4.2 Wide Field Camera 3 (WFC3)

The Wide Field Camera 3 (WFC3) was installed in May 2009 during Servicing Mission 4, replacing the highly accomplished WFPC2. This camera provides ultraviolet, optical, and near-infrared imaging through two independent channels. These channels cannot be operated simultaneously, although they can be operated sequentially within the same orbit. Their location in the HST field of view can be seen in Figure 2.2

The wide field ultraviolet-visible channel (WFC3/UVIS) is a high throughput, panchromatic camera with a field of view of 162" x 162", sensitive to wavelengths from 200 nm to 1000 nm. The total system throughput with this camera is 28% at 600 nm (see Figure 4.2). The detector is a pair of butted, 2K by 4K, thinned and backside-illuminated CCDs with an ultraviolet-optimized anti-reflective coating and 15 x 15 μm pixels. The plate scale is 0.04 arcsec/pixel. In addition to wavelength optimization, the primary differences between the WFC3 and ACS CCDs include a lower read noise (3 e- for WFC3, 5 e- for ACS) and a smaller interchip gap (465 μm rather than 750 μm). The UVIS channel provides 62 broad-, medium-, and narrow-band filters, and one grism.

The wide field high-throughput infrared channel (WFC3/IR) has a field of view of 136" x 123" over the wavelength range of 800 nm to 1700 nm. The total system throughput with this camera is 50% at 1600 nm (see Figure 4.2). The detector is a 1K by 1K HgCdTe Teledyne array with 18 x 18 μm pixels. The plate scale is 0.13 arcsec/pixel. The detector has 12 e- RMS read noise in a 16-sample non-destructive readout sequence, or 21 e- RMS read noise in the difference of two samples (a correlated double sample). The IR channel provides 15 broad-, medium-, and narrow-band filters and two grisms.

WFC3 has slitless imaging spectroscopic modes in both channels and five UVIS quad filters as described in Section 4.6.2.

Starting in Cycle 20, a major new capability was implement: adding a flash at the end of a UVIS exposure (post-flash). Post-flash greatly increases the detection of faint sources in low background observations where CTE losses would otherwise remove much or all of the flux from those sources. Most UVIS observers should consider using post-flash; it is useful for all UV, narrow-band, and relatively short medium- and broad-band exposures where the detection of faint sources is required. It should replace charge injection in essentially all circumstances. Users considering charge injection rather than post-flash should consult STScI via the helpdesk. Further information on CTE and post-flash is available in the WFC3 Instrument Handbook and at the WFC3 UVIS CTE Web page.

The observing technique of spatial scanning was introduced for WFC3 in Cycle 19. This mode can be used to turn stars into well-defined streaks on the detector or to spread a stellar spectrum perpendicular to its dispersion. It is useful for:

•	Observations requiring high temporal sampling and/or time resolution.

•	More efficient observation and higher S/N observations of exoplanet transits.

•	High precision relative astrometric observations.

•	Imaging and spectroscopy of brighter sources than previously possible.

See the WFC3 Instrument Handbook and WFC3 ISR 2012-08 for further discussion of spatial scanning.