| About ROSAT |
ROSAT Home Page | ROSAT Images |
|---|
A schematic diagram of the Wide Field Camera (WFC) showing the main
components - grazing incidence mirrors, filters and microchannel plate
(MCP) detectors - is shown in Figure 5.1 
.
The WFC optics consist of a nested set of 3 Wolter-Schwarzschild Type I mirrors,
fabricated from aluminium and coated with gold for maximum reflectance. The
mirrors provide a geometrical collecting area of 475 cm 
with a common
focal length of 525 mm. The grazing incidence angles chosen (typically
) allow the collecting area to be optimised whilst
retaining a wide ( 
radius) circular field of view and a low
energy reflectivity cut-off at 0.21 keV (60Å). The on-axis resolution
is 
HEW, but the response degrades to
HEW at
off-axis due to inherent optical aberrations. Hence, the average
resolution for the survey will be 
HEW.
The mirror parameters are summarized in Table 5.1 .
In order to take full advantage of the telescope resolution the
pair of MCPs in the detector are both curved, like a watchglass,
to match the optimum focal surface,
as is the resistive anode readout system. A CsI
photocathode is deposited directly onto the front face of the front
MCP to enhance the XUV quantum efficiency. The detector resolution
is substantially (a factor >2) better than that of the mirror nest and
consequently does not contribute significantly to the net performance of
the WFC. A focal plane turntable can be used to select one of two
identical detector assemblies in flight. Table 5.2
gives details of the detectors.
The filter wheel assembly contains eight filters, these are listed in
Table 5.3 . There are six science filters, any of these
can be selected to define the wavelength passbands and suppress
geocoronal background radiation which would otherwise saturate the
detector count rate. Another function of the filters is to prevent
the detection of UV radiation from hot O/B0 stars which would
otherwise be imaged indistinguishably from XUV sources.
The six science filters comprise two redundant pairs of
``survey'' filters (S1a/S1b and S2a/S2b), together with
two ``pointed'' phase filters (P1 and P2) which cover somewhat
different energy ranges. Since the differences between the filters within
each redundant pair are small,
elsewhere in this document these filter pairs are normally referred to
generically, e.g. as S1, implying S1a and/or S1b.
There may be some special circumstances where the differences in the
filters (e.g. between S1a and S1b - see
Figure 5.2 ) are scientifically
important. Note that observations with a specific, rather than generic
filter can be requested, see § 9.4 .
There are two sizes of filter, the large diameter survey (S) filters
and small diameter pointed (P) filters. Both survey and pointed
filters cover the full 
field
of view of the WFC, but the outer parts of the field are strongly
vignetted when the pointed filters are used.
S filters can also be used in the pointed
phase of the mission.
Of the remaining 2 filters, one is a narrow-band UV interference filter (UV in the table) used only in conjunction with the UV calibration system which permits in-flight monitoring of detector gain drifts and thermally-induced misalignment of the telescope axis. This filter may not be selected by the observer. The other is an ``opaque'' filter (OPQ in the table) designed to be opaque to photons, but to have a sensitivity to particle background very similar to the scientific filters. Originally it was intended that this filter be used to determine the particle component of the WFC background. In-orbit experience has demonstrated that the opaque filter suffers from an unexpected stray-light leak, seriously reducing its usefulness. It should therefore not be selected.
at 0.04 keV
m 
