Hubble Space Telescope Primer for Cycle 17

6.2 HST Visits

6.2.1 Defining New Visits and Optimizing Scheduling Efficiency and Flexibility

Guidelines and Rules:

The following guidelines were put in place to ensure scheduling efficiency and flexibility, and to maximize the number of scheduling opportunities during the HST observing cycle. We recommend that:

• Visits should not exceed five (5) orbits. For health and safety reasons, the STIS MAMAs in particular cannot be operated in or around SAA passages, so the five orbit duration limit is strictly enforced on such visits. The ACS/SBC has the same five orbit maximum duration.
• Exposures should be ordered and grouped such that instrument overheads occur between orbital viewing periods in multi-orbit visits.
• Changes in HST pointing within an orbit should not exceed ~2 arc minutes. This can be due to an explicit change in target position (e.g. POS TARG, pattern, aperture change) or the use of a new target. Changes larger than 2 arc minutes introduce major slews which may be accommodated but, only if science goals dictate and conditions allow it.

A new visit is required if any of the following conditions occur:

1. a change in HST pointing of greater than ~1 degree.
2. the interval of time between repeated or periodic exposures creates an empty visibility period (an orbit with no exposures).
3. there is a required change in telescope orientation between observations (e.g., for NICMOS coronagraphic observations, or for STIS long-slit spectra along different position angles on the sky).

A more complete explanation of the rationale behind these guidelines and rules can be found at

http://www.stsci.edu/hst/programs/recommendations

The practical implementation of these guidelines is dictated by the details of the telescope and instrument operating characteristics. Proposers should use the Phase I documentation and proposal tools to gain insight into how well a proposed observing scenario satisfies each of the guidelines.

In general, the rule of thumb is that "smaller is better". Thus, smaller visit durations, target separations, and instrument configurations are better, where "better" refers to telescope scheduling efficiency and flexibility. STScI will work with observers (in Phase II) to find the best observing strategy that satisfies the science goals while following these guidelines as closely as possible.

6.2.2 Instrument Specific Limitations on Visits

For all science instruments, there are instrument-specific restrictions on the definition of a visit.

ACS: Data Volume Constraints

If ACS data are taken at the highest possible rate (~ 5 WFC images per orbit) for several consecutive orbits, it is possible to accumulate data faster than it can be transmitted to the ground, even when using both HST data transmitters. High data volume proposals will be reviewed and on some occasions, users may be requested to divide the proposal into different visits or consider using subarrays. Users can achieve higher frame rates by using subarrays, at the expense of having a smaller field-of-view; see the ACS Instrument Handbook for details.

FGS: Astrometry

For astrometric observations using FGS1R, each individual set (consisting of target object and reference objects) may be contained in one visit if there is no telescope motion made during the sequence.

Coronagraphy

We anticipate that most ACS, STIS, or NICMOS coronagraphic observations will be single visits using the full orbit for science observations.

Analysis of past coronagraphic data has shown that there could be some advantage (i.e., a cleaner PSF subtraction under some circumstances) when obtaining two images of the same target within the same orbit with a roll of the telescope between observations. Executing this roll will require several minutes, including acquisition of new guide stars and the re-acquisition of the target.

Proposals requesting two coronagraphic observations at different roll angles in the same orbit will have the following requirements:

Each ACQ and its corresponding science exposures must be scheduled as a separate visit.

Each visit must not exceed 22 minutes, including guide star acquisition, ACQ, exposure time and overhead.

No more than two ACQs within one orbit will be allowed. The effectiveness of the roll-within-an-orbit technique has been shown to depend heavily on the attitudes of the telescope preceding the coronagraphic observation. Thus, using the technique is not a guarantee of cleaner PSF subtraction.

As an extra insurance policy, coronagraphic observers may want to consider adding an extra orbit for each new pointing. Thermal changes in the telescope are likely to be significantly smaller in the second and subsequent orbits on a target than they are in the first orbit.

Coronagraphic observations requiring particular telescope orientations (e.g., positioning a companion or disk between diffraction spikes) are time-critical and must be described in the `Special Requirements' section of a Phase I proposal (see Section 9.3 of the Call for Proposals).

STScI will provide standard calibration reference files, flat fields, and darks, which will be available for calibration purposes. Contemporary reference files in support of coronagraphic observations are not solicited or normally approved for GO programs, but coronagraphic observers who can justify the need for contemporary calibration observations must include the additional orbit request in the Phase I proposal. Acquisition of bright targets for which an onboard ACQ with NICMOS will not be feasible requires the observer to obtain flat field observations to locate the coronagraphic hole. This implies adding one or more orbits to the total time requested. All calibration data regardless of the program are automatically made public.

STIS: CCD and MAMA Observations in the Same Visit

In general, STIS programs that contain both CCD and MAMA science observations (excluding target acquisitions) must be split into separate CCD and MAMA visits. Exceptions to this rule may be allowed if one of the following two conditions is met:

• There is less than 30 minutes of science observing time (including overheads and target acquisition) using the CCD at a single target position.
• The target is observed for only one orbit.

If you believe your science requires CCD and MAMA science exposures in the same visit (e.g., for variability monitoring programs), you must explain this in the Special Requirements section of a Phase I Proposal.