2.1	Optical Design and Detectors

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 4096×2051 thinned, back-illuminated e2v Ltd. (formerly Marconi) CCD detectors to support imaging between 200 and 1000 nm. The IR channel uses a 1024×1024 Teledyne (formerly Rockwell Scientific) HgCdTe detector array, with the central 1014×1014 pixels useful for imaging, and covering the near-infrared between 800 and 1700 nm.

The primary characteristics of the two channels are summarized in Table 2.1.

Table 2.1: Characteristics of the two WFC3 channels.

Channel	f-ratio	Detector type	Spectral range (nm)	Detector pixel format	Pixel scale (arcsec)	Field of view (arcsec)
UVIS	31	CCD	200-1000	2×2051×4096	0.0395×0.0395	162×162
IR	11	HgCdTe	800-1700	1014×1014	0.135×0.121	136×123

Figure 2.1: Schematic optical layout of the WFC3 instrument. Note that for schematic simplicity, the incoming OTA beam and POM have been rotated into the plane of the optical diagram. The actual incoming OTA beam direction is into the page and then reflected by the POM into the instrument. Yellow indicates light from the OTA, which is sent into the camera by the pick-off mirror. The Channel Select Mechanism then either allows light to pass into the UVIS channel (blue path), or directs light into the IR channel (red path). Mechanisms and optics in both channels allow for focus and alignment, and correct for the OTA spherical aberration. Filters and grisms are contained in the UVIS SOFA and the IR FSM. The UVIS channel has a mechanical shutter, while the IR channel is shuttered electronically by the detector. Light is detected by either the UVIS CCDs or the IR focal-plane array. A separate subsystem provides flat- field illumination for both channels.