`mkexpmap' generates an exposure map which may be used
to convert a counts image of a source to an image in
flux units. An image in sky-coordinates, the exposure
map is essentially an image of the effective area
associated with each sky position as a result of the
aspect motion of the telescope over the course of an
observation. It accounts for the effects of dither
motion which are especially important near the edges of
the detector and also near bad pixels and bad columns.
The default units of the exposure map (M) are
M = [cm**(2)*counts/photon]
and are the same units as the effective area (ARF).
Optionally, the exposure time factor (t) may be
included, yielding a map
M' = M*t = [sec*cm**(2)*counts/photon].
To include the exposure time factor in the output map,
set normalize="no".
To compute an image in photon flux
units, divide the counts image (N) [counts/pixel] by
the product of the exposure map (M) and the exposure time (t):
photon flux [photons/cm**2/s/pixel] = N / (M*t) = N / M'
Dmimgcalc may be used to carry out this division.
As input, 'mkexpmap' requires an aspect histogram file
(computed by asphist) and an instrument map file
(computed by mkinstmap). The output exposure map is
generated for a specified sky-coordinate grid.
The input aspect histogram gives the total dwell time
for each sky pointing direction accumulated by the
telescope over the course of the observation.
The input instrument map is an image in detector
coordinates which, for a given incident spectrum, is
essentially the product of the mirror effective area
projected onto the detector surface with the QE of the
detector. It also incorporates bad pixels and ACIS
windows (if any) by setting the efficiency to zero at
the appropriate locations on the detector.
The exposure map is a dwell-time-weighted sum of
overlapping instrument maps, with one term in the sum
for each pointing orientation specified
in the aspect histogram. Because this repeated
shifting and adding of instrument map images can
require an extremely large number of floating-point
operations, the exposure map calculation may take a
long time. The running time scales roughly as the
product of the number of aspect histogram records and
the number of sky pixels. In cases where computation
speed is more important than a detailed treatment of
the pointing motion, one can greatly speed up the
computations by computing an exposure map for the
average pointing direction; to select this option,
set useavgaspect=yes.
For a detailed, quantitative definition of the exposure
map, see Davis, John E. 2001, ApJ, 548, 1010.
See also
http://space.mit.edu/CXC/docs/expmap_intro.ps.gz
.
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