|WFC3 Instrument Handbook for Cycle 24|
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.First, from your science requirements, determine the desired wavelength(s) of observation. Those requirements may include considerations of the spectral energy distribution (SED) of the target, or the required angular resolution, which also varies as a function of wavelength. Typically, if the wavelength of observation is less than 900 nm, then the WFC3 UVIS channel will be used; or if the wavelength is greater than 900 nm, then the WFC3 IR channel will be used. Your program may involve use of both channels.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.1 for detailed filter passbands), or grisms (see Chapter 8).To match continuum features, wide-, medium-, and/or narrow-band filters may be selected, presenting the possibility of a trade-off between detector signal and spectral resolution. Note that the UVIS quad filters limit the field of view to about one sixth of the full field.4.2.2 What Exposure Times?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.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.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.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 a problem with the UVIS channel.For UVIS observations, it will almost always be desirable to use multiple exposures to remove cosmic-ray impacts. Dithering is generally preferable to CR-SPLIT for the reasons discussed above under Dithering and Mosaicking. For observations with the UVIS channel of faint targets on low levels of background emission, the effects of charge transfer efficiency (CTE) during readout of the detector must be considered (see Sections 5.4.11 and 6.9). Post-flash, implemented for Cycle 20, can greatly reduce CTE losses.For observations with the IR channel you must choose a readout method from the 12 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.Next, from considerations of the target's angular size and structure, and of data volume, you should determine the WFC3 aperture or subarray you will use. The available UVIS apertures and subarrays are presented in Chapter 6, and those for the IR channel in Chapter 7.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), 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).4.2.4 What Overheads and How Many HST Orbits?Fourth, determine the overhead times required, in addition to the exposure times, in order to operate the spacecraft and the camera (see Chapter 10).The spacecraft overhead includes the time needed for guide-star acquisition or re-acquisition at the beginning of each orbit. The camera overheads include time needed to change filters, change between the UVIS and IR channels, read out the exposure to the WFC3 data buffer, and transfer the images from the buffer to the HST science data storage. Note that overheads are especially severe for sequences of short exposures, but these can sometimes be mitigated by using small subarrays or by alternating short and long exposures. For Phase II proposals, the APT provides tools for detailed modeling of complete observation sequences.Finally, you will add the overhead times to the exposure times to find the total time needed for your program, which is what you will request in your Phase I proposal.This total time is expressed as the (integer) number of HST orbits required to obtain the observations.Most observers will not need to worry about special calibration observations. As a result of ground based and SMOV testing, WFC3 is fairly well-characterized as described in this Handbook and in more detail in WFC3 Instrument Science Reports. Instrument characterization and calibration will be maintained and improved as part of the ongoing calibration program conducted by STScI (Appendix E).The main reasons an observer would need to consider special observations are situations where a program requires greater precision than is provided through the standard calibration program. These additional observations must be justified in your Phase I proposal submission, and the orbits required to carry out the special observations must be included in the overall orbit allocation requested. Proposers are advised to discuss their need for special observations with the helpdesk.Having determined the content of the science and supporting observations necessary to achieve your scientific objectives, you must finally determine the total amount of HST time to carry out those activities by including the appropriate amount of time for spacecraft and instrument overheads.Detailed procedures for determining the total amount of time to request in your Phase I proposal are presented in Chapter 10.