6.1 Overview of an Observing Program

6.1.1 General Observer (GO) Programs

Definitions (HST Orbits, Visibility Periods, and Visits)

HST GO observing time is counted in terms of orbits. Each 96 minute orbit contains a certain amount of useful time when the target can be observed, called the visibility period. The length and timing of the visibility period depends on the declination of the target and on whether there are any special scheduling constraints. Orbits are grouped into larger units called visits; a visit is a series of one or more exposures on a target, including the overheads, that will execute in one or more consecutive orbits.

Components of a Visit

The orbits in a visit generally contain the following components:

• Guide star acquisition (needed in the first orbit of a visit) or re-acquisition (needed in the subsequent orbits of a visit), to ensure that HST can maintain adequate pointing during each orbit. See Section 3.2 for details on guiding.
• Target acquisition. This is required if the target must be placed in an instrument aperture. Imaging observations (unless they are coronagraphic) generally do not require a target acquisition. See Section 5.2 for details on target acquisition strategies.
• Science exposures.
• Instrument overheads (e.g., the time required to set up the instrument and read out the data).
• Telescope repositioning overheads for small angle maneuvers such as pattern dithers and offsets. Note that it is generally not possible to observe multiple targets within a single HST orbit in two-gyro mode unless the separation of targets is small enough (<2') that the telescope can be repositioned with a small angle maneuver that does not break guide star tracking. Maneuvers that require a full guide star acquisition will be forced into a new orbit.
• Special calibration observations, which may be required if the accuracy provided by the standard calibrations is inadequate for the goals of the project (see Section 4.3 of the Call for Proposals).

Preparing your Program

To calculate the resources required for your GO program you must take the following steps:

1. Define the observations (instrument setup, number of exposures, exposure times, etc.) you wish to execute on each target. Use the Instrument Handbooks and the Exposure Time Calculator (ETC) tools that are available on the HST Instruments Web page as primary resources in this stage of your proposal preparation.
2. Group your observations into separate visits following the guidelines in Section 6.2.
3. Determine the visibility period of each target in your proposal (described in Section 6.3).
4. Compute the times required for guide star acquisitions, target acquisitions, instrument overheads, and telescope repositioning overheads (described in Section 6.4 and Appendix A). The observations for a Survey target, including overheads, must not exceed 48 minutes. See Section 3.2.3 of the Call for Proposals
5. Lay out all the exposure and overhead times for your program into visits (described in Section 6.5) and add up the number of orbits from each visit to obtain your total orbit request. Each visit must consist of an integer number of orbits. Partial orbits are not granted.

6.1.2 Snapshot Programs

In a Phase I Snapshot proposal, the PI specifies a requested number of targets, rather than a requested number of orbits. The exposure times and overhead times for Snapshot observations are calculated in similar fashion as for GO observations. The observations for a Snapshot target, including overheads, must not exceed 40 minutes. See Section 3.3 of the Call for Proposals for detailed policies and procedures regarding Snapshot observations.