Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

9.1 Summary

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Overheads are the times required to execute various instrumental functions that are over and above an actual exposure time. For instance, mechanisms take a finite time to move into place, and electronic components must be configured properly for use.

This chapter helps you determine the total number of orbits that you need to request in your Phase I observing proposal. This process involves compiling the overheads for individual exposures or sequences of exposures, packing the exposure plus overhead time into orbits, and adding up the total number of orbits required. This will most likely be an iterative process as you modify exposures or their order to efficiently use orbital visibilities.

The Phase I Call for Proposals includes 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 generic observatory overheads, the specific COS overheads, and several examples that illustrate how to calculate your orbit requirements for a Phase I proposal.

All overheads provided here are accurate as of the writing of this Handbook and reflect both the specifications of the COS instrument commanding and the results of actual Phase II runs of APT. These numbers may be used in conjunction with the values in the HST Primer to estimate the total number of orbits for your Phase I proposal. After your HST proposal is accepted you will be asked to submit a Phase II proposal to support scheduling of your approved observations. At that time you will use the APT scheduling software which will contain the most up-to-date COS overheads. Allowing sufficient time for overhead in your Phase I proposal is very important; additional time to cover unplanned or overlooked overhead will not be granted later.

Please note that the information provided here on overheads is illustrative and meant as an aid to approximation. Proposers are urged to use APT and its capabilities to derive accurate determinations of these times rather than the rougher numbers presented here.