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 moved out of the incoming beam's path, which allows the light to enter the UVIS channel. As a result 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, only one channel switch per visit is permitted to minimize the total number of CSM moves (see Section 10.3.1 for channel switching instrument overhead times).

WFC3 uses two different types of detectors. The UVIS channel contains 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 and covers the near-infrared between 800 and 1700 nm. Optical elements (anamorphic aspherical correctors) in each channel correct separately for the ~1/2 wave spherical aberration of the HST primary mirror. Both channels also have internal flat-field illumination sources.

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

 

In Figure 2.1, 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 WFC3 by the pick-off mirror. The CSM 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 Selectable Optical Filter Assembly (SOFA) and the IR Filter Select Mechanism (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.

 

Table 2.1: Characteristics of the two WFC3 channels.