An HST program is a set of exposures specified so as to achieve one or more scientific objectives. We can break down the development of a WFC3 observing program, imaging and/or spectroscopic, into a six-step process. Often there is not a unique way in which to achieve the scientific objectives, and you must assess the trade-offs and feasibilities of multiple approaches. Furthermore, you will wish to use
HST and WFC3 efficiently, in order to obtain as much science within as small an orbit allocation as possible. Therefore, you may need to iterate these steps in order to achieve a final feasible program that is also optimal.
In this chapter we introduce issues that you may need to consider in designing your observations. Later chapters in this Handbook will present detailed information for your use. These six steps, and the considerations they entail, are described in the following subsections.
The angular resolution, field of view, and sensitivity of the two channels differ appreciably, and may also influence your selection of the WFC3 channel(s) to use (see Chapter 2 for an overview of the UVIS and IR channels). Features of interest in the target's SED can be matched to the spectral resolution of the observation by selecting appropriate filters (see
Chapter 6 for the UVIS channel,
Chapter 7 for the IR channel, and
Appendix A:WFC3 Filter Throughputs for detailed filter passbands), or grisms (see
Chapter 8).
Second, you should determine the exposure time and exposure sequences needed to achieve the required signal-to-noise (S/N) with the chosen filter(s) or grism(s). A full discussion of exposure time calculation is presented in Chapter 9, but, as mentioned in that chapter, in most cases you will use the online
Exposure Time Calculator (ETC). The S/N depends upon the target's incident flux and the noise from 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.
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:Dithering and Mosaicking of this
Handbook and in the Phase II Proposal Instructions.
In some cases, correct placement of an extended target within an aperture may require you to specify a special HST pointing and possibly the orientation of the field of view (which is determined by the spacecraft roll angle). Additional considerations may include detector imperfections such as the UVIS inter-chip gap (
Chapter 5), diffraction spikes (Chapters
6 &
7), filter ghost images (see Chapters
6 &
7), detector saturation (i.e., for bleeding in a UVIS image along a detector column;
Chapter 5), detector charge transfer (
Chapter 5), distortion of the image (
Appendix B:Geometric Distortion), or dispersion direction of the grism (see
Chapter 8). However, most of these only need to be considered at the Phase II stage, unless they affect the number of orbits needed for the proposal.
Note that selection of a WFC3 aperture without specifying further constraints implicitly specifies: (1) the full image will be read out; (2) the target coordinates will be placed at a default location on the detector (see Chapters 6 &
7; generally the target will be placed at the center of the chosen field of view); and (3) the telescope roll angle will be unspecified, as it will depend on the date the exposure is executed. You may override any of these defaults, however.
You can reduce the size of the image read out and thus the volume of data obtained by selecting a subarray. For the UVIS detector, on-chip binning of the pixels will also reduce the data volume, but at the expense of angular resolution (see Chapters 5 &
6). Reducing the data volume will reduce the overhead to read out and transfer images, which may be desirable in order to allow more images of the target of interest to be obtained during an
HST orbit. During Phase II preparation, the location of the target can be specified with the POS TARG Special Requirement and the rotation of the image can be specified with the ORIENT Special Requirement (see Chapters
6 &
7).
