| About ROSAT |
ROSAT Home Page | ROSAT Images |
|---|
The HRI response is basically flat over the entire active area of the detector. The telescope point spread function degrades with off-axis angle and there is a slight vignetting of the telescope so that the effective area decreases. For field angles of less than 20', the mirror point spread function can be represented by a radial Gaussian of the form
where
arc seconds (independent of
energy), and 
is the field angle in arc minutes.
Users concerned with sensitivity off-axis will need to estimate the cell
size from the off-axis PSF by combining in quadrature
from the above expression, 
,
and 
to give 
. The
cell size is then given by the value 
, which
is the size of a square region where 50% of the source counts will
fall. The background rates must also be scaled to the new cell size.
The vignetting correction in Figure 10.18 
can be used to
scale down the
number of counts detected from off-axis sources due to the decrease in
telescope area; note that the HRI field of view is only 32' and
that the energy dependence of the vignetting correction is small and can
be neglected.