| About ROSAT |
ROSAT Home Page | ROSAT Images |
|---|
This section describes the expected performance of the complete WFC
telescope ( 
for the full aperture).
Values for the point response function, vignetting
and effective area were obtained
directly from in-flight calibration and
full aperture test at MPE's Panter X-ray beam line
facility.
Figure 5.2 
shows the on-axis effective area of the
complete WFC for each filter
design. The effective area decreases with increasing off-axis angle, dropping
linearly to 67% of its on-axis value at 2.5 degrees off-axis. The vignetting
function (Figure 5.3 ), which describes the
relative efficiency as a function of
off-axis angle, is dominated by geometric baffling. Rejection of scattered flux
from bright sources outside the field of view is good. Less than 0.5% and
0.01% of the source flux is detected for sources 3 degrees off-axis and 5
degrees off-axis respectively. Note that some of the available WFC filters have
a secondary transmission band in the far-ultraviolet. Although the far-UV
transmission is much weaker than that in the primary XUV bands, problems may
potentially arise when observing objects that have a particularly strong far-UV
component in their spectrum. Details of the far-UV sensitivity, the
``UV leak'', are given in § 12.6 .
The point response function of the WFC (Figure 5.4 ) is dominated
by the performance of the mirrors. Figuring errors determine the on-axis
response but aberrations inherent in the optical design dominate at the edge of
the field of view. As the energy of the incident photons increases the
scattering wings becomes more pronounced. This is illustrated in
Figure 5.5 . At 277 eV and 40.8 eV the on-axis
half energy widths (HEW) are
2.9' and 1.7' respectively. However, at the edge of the field
of view the HEW is 4.4' and independent of photon energy. The average
HEW over the field of view is 3.5'.
The resistive anode image readout system of the MCP detectors has a small
but significant intrinsic distortion. This is determined largely by the
physical parameters of the system (i.e., anode resistance and capacitance)
but assymmetries are introduced by variations in the resistivity of the
anode as a result of the manufacturing process. Consequently, the distortion
needs to be removed during the data processing. This task,
termed ``linearization'', is performed by means of a look-up table that
gives the relationship between the detector coordinate system and that of
the telescope. Figure 5.6 a shows the image from an early
calibration observation displayed in raw detector coordinates.
Figure 5.6 b shows the corresponding image after correction
for the anode distortion, i.e. ``linearization'', whilst
Figure 5.6 c shows the
image in celestial coordinates after correction for the spacecraft
attitude changes during the observation (the main attitude changes here
are the intentional aspect ``wobble'' - see § 3.4 ).
The ``linearization'' correction introduces positional errors which are
negligible compared with other sources of uncertainty
(see § 12.8 ).