WFC3 Data Handbook V. 4.0

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1.1	Instrument Overview

Wide Field Camera 3 (WFC3) is a fourth-generation imaging instrument installed in 2009. It replaced the extraordinarily successful WFPC2, thereby ensuring and enhancing the imaging capability of HST in the remaining years of its observing lifetime. WFC3 is the only HST instrument developed as a facility instrument by the HST Project. WFC3 provides HST with high-sensitivity, high-resolution, wide-field survey capability covering a broad wavelength range, from the near-UV at 200 nm to the near-IR at 1700 nm. WFC3 comprises two channels, each optimized for a specific goal:

•	Ultraviolet-Visible channel (UVIS): 162 x 162 arcsecond field of view from 200-1000 nm with a plate scale of 0.040 arcsec/pixel and a focal ratio of f/31.

•	Infrared channel (IR): 136 x 123 arcsecond field of view from 800-1700 nm with a plate scale of 0.13 arcsec/pixel and a focal ratio of f/11.

In addition to these capabilities, WFC3 provides:

•	62 wide-, medium-, and narrow-band filters in the UVIS channel

•	15 wide-, medium-, and narrow-band filters in the IR channel

•	3 grisms: 1 in the UVIS channel and 2 in the IR channel

WFC3 occupies WFPC2's spot in HST's radial scientific-instrument bay, where it obtains on-axis direct images. Light coming from the HST Optical Telescope Assembly (OTA) is intercepted by the flat 45 degree WFC3 pick-off mirror (POM) and directed into the instrument. A channel-select mechanism (CSM) inside WFC3 then diverts the light to the IR channel via a fold mirror, or allows the light to enter the UVIS channel uninterrupted. Because of this design, only a single channel, either UVIS or IR, can be used at any one time. 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. Figure 1.1 shows a schematic diagram of the instrument's optical and mechanical layout. The main characteristics of each channel are summarized in the following sections. For a technical description of the instrument's properties, performance, operations, and calibration, please refer to the WFC3 Instrument Handbook.

Figure 1.1: Schematic optical layout of the WFC3 instrument

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 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 selection 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.