|ACS Instrument Handbook for Cycle 25|
8.1 After you establish a set of scientific exposures, as well as any additional target acquisition or calibration exposures required for your program, you are ready to determine the total number of orbits to request. Generally, this straightforward exercise involves compiling the overheads for individual exposures, packing exposure plus overhead time into individual orbits, and tallying up the results to determine the total number of orbits needed. It may be an iterative process as you refine your exposures to better use the orbit visibility times.The Phase I Call for Proposals includes instructions that provide information on the Observatory policies and practices with respect to orbit time requests. The HST Primer provides specific advice on orbit determination. Below, we provide a summary of the ACS specific overheads, and give several examples that illustrate how to calculate your orbit requirements for a Phase I Proposal.