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ROSAT Status Report #178: HRI ASPECT TIME BUG IN SASS PROCESSING PRIOR TO SASS7_B
Date: Mon, 3 Mar 1999 14:09:31 -0500 (EST)
Subject: HRI ASPECT TIME BUG IN SASS PROCESSING PRIOR TO SASS7_B
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[ HINTS & POINTERS for PROS Users ]
[ 19 FEBRUARY 1999 #26 ]
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This issue of H&P is essentially a bug report. The first item
describes a bug in the standard processing, SASS, which affects all
ROSAT HRI observations prior to 17 Jan 1997. An off-line fix is
available and once SASS7_B is released, newly processed data will be
correct. We also report on 3 bugs in PROS which are of minor
importance since they affect only a few users.
1.0 ROSAT HRI ASPECT TIME BUG IN SASS PROCESSING PRIOR TO SASS7_B
2.0 PROS BUGS AND PROBLEMS REPORTED BY USERS
1.0 ROSAT HRI ASPECT TIME BUG IN SASS PROCESSING PRIOR TO SASS7_B
In late 1998, W. Voges and S. Doebereiner were working on the
implementation of the WFC star tracker data for the aspect solution
required for observations between the 1998Apr loss of the second
tracker and the 1998Sep destruction of the HRI detector. During this
investigation, they developed an aspect solution routine and comparing
photon positions to those in archival SASS output, found an average
offset of circa 3.5 pixels for a test observation. This effect
vanished if a time shift was introduced into the aspect table. The
value of the time shift was close to the fractional part of aspect
times. At SAO, we found a bug in the SASS code which explains the
results obtained at MPE.
1.1 Bug description
The bug occurs because the fractional time (an integer) assigned to
the aspect time was divided by 8192 to get decimal seconds to add to
whole integer seconds, whereas it should have been divided by 64.
This means that the listed aspect times are wrong by up to one second
(for a particular observation, the magnitude of this error is fixed).
When a photon arrives, and does the interpolation to find the x,y.roll
to apply to itself, it will often be interpolating between wrong
entries (i.e. the wrong interval) and it will always get the fraction
of the interval wrong. Thus the events have a wrong position, with
mean offset of 3.5 pixels for one observation (and similar means for
about 4 other sequences so far examined). Preliminary evaluation of
one observation (Cen A) indicates an improvement in the width of the
PRF by 10%, with an similar value for the increase in peak intensity
(i.e. a sharpening of the PRF). For Cen A, the bug contributes
something of order 3" as an added error term to the PRF;
i.e. FWHM(bad)=SQRT[FWHM(good)**2 + 3**2].
1.2 Affected Data
The (RDF) ...anc.fits file contains the aspect table which lists the
pointing position (RA, DEC, roll) read from the star tracker every
second. The fractional time assigned to each set of aspect solutions
depends on the clock reset which occurs each January. Thus, for a
year (more or less) the fractional time will be a constant. The RDF
...anc.fits files contain the correct fractional times. NB: The
error occurred during the correction of the photon positions by SASS;
thus the photon positions in the RDF *_bas.fits file (and quantities
derived from these photon positions) are affected, but the aspect
information contained in the ASPECT extension of the RDF *_anc.fits
file is correct. Data taken from the clock reset on 17 Jan 1997 to
the next reset on 20 Jan 1998 had a zero fractional time so the bug
had no effect. After 20 Jan 1998, the fractional time was 0.047s,
so the effects of the bug are minimal and difficult to detect.
1.3 An FTOOLS Offset Approach
At SAO, F. Primini has developed an approximate solution which is
accurate if the roll angle is not changing rapidly (i.e. for normal
observations). Since the standard SASS output includes the aspect
table (in the ...anc.fits file), we create a 'bad' version of the
table by replacing the aspect times with bad times (calculated
simply by dividing the fractional part of the aspect time by 128).
For each event, we then read (interpolate) RA,DEC from both the
good (original) aspect table and from the bad table. Taking
the differences, we then find a correction directly for x,y.
Finally, we add the new keyword plus two history cards to the header.
ASP_FIX= 'T' HRI ASPECT CORRECTION performed
HISTORY HRI Aspect bug fix incorporated in these data
HISTORY executed by FTOOLS HRI Aspect Fix SAO-Vs1.0/1999feb
1.4 Availability of Fix
The scripts for running the ftools version are available via anonftp
and (eventually**) web sites at SAO and GSFC. SAO will offer a limited
'service option', designed to assist ROSAT users at institutions with
only a few ROSAT observations. This service will be provided on a "as
resources permit" basis. Please send requests to rsdc@cfa.harvard.edu
providing the sequence number, plus the start and end dates of the
observation requested. We will send you a short form to fill in.
To obtain the scripts, examples of 'before and after' and instructions:
ftp -i sao-ftp.harvard.edu (see particulars at bottom of this doc)
then cd to pub/rosat/aspfix
** the website is now active (1999 April):
http://hea-www.harvard.edu/rosat/rsdc_www/aspfix.html
2.0 PROS BUGS AND PROBLEMS REPORTED BY USERS
2.1 qpsim -- Generate simulated events for a ROSAT HRI field.
qpsim resides in xray.xspatial. There was a problem with the 'rename'
call. It has been corrected and will be fixed in our next release.
2.2 wcsqpedit -- Edit the WCS parameters in a QPOE file.
This task is in xray.xproto. The bug did not allow changing to negative
declinations (CRVAL2). It has been fixed and will be correct in the
next release.
2.3 qpsort for extremely large files
The user was attempting to get a qpoe file from a fits file of an
observation of the Crab Nebula. The bas.fits file was 263 Mb and the
udummy.qp file was just under 200 Mb. The task crashed when qpsort
was called. The problem was solved by increasing the hidden parameter
'sortsize'.
ABOUT HINTS & POINTERS
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Mailings will be sent as the need arises. If you would like to
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distribution list, please contact us at the following address:
**************************************************************************
RSDC MS-3 tel: (617) 495-7134 FAX: 495-7356
Center for Astrophysics Internet - rsdc@cfa.harvard.edu
60 Garden St. Cambridge MA 02138 USA
**************************************************************************
* to access our anonymous ftp service:
o ftp -i sao-ftp.harvard.edu # node address: 131.142.41.9
[Name: anonymous]
[Password: <yourusername@yoursite>]
ftp> cd pub/pros/Hints
N.B.: you can obtain your own copy of the PROS USERS GUIDE by
anonftp from subdir /pub/pros/PUG. The PUG is also on the WorldWideWeb:
http://hea-www.harvard.edu/PROS/pros.html
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