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Coordinate Correction Table and Software

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!).


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