC Science Data Systems Planning Group (Dan Harris). Assignments of local contact scientists for visitors coming to the CXC to analyze their Chandra data will be co-ordinated by the same individual.
Last modified: 11/18/20
The Chandra X-Ray
Center (CXC) is operated for NASA by the Smithsonian Astrophysical Observatory.
60 Garden Street, Cambridge, MA 02138 USA. Email: firstname.lastname@example.org
Smithsonian Institution, Copyright © 1998-2004. All rights reserved.