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

34 Chapter 5: WFC3 Detector Characteristics and Performance
5.3.2 Subarrays
The default CCD readout mode is to read all pixels of both chips, including all
available overscan regions. It is also possible to restrict the readout to rectangular
subarray regions. Only data from the area within the subarray are stored in buffer
memory, and the rest of the image is discarded. The subarray can be chosen from
several pre-defined configurations.
UVIS subarray images contain no virtual overscan data and serial physical
overscan is present only if the defined subarray boundaries overlap the physical
overscan columns on either end of the chips. Thus all corner subarrays contain
physical overscan data, while centered subarrays do not. (Table 6.1).
Subarrays are discussed in detail in Section 6.4.4.
5.3.3 On-Chip Binning
The UVIS CCDs also provide an on-chip binning capability, in which several
adjacent pixels may be read out as a single pixel. The available choices are 2×2 and
3×3 on-chip binning. On-chip binning and subarrays can not be used simultaneously.
See Section 6.4.4 for details on the use of on-chip binning in WFC3/UVIS
observations. If on-chip binning is used, the overscan geometry is complicated by the
need to truncate “odd” pixels, and each half of a row must be considered separately.
As a result, depending on the binning mode, some science pixels adjacent to the
overscan region may be binned together with overscan data. Details are given at the
end of Section 6.7.2.
5.4 WFC3 CCD Characteristics and Performance
5.4.1 Quantum Efficiency
The quantum efficiencies (QEs) of the two WFC3 CCDs are plotted against
wavelength in Figure 5.2. Here the QE is defined as electrons yielded per incident
photon. The solid curves illustrate the QEs as measured at the Detector
Characterization Laboratory (DCL) at Goddard Space Flight Center, slightly corrected
downward by the TV3 ground tests. The plots demonstrate the high sensitivity of the
CCDs in the UV down to 200 nm. On the other hand, the peak QE at ~600 nm is less
than that of the ACS/WFC detectors which reach ~85% at their peaks. The QE
measurements were made with the detectors perpendicular to the incident light. As
installed in WFC3, the CCDs are tilted by 21 degrees with respect to the normal. The
nominal change in optical thickness is ~6%, but the QE variations, as measured at the
DCL on similar devices, turn out to be negligible.
The integrated system throughput of WFC3 depends on many factors including the
HST OTA, pickoff mirror, filter transmission functions, QE, etc. Based on ground