HST Call for Proposals and HST Primer for Cycle 24

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6.2	HST Visits

6.2.1 Defining New Visits and Optimizing Scheduling Efficiency and Flexibility

6.2.2 Instrument Specific Limitations on Visits

ACS and WFC3: Data Volume Constraints

FGS: Astrometry

Coronagraphy

STIS: CCD and MAMA Observations in the Same Visit

6.2.1 Defining New Visits and Optimizing Scheduling Efficiency and Flexibility

Guidelines and Rules

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

1.	For health and safety reasons, observations using the STIS MAMAs and ACS/SBC should not exceed five orbits.

2.	Exposures should be ordered and grouped such that instrument overheads occur between orbital visibility periods in multi-orbit visits.

3.

Changes in HST pointing within an orbit should not exceed ~2 arcminutes. This can be due to an explicit change in target position (e.g., POS TARG, dither pattern, aperture change) or the use of a new target. Changes larger than 2 arcminutes introduce major slews which may be accommodated 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°.

2.	The interval of time between repeated or periodic exposures creates an empty orbital visibility period (an orbit with no exposures).

3.	There is a required change in telescope orientation between observations (e.g., 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 and WFC3: Data Volume Constraints

If full frame ACS or WFC3 data are taken at the highest possible rate (~6 ACS/WFC or WFC3 images per nominal orbit, or ~12 images per CVZ orbit) for several consecutive orbits, it is possible to accumulate data faster than it can be transmitted to the ground. 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 or WFC3 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

Most STIS coronagraphic observations will likely be single visits that use the full orbit for science observations.

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

•	Each acquisition (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 attitude of the telescope preceding the coronagraphic observation. Thus, using this technique does not guarantee 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 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 such as flat fields and darks 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. All calibration data (regardless of the program) are automatically made public.

STIS: CCD and MAMA Observations in the Same Visit

In order to preserve SAA-free orbits for MAMA observations, 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 three conditions is met:

•	There is a well justified scientific need for interspersed MAMA and CCD observations.

•	There is less than 30 minutes of science observing time (including overheads) using the CCD.

•	The target is observed for only one orbit.