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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 .
|Mirror type||Wolter-Schwarzschild I|
|Mirror material||Ni plated Aluminium|
|Number of shells||3|
|Field of view||diameter|
|Geometric area||456 cm|
|Aperture diameter||576 mm|
|Focal length||525 mm|
|Focal plane scale factor|
|High energy cutoff (10% of peak)||0.21 keV|
|Half energy width (on-axis)||1.7 at 0.04 keV|
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.
|Active diameter||45 mm|
|Radius of curvature||165 mm|
|Channel diameter||12.5 m|
|Channel pitch||15.0 m|
|Open area||63 %|
|Front MCP bias angle|
|Rear MCP bias angle|
|Photocathode (front MCP only)||14000 Å CsI|
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 .
|Filter name/type||Survey/Pointed||Mean energy(eV)||Bandpass(eV)|
|(at 10% of peak)|
|UV: UV interference||-||-||-|
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.