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

26 Chapter 4: Designing a Phase I WFC3 Proposal
the background and detector sources. These sources include zodiacal light, detector
dark current, and stray light from both Earth and bright targets in the field of view.
Having determined the basic exposure time necessary to achieve the required S/N,
you will in most cases find it necessary to achieve that total exposure time through a
sequence of shorter exposures. For instance, if the exposure time is greater than the
maximum orbital target visibility, it will be necessary to obtain a sequence of
exposures. UVIS exposures exceeding 3,600 s require more than one exposure as do
IR exposures greater than 2,800 s (see Chapter 6 and Chapter 7 for a fuller discussion).
Additional reasons to structure the total exposure time are described in the
following paragraphs, as well as considerations peculiar to each of the two WFC3
channels.
Dithering and Mosaicking
A sequence of exposures obtained in a dither pattern of HST pointings will often be
used to reduce the noise from flat-field calibration error, cosmic rays, and residual
images. Including sub-pixel displacements in the dither pattern will allow better
sampling of the point-spread function (PSF). You may design and specify a dither
pattern, or use one of the pre-defined patterns already designed to sub-sample the PSF,
to cover the UVIS inter-chip gap, or to mosaic a large field. The pre-defined sequences
and information on designing your own patterns, are presented in Appendix C of this
Handbook and in the Phase II Proposal Instructions.
Bright Targets
For bright targets, a sequence of shorter exposures may be needed to avoid entering
the non-linear or saturation regimes of the detectors (see Chapters 5, 6, and 7).
Bright objects do not cause safety concerns for either UVIS or IR observations
with WFC3. Image persistence can be a concern for IR observations (as discussed in
Section 7.9.4 and Appendix D) but is not generally a problem with the UVIS channel.
UVIS Exposures
For UVIS observations, it will almost always also be desirable to use a sequence of
exposures, in order to remove cosmic-ray impacts. For observations with the UVIS
channel of faint targets, the effects of charge-transfer efficiency (CTE) during readout
of the detector must be considered (see Chapters 5 and 6). Charge injection for such
images will mitigate the non-ideal CTE, and will be offered in Cycle 19.
IR Exposures
For observations with the IR channel you must choose a readout method from the
11 available sample sequences, each of which may comprise from 1 to 15
non-destructive readouts. These include RAPID (linear), SPARS (linear), and STEP
(linear-log-linear) sequences (see Chapter 7). The exposure time is dictated by the
sequence chosen. The ability to remove cosmic-ray impacts will depend upon the
sequence chosen.