HST Primer: Orbit Calculation Overview

An overview in determining the observation time with HST.

Overview of Observer Programs

Definitions (HST Orbits, Orbital 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 orbital visibility period. The length and timing of the visibility period depend 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 overheads, that will execute in one or more consecutive orbits.

While exposures will execute in the order specified by the user, they are not guaranteed to be contiguous. A sequence of exposures may be scheduled across multiple orbits even if APT's Orbit Planner places them in a single orbit. If a program requires that the exposures must occur without gaps in time coverage, the exposures must be placed in a Sequence Non-Int container, and this special requirement must be justified in the Phase I proposal.

Components of a Visit

The orbits in a visit generally contain the following components:

  • Guide star acquisition and re-acquisition of a target are required to ensure that HST can maintain adequate pointing during each orbit. Guide star acquisition is needed for the first orbit in a visit, and re-acquisition is needed for subsequent orbits in that same visit. See HST Primer: Optical Performance, Guiding Performance, and Observing Efficiency
  • Target acquisition is needed if the target must be placed in an instrument aperture. Imaging observations (unless they are coronagraphic) generally do not require a target acquisition. See HST Primer: Observing Considerations
  • The 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. 
  • Special calibration observations which may be required if the accuracy provided by the standard calibrations is inadequate for the goals of the project (see HST Observation Types).

Preparing a Phase I Program

Use the following steps to calculate the resources required for a GO program:

Snapshot Programs

In a Phase I Snapshot proposal, the PI specifies a requested number of targets, rather than a requested number of orbits, and for reasons of temperature-induced elevated dark current rates with increased time of operation, any ACS/SBC observation needing to be longer than ~2 orbits at a time should consider using the special SBC-LODARK aperture if the target of interest is small enough to fit within that corner of the detector where the dark rate remains constant at all temperatures. There is also a small cluster of bad pixels in that area of the SBC which should be avoided. See ACS ISR 2018-07 for more.

The exposure times and overhead times for Snapshot observations are calculated in a similar fashion as for GO observations. The observations for a Snapshot target, including all overheads and the final data dump, are typically 45 minutes or less. See UIR-2014-001 for guidance on orbit length for SNAP programs.

Defining New Visits and Optimizing Scheduling Efficiency and Flexibility

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 to 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 complete explanation of the rationale behind these guidelines and rules can be found at http://www.stsci.edu/hst/proposing/phase-ii/visit-size-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.

Constructing the Program

The final step is to efficiently pack all science exposures and overheads into the orbital visibility period of each orbit for all visits. For particularly complex programs, the APT Phase II orbit planner can be used for assessing the orbit layout. Please contact the HST Help Desk at http://hsthelp.stsci.edu for assistance.

When placing observations into orbits and visits, note that exposures cannot be paused across orbits. For instance, if there are 20 minutes left in an orbit, only an exposure that takes 20 minutes or less (including overhead) can be placed in it. For a 30 minute exposure, all of it can be placed in the next orbit, or a 20-minute exposure can be placed at the end of the first orbit, and a second 10-minute exposure in the next orbit.

As an example, The table below shows the layout of a visit with two orbits of spectroscopic observations that require a target acquisition, but no SAMs and no special calibration observations. For simplicity, overheads are shown to occur after each exposure; in reality, some overheads occur before an exposure (e.g., a filter change) while others appear afterward (e.g., readout time).

Orbit 1

Guide star acquisition

Target acquisition


Science exposure


Earth occultation

Orbit 2

Guide star reacquisition

Science exposure


Science exposure


Earth occultation

More detailed examples for each of the SIs are given in HST Primer: Orbit Calculation Examples. Those examples are for common, simple uses of the instruments. For more complex examples and observing strategies, please consult the instrument handbooks.

Coordinated Parallel Observations

For a program with coordinated parallel observations, laying out the parallel observations into orbits and visits is fairly straightforward. The primary observations determine the orbit and visit structure, and the coordinated parallels should conform to the visit structure of the primary observations.

Instrument Specific Limitations on Visits

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 the target object and reference objects) may be contained in one visit if there is no telescope motion made during the sequence.


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:

  1. Each acquisition (ACQ) and its corresponding science exposures must be scheduled as a separate visit.
  2. Each visit must not exceed 22 minutes, including guide star acquisition, ACQ, exposure time, and overhead.
  3. 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.

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 are less than 30 minutes of science observing time (including overheads) using the CCD.
  • The target is observed for only one orbit.

Next: HST Primer: Orbital Visibility, Acquisition Times, and Overheads