The source positions measured with ASCA are found to contain a
larger than expected error (~1 arcminute). We have identified the
cause of this systematic error and developed an algorithm to reduce
the absolute pointing uncertainty of the ASCA satellite by a factor of
four. The revised 90% error circle
radius for SIS coordinates is then 12 arcsecs, consistent with
pre-flight specifications, effectively restoring the full ASCA
pointing accuracy.
A comprehensive calibration of the ASCA X-ray pointing
accuracy is given in
Restoration of the ASCA Source Position Accuracy (ApJ, Nov 10
2000), the primary reference for this web page. Here we provide the
look-up table and software
referred to therein. These can be used to easily correct the measured
sky coordinates derived from any given ASCA observation.
Below, we provide a brief summary of the ASCA pointing problem and
show how to update your ASCA sky coordinates.
This work is a collaborative effort involving the following ASCA team
members: Y. Ueda, T. Kii, R. Fujimoto & K. Yamaoka (ISAS) and
E. V. Gotthelf (GSFC; Columbia Univ). We would like to thank
K. Ishibashi and E. Pier for their assistance at various stages of
this endeavor.
If you find this result useful, kindly reference the
above paper (not this web page!).
REFERENCES:
-
"Restoration of the ASCA Source Position Accuracy, " Gotthelf,
E. V., Ueda, Y., Fujimoto, R., Kii, T., & Yamaoka, K. 2000, ApJ, 543,
p. 417.
-
"Improvement of the ASCA Position Accuracy, " Ueda, Y., Inoue, H.,
Ogawara, Y., Fujimoto, R., Yamaoka, K., Kii, T., & Gotthelf, E. V. 1999, ISAS
Research Note, No. 688.
- "Measuring the ASCA X-ray Pointing Accuracy, " Gotthelf, E. V. &
Ishibashi, K. in "X-Ray Imaging and Spectroscopy of Cosmic Hot
Plasmas", Ed. F. Makino & Mitsuda, Frontiers Science Series (Univ. of
Tokyo Press) 1997, No. 19, p. 631.
- "The ASCA Source Position Uncertainties, " Gotthelf, E. V. in "ASCAnews" (NASA/GSFC Greenbelt, Maryland) 1996, No. 4, p. 31.
- "In-Orbit Performance of ASCA Satellite Attitude Control System, " Ninomiya, K., Hasimoto, T., Kii, T., Muranaka, N, Uo, M., Maeda, K., Saitoh, T., 1994, in "Advances in the Astronautical Sciences" ed. Culp, D. & R. Rausch (Univelt Corp., San Diego), Guidance and Control (Amer. Astonautical Society), Vol 86.
PROBLEM: Erroneous Star-tracker Reading
A systematic error has been identified in the apparent absolute
pointing of the ASCA satellite. This effects all derived ASCA source
positions - the celestial coordinates of a given observation are
effectively shifted by a constant offset (see Section 4,
ASCAnews #4, 31).
The cause of this offset error is the motion of the star trackers
relative to the satellite. The apparent position of the (optical)
guide stars as imaged by the star-trackers wanders along the satellite
Y-axis by up to one arcminute. This generates a false reading used in
determining the X-ray pointing (see Fig. 3a
of Ueda et al. 1999). A problem was recognized early in the mission
and the star trackers are not used to guide the instantaneous
pointing, which is otherwise stable.
The above effect is found to be related to the orbital variation in
temperature of the baseplate on which the star tracker sits. A simple
relationship exists between the baseplate temperature and the
mis-pointing along the detector axes (see Fig. 1
of the reference paper). This can be use to compute a correction
offset which restores the pointing to within the expected (random)
pointing uncertainty (see Fig. 2
of the reference paper).
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SOLUTION: Correction Table and Software
The following look-up
table lists the offsets needed to compensate for the erroneous
star tracker reading. For each ASCA pointing (e.g. the time spanned by
a single attitude file in "pointing" mode) a pair of numbers is
provided for each detector type (SIS or GIS) which gives the
correction to the sky (X/Y) coordinates derived from that
observation. These corrections apply strictly to data processed with
the standard REV2 processing.
To correct your ASCA coordinate, choose the appropriate offset from
the look-up
table for the given observation and detector type and apply them
in the following sense (but see NOTES below):
RA(corr) = RA(uncorr) + ra_offset / 60.0
DEC(corr) = DEC(uncorr) + dec_offset / 60.0
where RA/DEC are in degrees and offsets are given in arcmins.
The program offsetcoord
is available in the FTOOLS package to conveniently update the
coordinates system (CRVAL) keywords in an ASCA FITS image file using
the above algorithm. The program parameters are: 1) the input FITS
file (+ extension number) and 2) the RA/DEC offset pairs.
The comment string for the CRVAL keyword gives both the old CRVAL
values and the offsets used. An error message is generated if the
program is run twice on the same extension of a FITS file (as long as
the comment string is not overwritten). The FTOOL FDUMP can be used to
check these keyword.
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NOTES: Important Usage Info
Formally, the corrections discussed in this work reduces the
uncertainty in the ASCA satellite attitude in "pointing" mode by a
factor of four. However, this correction in no way compensates for the
measurement error associated with any particular source - this depends
on measurement method, source count rate, detector calibration,
off-axis position, etc...
For the SIS, which faithfully reproduces the ASCA mirror point-spread
function, the 90% error circle radius is 12 arcsecs (see Fig. 2
of the reference paper). This result is derive using a selected set of
100 suitable AGN observations by ASCA.
For the GIS, the measurement error dominate over the above correction,
and this error increases as a function of off-axis angle. The GIS
error circle radius derive using the same set of 100 AGNs as for the
SIS (all in the central 20 arcmin radius of the GIS) is 24 arcsecs
(see Fig. 4
of the reference paper).
The coordinates for GIS data obtained in reduced spatial resolution
mode (i.e. RAWXBINS = 64) are not well calibrated. GIS source positions
obtained in this mode likely contain an additional uncertainty of one
arcminute.
Most ASCA observations involved a single attitude file (i.e. seq#
54000000 used the attitude file fa960313_2043.0610). However, a few
longer observations span multiple attitude files. For these cases,
source position measurements from each segment of the observation
involving a new attitude file may need to be corrected separately.
The calibration which produced the corrections given in the look-up
table used ASCA data processed by the ASCA REV2 scripts. These
corrections are suitable for updating the coordinates of data obtained
from the HEASARC REV2 archival data and analyzed using the standard
HEASARC software tools.
The look-up table contain observations from the beginning of the
mission to 28 July 2000 and is 99% complete for this range of
dates. Please e-mail the ASCA help desk if your favorite observation
is missing from the list. The table will be updated periodically to
cover new ASCA observations as they become available.
The observation time and date are encoded in the attitude file name as
followed: faYYMMDD_HHMM.HHMM, where YYMMDD_HHMM is the start date and
time (YY=year, MM=mouth, DD=day, HH=hour, MM=minute) and .HHMM is
the stop time (the next day, if less then the start time).
If you have any problems using the look-up table and/or software
please write to the ASCA help desk.
A copy of the look-up table is available
here.
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Document author: Eric Gotthelf, eric@astro.columbia.edu
