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Filter name/type FOV diameter Mean energy (eV) Bandpass (eV)
(degrees) (at 10% of peak)
S1a (C/Lexan/B) 5 124 90-210
S1b (C/Lexan/B) 5 124 90-210
S2a (Be/Lexan) 5 90 62-111
S2b (Be/Lexan) 5 90 62-111
P1 (Al/Lexan) 2.5 69 56-83
P2 (Sn/Al) 2.5 20 17-24
OPQ (Opaque) 2.5 - -
UV (UV interference) 2.5 - -
Table 6.3: WFC filters

The filter wheel assembly contains eight filters, these are listed in Tab. 6.3.   

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 EUV 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. There are two sizes of filter, large diameter ( tex2html_wrap_inline17891 field of view, ``S'') for the survey and pointed phases, and small diameter ( tex2html_wrap_inline17893 field of view, ``P'') for pointed observations only. The S2a filter was damaged in 1991 January and has not been used for the remainder 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.     The other is an ``Opaque'' filter (``OPQ'' in the table) which is opaque to photons, but has a sensitivity to particle background which is very similar to the scientific filters.  Originally it was intended that this filter be used in 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.    

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