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             |              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
             |                                            |
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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

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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. 

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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). 

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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.