7.3	IR Channel Characteristics

The WFC3 IR channel has been optimized for observing over the wavelength range 800-1700 nm. All the IR reflective optics (except for the WFC3 pick-off mirror) are coated with a silver layer for maximum IR throughput.

A schematic mechanical diagram showing both channels of WFC3 is given in Figure 2.1. Components of the IR channel include: the Channel Select Mechanism (CSM), which directs on-axis light from the HST OTA to the IR channel; a flat folding mirror; a two-mirror mechanism for providing focus and alignment adjustments; the Refractive Corrector Plate (RCP), which applies the spherical-aberration correction; the IR filter wheel (FSM or Filter Selection Mechanism); and finally the HgCdTe IR detector package.

The WFC3 IR detector is a HgCdTe 1024×1024 array, with 18 micron pixels, bonded to a silicon multiplexer, with 1014×1014 pixels sensitive to incoming light. It is a direct descendant of the NICMOS 256×256 and Hawaii-1 1024×1024 arrays (more details on the detector are given in Chapter 5).