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The procedures described in this section are still under development.
Users with problems or suggestions are urged to contact us:
rsdc@cfa.harvard.edu. The scripts described below are available from
our anonftp area: sao-ftp.harvard.edu. cd to pub/rosat/dewob. A
paper containing further discussion and examples is in preparation
(Harris, Silverman, and Hasinger, 1998, to be submitted to Astron. &
Astrophys.).
Throughout this discussion, we use the term ``PRF'' in the dynamic
sense: it is the point response function realized in any given
situation: i.e. that which includes whatever aspect errors are
present. We start with an observation for which the PRF is much worse
than it should be; caused by residual wobble errors or other causes.
We seek to improve the PRF by isolating the offending contributions
and correcting them if possible.
Spatial analysis of ROSAT HRI observations are often plagued by poor
aspect solutions, precluding the attainment of the potential
resolution of about 5''. In many cases (but not all), the major
contribution to the degradation in the effective Point Response
Function (PRF) comes from aspect errors associated with the ROSAT
wobble. This work is similar to an analysis by Morse (1994)
and provides a set of routines to minimize these effects. The
procedure in section 13.2.4 will also fix most cases of ``displaced
obis''. Occasionally a multi-obi observation consists of two quite
distinct aspect solutions. A recent example showed one obi for which
the source was 10 north of its position in the other
17 obis.
The 'model' assumes that the star tracker has some pixels with
different gain than others. As the wobble moves the de-focused star
image across the CCD, the centroiding of the stellar image gets the
wrong value because it is based on the response from several
pixels. If the roll angle is stable, it is likely that the error is
repeated during each cycle of the wobble since the star's path is over
the same pixels (to a first approximation if the aspect 'jitter' is
small compared to the pixel size). What is not addressed is the error
in roll angle induced by erroneous star positions. We have reason to
believe that this error may be rather large so that the centroiding
technique with one strong source 'fixes' only that source.
The method given here assigns a 'wobble phase' to each event; then
divides each obi into a number of wobble phase bins. Each bin is used
to create an image and the centroid of the source is measured. The
data are recombined after applying x and y offsets in order to ensure
that the centroids of sub images are aligned. What is required is
that there are enough counts in the source to obtain a reliable
centroid. Variations of this method for sources weaker than approx
0.1 count/sec involve using all obis together before dividing into
phase bins. This is a valid approach so long as the nominal roll
angle is stable (i.e. within a few tenths of a degree) for all obis.
We have seen misalignment of 5 to 8 arcsec between obis and when
different combinations of guide stars are used in the aspect solution.
It is important that the phase of the wobble is maintained during the
observation. This is ensured if there is no 'reset' of the space craft
clock during an observation. If your qpoe file has a begin and end
time/date that includes a reset, it will be necessary to divide the
qpoe file into two with a time filter before running qp-phase. Dates
of clock resets are provided by MPE:
http://www.rosat.mpe-garching.mpg.de/prp/timcor.html. Currently they
are: