|WFC3 Instrument Handbook for Cycle 25|
10.1 This chapter provides information for Phase I proposers who plan to use WFC3 for some or all of their planned HST observations. Because your Phase I proposal must state a specific integer number of HST orbits that are requested for your program, it is important that this number be as accurate as possible.After you establish a set of scientific exposures, as well as any additional calibration exposures required for your program, you are ready to determine the total number of orbits to request in your Phase I proposal. The time requested should include not only the total exposure time, but also the additional time for the observatory and instrument operations needed to support those exposures. Those operations usually include acquiring (and possibly re-acquiring) guide stars, configuring WFC3 in preparation for an exposure, transferring data, and possibly repositioning the target in the focal plane between exposures.It will often be necessary or desirable to achieve the total exposure time through a sequence of shorter exposures. For example, an exposure totalling several hours will be interrupted by target occultations. Moreover, UVIS exposures will almost always be obtained as a sequence of shorter exposures, in order to remove cosmic-ray hits. For your Phase I proposal, you should plan the sequences of exposures and overhead activities for each of the requested orbits. An overview of observation planning is given in Chapter 4.Generally, you will need to compile the overheads for your individual exposures, having packed an integer number of exposures and their supporting activities into individual orbits. Some activities may be executed while the target is occulted by the Earth, allowing more efficient utilization of an orbit. Finally, you will tally the exposure times and resulting overhead times required during visibility of the target in order to determine the total number of orbits needed. This may be an iterative process as you refine your exposures to better use the targets’ orbit visibility (for non-CVZ targets).The Phase I Call for Proposals includes instructions on observatory policies and practices regarding orbit time requests; see:The HST Primer provides additional specific advice on how to determine the number of required orbits for your scientific program; see:In the following sections, we provide a summary of the WFC3-specific overheads and give several examples that illustrate how to calculate your orbit request for a Phase I proposal.The overheads presented below are approximate. These overhead times are to be used, in conjunction with the exposure times you determine and the instructions in the HST Primer, in order to estimate the total number of orbits for your Phase I proposal. If your HST proposal is accepted, you will then develop and submit a Phase II proposal to support the actual scheduling of your approved observations. At that time you will use the APT scheduling software (which employs more precise values for the various overheads) to craft exposure sequences that match orbital target visibility intervals and your allocation of orbits. Therefore, requesting sufficient time in your Phase I proposal for overhead operations is important; additional time to cover unplanned overhead will not be granted later.