In the default spectroscopy mode, the most important parameters for each detected photon in the event lists are the two spatial coordinates and one energy coordinate. These go through a series of improvements in the initial stages of rgsproc in which the initial raw set of
[CCDNR, CCDNODE, RAWX, RAWY, ENERGY]
BETA
, and the perpendicular
cross-dispersion direction, or XDSP
,
[BETA, XDSP, PHA]
[BETA_CORR, XDSP_CORR, PI]
BETA_CORR
and XDSP_CORR
reconstruct the angular distribution of photons emerging from the
gratings and include randomisation of the quantised CCD coordinates in
order to simulate a continuous distribution. The coordinates of the
nominated source make a subtle appearance at this stage as they are used
in the correction of BETA
to BETA_CORR
(and hence to
MLAMBDA
) caused by variations in the angle of incidence on the
gratings due to spacecraft pointing jitter. The otherwise redundant
PI
energy column is used to distinguish between spatially
overlapping orders.
Once the event list has been filtered for bad pixels of the various
types signaled by rejflags
in
Table 7, the source list plays a central
role. The source list here has 5 binary-table extensions
P0136540101R2S002SRCLI_0000.FIT:SRCLIST 1=PROPOSAL 2=ONAXIS 3=MKN421 P0136540101R2S002SRCLI_0000.FIT:RGS2_BACKGROUND LAMBDA-XDSP_CORR region P0136540101R2S002SRCLI_0000.FIT:RGS2_SRC3_SPATIAL LAMBDA-XDSP_CORR region P0136540101R2S002SRCLI_0000.FIT:RGS2_SRC3_ORDER_1 LAMBDA-PI region P0136540101R2S002SRCLI_0000.FIT:RGS2_SRC3_ORDER_2 LAMBDA-PI region
which specify selection regions for the nominated source #3 and the
orders 1 and 2 chosen by default and the background region. The
calculation of the position and width of the selection regions depends
on the source's celestial coordinates, and the fractions of the
XDSP_CORR
and PI
response curves specified by the
parameters xpsfbelow
, xpsfabove
, xpsfexcl
and
pdistincl
. The events which contribute to source and background
spectra are those that fall in selection regions defined, for example,
for 1st order in RGS2 :
(MLAMBDA,PI) IN REGION(P0136540101R2S002SRCLI_0000.FIT:RGS2_SRC3_ORDER_1)) && ((MLAMBDA,XDSP_CORR) IN REGION(P0136540101R2S002SRCLI_0000.FIT:RGS2_SRC3_SPATIAL))
Finally, the simulated continuous MLAMBDA
distribution of
selected events are rebinned into channels to yield the final binned
counts spectra. Before SAS v8.0, spectra were always accumulated in
intervals of the dispersion angle BETA_CORR
. Starting in SAS
v10.0, spectra are calculated by default on a uniform wavelength
grid. Wavelength-based spectra facilitate the combination of two or more
spectra and their associated background and response files.