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WFC3 Instrument Handbook for Cycle 24 |
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The optical design of WFC3 was driven by the need to provide a large field of view and high sensitivity over a broad wavelength range, excellent spatial resolution, and stable and accurate photometric performance. WFC3 features two independent imaging cameras: the UV/Visible channel (UVIS) and the near-infrared channel (IR). Figure 2.1 shows a schematic diagram of the instrument’s optical and mechanical layout.On-axis light coming from the HST optical telescope assembly (OTA) is intercepted by the flat 45° WFC3 pick-off mirror (POM) and is directed into the instrument. For IR observations, the channel select mechanism (CSM) then diverts the light into the IR channel; for UVIS observations, the CSM mirror is simply removed, which allows the light to enter the UVIS channel. Because of this design, only a single channel, either UVIS or IR, can be used at any one time, although it is possible to switch between them fairly quickly.Optical elements in each channel (anamorphic aspherical correctors) correct separately for the ~1/2 wave spherical aberration of the HST primary mirror. Both channels also have internal flat-field illumination sources.WFC3 uses two different types of detectors. The UVIS channel uses two butted 4096в2051 thinned, back-illuminated e2v Ltd. (formerly Marconi) CCD detectors to support imaging between 200 and 1000 nm. The IR channel uses a 1024в1024 Teledyne (formerly Rockwell Scientific) HgCdTe detector array, with the central 1014в1014 pixels useful for imaging, and covering the near-infrared between 800 and 1700 nm.Table 2.1: Characteristics of the two WFC3 channels.
f-ratio 0.0395в0.0395 162в162 1014в1014 0.135в0.121 136в123