Wide Field Camera 3 Instrument Handbook for Cycle 19 - Space ...

20 Chapter 3: Choosing the Optimum HST Instrument
samplings offered by the NICMOS/NIC1 and NIC2 channels. Further, the WFC3/IR
channel covers 45 times the area of the NICMOS/NIC2 channel with slightly coarser
sampling, but arguable better photometric precision.
Table 3.1 presents a comparison of the wavelength coverage, pixel scale, and field
of view of WFC3 and of the other HST imaging instruments that are currently
available.
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. The
WFC3/IR dark current includes the instrument thermal background.
Detector
Read-out noise
(e – rms)
Dark current
(e – /pix/s)
Mean well Depth
(e – )
WFC3/UVIS 3.1–3.2 0.0008 75,000-80,000
ACS/WFC 4.8 0.0062 84,700
STIS/CCD
5.4 (gain=1),
7.7 (gain=4)
0.009 114,000
WFC3/IR ~12.0 1
0.05 77,900
NICMOS/NIC1 26 0.3 145,000
NICMOS/NIC2 26 0.3 152,000
NICMOS/NIC3 29 0.3 213,000
1. WFC3/IR read noise is for a 16-read linear fit; NICMOS read noise is
for a pair of reads (double sampling). 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 to 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