3.3 Comparison of WFC3 with Other HST Imaging Instruments

3.3.1 Wavelength Coverage

The WFC3 UVIS channel is similar in design to the Wide Field Channel (WFC) of the ACS. There are, however, a few differences. While ACS/WFC is unable to detect wavelengths shorter than about 370 nm (i.e., shortward of the B band), WFC3/UVIS has excellent sensitivity extending down to 200 nm. The design trade-offs adopted to achieve this extended UV wavelength coverage (primarily the CCD coating and the use of aluminum coatings for the reflective optics) lead to a reduced sensitivity of WFC3 at longer optical wavelengths compared to that of ACS/WFC. WFC3/UVIS has no sensitivity in the far-UV region below 200 nm. The far-UV is covered by three MAMA detectors in ACS and STIS.

3.3.2 Field of View

Figure 3.1 illustrates the fields of view, at the same scale, for all of the HST imaging instruments currently available on HST. Note that the pixel scale of the WFC3 UVIS channel is 20% finer in comparison to the ACS/WFC, obtained at the cost of covering only about 64% of the area of the ACS field of view.

3.3.3 Detector Performance

Table 3.2 summarizes the on-orbit measurements of read-out noise and dark current for the WFC3 detectors, and compares them with the parameters for the other currently available HST imaging detectors. Chapter 5 gives more detailed information about the detectors in both channels. Chapter 9 discusses sensitivities, limiting magnitudes, and exposure times.

Table 3.2: Characteristics of HST CCD and HgCdTe imaging detectors currently available.

¹ WFC3/IR read noise is for a 16-read linear fit. WFC3/IR double sampling read noise is 20.2–21.4 e^–.

3.3.4 System Throughputs and Discovery Efficiencies

Figure 3.2 plots the measured on-orbit system throughputs of the two WFC3 channels as functions of wavelength, compared to those of ACS, NICMOS, and WFPC2. These curves include the throughput of the OTA, all of the optical elements of the instruments themselves, and the sensitivities of the detectors. Throughputs were calculated at the central wavelength (the “pivot wavelength”; see footnote 3 in Table 6.2) of each wide-band filter of each instrument.

As Figure 3.2 shows, WFC3 offers a unique combination of high sensitivity and wide spectral coverage ranging from the UV to the near-IR. WFC3 extends and complements, over a large field of view, the optical performance of ACS/WFC at wavelengths shorter than ~400 nm and longer than 1000 nm. The good degree of functional redundancy with ACS will help ensure that the unique scientific capabilities of HST, at optical wavelengths, will remain available until the end of its mission.

Another quantity that is useful when comparing different instruments, especially in the context of wide-angle surveys, is the “discovery efficiency,” defined as system throughput times area of the field of view as projected onto the sky. In Figure 3.3 we plot the discovery efficiencies of the HST imaging instruments, again vs. wavelength. Note that the y-axis is now logarithmic. This figure dramatically illustrates the enormous gains that WFC3 offers, compared to current HST instruments, both in the optical/UV below 400 nm, and in the near-IR.

Finally, we present WFC3’s strengths by including detector noise and thus showing how its efficiency, wide wavelength coverage, and large field of view apply to general problems: the limiting point-source magnitude reached in 10 hours of observing time (Figure 3.4); and the time needed to survey a sky area about 9 times larger than the Hubble Ultra Deep Field, to a limiting ABMAG of 26 (Figure 3.5).