My BACKSCAL (or effective area) has crazy values: help!
look at the spatial region used to extract
a particular spectrum, they expect to find a region defined in
or EQPOS(RA,DEC) coordinates. However in some circumstances
produce a region with the units AXIS2D1(X,Y) or
AXIS2D1(DETX,DETY). In general this axis is misinterpreted by the
and causes them to calculate wrong values (often
If bad pixel correction is switched on (withbadpixcorr=yes)
this can sometimes be
detected by an unfeasibly high bad pixel correction factor in the task
output (when SAS_VERBOSITY>=4), e.g.:
backscale::arfgen:- Bad pixel & chip gap fraction: ~43.23 %
This problem has so far been detected in two cases:
Its probable that more lines of processing will run into this error.
Please check the DSTYPEn keywords in the header of the spectrum if
you suspect that your BACKSCAL value of effective areas are
wrong, e.g. with the ftools command:
- A spatial region was extracted with the axes reversed,
i.e.what should have been
a region of type POS(Y,X) became AXIS2D1(Y,X).
- A spectrum was extracted from a filtered event list where the filtering
was performed with a region file, e.g.:
evselect expression="region(bkg_region.fits)&&(PATTERN == 0)&&(PI in [200:10000])"
Nonetheless, spectra extracted from a normal unfiltered event file using a
region file don't seem to suffer from this problem.
fkeyprint spectrum.ds DSTYP
The workaround for this problem is straightforward. It is sufficient to use a
FITS editor (such as fv) to change the AXIS2D1 string to
the right value, e.g. change DSTYP3 = AXIS2D1(Y,X)
to DSTYP3 = POS(Y,X), or
DSTYP5 = AXIS2D1(DETX,DETY) to DSTYP5 = DET(DETX,DETY),
and then run
For complicated spatial regions the identifier may be
AXIS2D2 or AXIS2D3, but
the same principle applies.
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June 18, 2009