next up previous contents



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 gif.

 fig5-1 figure380


Mirror type Wolter-Schwarzschild I
Mirror material Ni plated Aluminium
Reflective coating Au
Number of shells 3
Field of view tex2html_wrap_inline2122 diameter
Geometric area 456 cm tex2html_wrap_inline1870
Aperture diameter 576 mm
Focal length 525 mm
Focal plane scale factor tex2html_wrap_inline2126
High energy cutoff (10% of peak) 0.21 keV
Half energy width (on-axis) 1.7 tex2html_wrap_inline2128 at 0.04 keV
Table 5.1:   WFC Mirror parameters

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 tex2html_wrap_inline1870 with a common focal length of 525 mm. The grazing incidence angles chosen (typically tex2html_wrap_inline2132 ) allow the collecting area to be optimised whilst retaining a wide ( tex2html_wrap_inline2134 radius) circular field of view and a low energy reflectivity cut-off at 0.21 keV (60Å). The on-axis resolution is tex2html_wrap_inline2136 HEW, but the response degrades to tex2html_wrap_inline2138 HEW at tex2html_wrap_inline2134 off-axis due to inherent optical aberrations. Hence, the average resolution for the survey will be tex2html_wrap_inline2142 HEW. The mirror parameters are summarized in Table 5.1 gif.

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 gif gives details of the detectors.


Diameter 55 mm
Active diameter 45 mm
Radius of curvature 165 mm
Channel diameter 12.5 tex2html_wrap_inline1892 m
Channel pitch 15.0 tex2html_wrap_inline1892 m
Open area 63 %
Length-to-diameter ratio 120:1
Front MCP bias angle tex2html_wrap_inline2150
Rear MCP bias angle tex2html_wrap_inline2152
Photocathode (front MCP only) 14000 Å CsI
Table 5.2:   WFC Detector characteristics

The filter wheel assembly contains eight filters, these are listed in Table 5.3 gif. 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 gif) are scientifically important. Note that observations with a specific, rather than generic filter can be requested, see § 9.4 gif.

 fig5-2 figure421


Filter name/type Survey/Pointed Mean energy(eV) Bandpass(eV)
(at 10% of peak)
S1a: C/Lexan/B S+P 124 90-185
S1b: C/Lexan S+P 124 90-210
S2a: Be/Lexan S+P 90 62-111
S2b: Be/Lexan S+P 90 62-111
P1: Al/Lexan P 69 56-83
P2: Sn/Al P 20 17-24
OPQ: opaque (S+P) - -
UV: UV interference - - -
Table: 5.3   WFC filters

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 tex2html_wrap_inline2122 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.

next up previous contents

Michael Arida
Tue Jun 11 16:18:41 EDT 1996