Figure 7:
The effective area (or vignette function) as a function of off-axis
angle. The solid curve was derived from C K and Al K data
and the dashed curve from Ag L data.
The SXT X-ray effective area varies across the 
arcmin
field-of-view. The effective area and response functions that are
reported in other sections of this manual always refer to values for the
optical axis of the telescope, where the effective area is maximum. The
off-axis response has been measured during pre-launch calibration and
has been refined with in-flight data. To a first approximation, the
off-axis variation of effective area, or the vignette function, can be
approximated by two non-concentric cones (see SXT Calibration Note 37).
The inner- and outer-cones intersect one another at a radius of about 21
arcmin. Analysis of flight data indicates that the vignette function
is actually more complicated and should at least be approximated by a
cone with some flat distortions. These distortions were not detected in
pre-launch calibration, and several questions about them remain. The
vignette function is known to vary as a function of incident photon
energy. Pre-launch measurements were made at three energies: 0.277 keV
(C K), 1.49 keV (Al K), and 2.98 keV (Ag L). Measurements at the two
lower energies gave results that were consistent with one another.
Thus, we have characterized the SXT vignette functions as either ``low"
or ``high" energy. The shape of the vignette function can be seen using
the IDL routine HELP_VIGNETTE for which a sample plot is shown in
Fig. 7.
There are no fully approved routines to remove or correct for the SXT vignette function, however, there are a two preliminary routines. SXT_VIGNETTE will compute the vignette function as it is currently understood. The form and parameters of the shape may be changed as additional data and analysis become available. The routine SXT_OFF_AXIS will correct for the vignette function by calling SXT_VIGNETTE. Users are warned that these routines are preliminary and may not be the most appropriate technique to use, depending on the scientific application. However, they are certainly more accurate than no correction when intensities from different parts of the CCD must be compared.