9.1 Introduction

Once you have established the series of target-acquisition, scientific, and (if necessary) calibration exposures required for your program, you are ready to determine the total number of orbits to request. Generally, this is a straightforward exercise: compile the overheads on the individual exposures, assign the exposures to orbits, and tally the results to determine your orbit request. In some cases, it may be an iterative process, as you refine your observing plan to use each orbit most efficiently. This chapter provides simple tools to help you perform these calculations.

Once your proposal has been accepted you will return to this chapter to construct your Phase II proposal, which is the detailed observing plan that will eventually be transmitted to the telescope. Our goal is to help you schedule your observations as efficiently as possible.

Table 9.1: Phase I Estimates of Observatory and Instrument Overheads.

Acquisition or ExposureOverhead Time (minutes)Notes

Guide-star acquisition


First orbit of each visit

Guide-star re-acquisition


Each subsequent orbit



Typical imaging acquisition



Imaging or dispersed light, NUV or FUV



Typical dispersed light acquisition, NUV or FUV

Science (Imaging or Dispersed Light)


First exposure in series


Each subsequent exposure in series


Add 1 minute for each instrument change (except to increment FP-POS)

9.1.1 Phase I Proposal

The HST Primer provides simple guidelines for estimating the overheads associated with a COS observation. These guidelines are summarized in Table 9.1. These numbers are estimates only, and will usually overestimate the amount of time needed for overheads. However, additional observing time will not be granted in Phase II if the Phase I overheads were underestimated, so it is important that overhead times not be underestimated.

Each orbit must begin with a guide-star acquisition, which takes 6 minutes on the first orbit of a visit and 4 minutes on subsequent orbits. Next, the target must be acquired via an ACQ/SEARCH (7 min) and/or centered in the aperture via an ACQ/IMAGE (3 min) or a pair of ACQ/PEAKXD and ACQ/PEAKD exposures (7 min for both). The first science exposure requires 5 minutes of overhead. Subsequent identical exposures incur 2 minutes of overhead each. Add 1 minute for each instrument change (e.g., new grating or central wavelength; incrementing the FP-POS takes only 3 s, if ordering is efficient—see Table 9.5). If the same target is observed on contiguous orbits, a target acquisition is not required on the second and subsequent orbits.

These simple rules are remarkably successful at reproducing the total time required for a COS observation. To demonstrate, we list in Section 9.7 both the Phase I times and the final times predicted by APT for a series of observing scenarios. These rules assume that acquisition exposure times are on the order of 20 s. If your targets are extremely faint, you must increase the length of the acquisition exposures accordingly (Section 9.4). Finally, note that some instrument changes, such as turning a detector segment on or off, take considerably longer than 1 minute (Section 9.5).

Allowing sufficient time for overheads in your Phase I proposal is important; additional time to cover unplanned or overlooked overheads will not be granted later. Do not underestimate the amount of time that will be needed for overheads.

9.1.2 Phase II Proposal

Once your proposal is approved, you will be responsible for building the observing sequences that will be executed by the telescope. The APT (Astronomer’s Proposal Tool) scheduling software is used to prepare the Phase II; it automatically incorporates the appropriate overheads into your observing plan. While all COS overheads are automatically scheduled by APT, it is useful to understand where they come from. To that end, this chapter discusses the various observatory and instrument overheads in some detail, and Section 9.7 provides observing scenarios as examples. Note that, when this chapter and the APT disagree, the APT overheads are the definitive values.

Accounting properly for all the overheads involved in an observation can be complicated. The information provided here is meant only to be illustrative. Proposers are urged to use APT to derive a complete and accurate determination of overhead times.