A description of orbital visibility, acquisition times, and overheads.

The Orbital Visibility Period

The orbital visibility period is the amount of unocculted time per orbit during which observations of a given target can be made. The table below gives the visibility period for fixed targets of given declination, for moving targets (assumed to be near the ecliptic plane), and for cases in which the special requirements CVZ, LOW SKY, and SHADOW are used. See HST Preparation of the PDF Attachment for more information.

The listed orbital visibility for the CVZ (96 minutes, i.e., the entire HST orbit) assumes that there are no SAA intersections in these orbits. This is the orbital visibility that should be used for planning CVZ observations unless the proposer knows that he or she may have to observe in orbits that are SAA-impacted. In the latter case, the visibility time is approximately 70 minutes per orbit. During a CVZ window, only 5-6 true CVZ orbits (96 minutes) can be scheduled each day. Note that CVZ orbital visibility should not be requested if there are special background emission or timing requirements. 

Also included in the table are orbital visibilities suitable for use in Large Programs. Proposers submitting Large Programs should consult the Large Program Scheduling User Information Report (UIR-2012-002). This document contains a discussion of the issues surrounding Large Program scheduling. The table below summarizes orbital visibility in three-gyro mode

Orbital Visibility Period for Pure Parallel Observations

For Pure Parallel observations, proposers may not know the prime target declinations. One of the following two options should be used for planning observations:

• Use the minimum allowed orbital visibility period for the target’s selection criteria. For example, if the requirement calls for fields around M31 (at a declination of 50°), use 57 minutes.
• Map out the desired exposures (plus overheads) in an orbit for any legal orbital visibility period (54 to 61 minutes). If this method is selected, note that longer total exposure times typically have fewer scheduling opportunities.

Acquisition Times and Instrument Overheads

The entire target orbital visibility for actual science exposures cannot be used because of the required times for guide star acquisition, target acquisition, and SI overheads. The following subsections discuss the amounts of time that should be budgeted for these items; they are conservative approximations suitable for use in a Phase I proposal and may differ slightly from the numbers in the instrument handbooks.

Guide Star Acquisition Times

A normal guide star acquisition, required in the first orbit of every visit, takes 6.5 minutes. At the start of subsequent orbits in a multi-orbit visit, the required guide star reacquisition takes 6.5 minutes. For CVZ observations, guide star reacquisitions are not required but, if an observation extends into SAA-impacted orbits, then guide star reacquisitions will be necessary for those orbits.

Table of Guide Star Acquisition Times

Target Acquisition Times

A target acquisition may be required after the guide star acquisition, depending on the science instrument (SI) used and pointing requirements. Consult the various instrument handbooks to determine whether a target acquisition is required for particular types of observations, and which acquisition type is most appropriate. 

Target acquisitions are commonly used for COS and STIS spectroscopy. A number of target acquisition options are provided for each instrument. In some cases, an additional set of peak-up observations can follow the initial instrument acquisition.

Most normal imaging observations with ACS, STIS, and WFC3 do not require target acquisitions (assuming that the coordinates delivered by the observer in Phase II have sufficient accuracy of 1" to 2"). Since the COS aperture has a small 2.5" diameter field of view, a target acquisition is recommended for COS imaging unless the coordinates supplied by the observer are accurate to 0.4" or better. For COS imaging, the same acquisition strategies are available as for spectroscopy.

Due to increased COS/NUV detector background and mechanism position uncertainties, certain NUV central wavelength settings have proven to be unreliable in the ACQ/PEAKXD target acquisition step. The central wavelength settings below are settings that have been proven reliable. Spectroscopic NUV target acquisitions that use ACQ/PEAKXD must use one of the settings listed below. The most reliable settings for each grating are shown in bold, but any setting below may be used.

List of NUV settings to use for ACQ/PEAKXD TAs with COS

For coronagraphic imaging with STIS, a target acquisition is required to place the target behind the coronagraphic hole or feature. For STIS, the same ACQ and ACQ/PEAK strategies are available as for spectroscopy. 

FGS observations use a so-called “spiral search” location sequence for target acquisitions. This is part of a science observation, and the time required for the acquisition is considered to be part of the overhead associated with the science observation.

Table of Target Acquisition Times

Generally, a target acquisition does not need to be repeated for separate orbits of a multi-orbit visit. However, for observers planning multi-orbit observations in 0.1" or smaller STIS slits, insertion of a target peak-up maneuver for every four orbits is recommended (see HST Primer: Optical Performance, Guiding Performance, and Observing Efficiency).

A target acquisition, if necessary, should usually be inserted in each visit. However, programs with multiple visits to the same target within a six-week period (from start to finish) may be able to use the Reuse Target Offset function. If “reuse target offset” is appropriate for a program, then the observer should include the full target acquisition sequence only in the initial visit; subsequent visits will not need a full target acquisition. However, a Small Angle Maneuver (SAM) may be required for the offset maneuver, usually followed by the final peak-up stage used in the original acquisition. Please contact the STScI Help Desk if this capability would benefit your program.

Instrument Overhead Times

There are a variety of instrument overheads associated with science exposures. The tables below summarize, for each instrument, the amount of time needed to budget for overheads, depending on the observing strategy.

For several years, many observers have been using dithering, or small spatial displacements, to allow for better removal of detector defects and the reconstruction of sub-pixel resolution images. In general, undithered observations with the ACS CCD and WFC3 detectors will not be approved without strong justification that such observations are required for the scientific objectives.

ACS

ACS exposure overheads are listed. The overhead per exposure is shorter if the exposure is the same as the previous exposure (i.e., the exposures use the same aperture and spectral element, but not necessarily the same exposure times). If there is uncertainty about whether the shorter overhead time is appropriate, then use the longer overhead time (to avoid a possible orbit allocation shortfall).

Table of ACS Exposure Overheads

Note that if AUTOIMAGE=NO is invoked and a different direct image is specified for the WFC spectroscopic calibration, and in all cases for the SBC calibration (for which there is no AUTOIMAGE due to the safety issue), these direct images must be included explicitly in the Observing Summary and the observing time (orbit) request of the Phase I proposal.

COS

An ACQ/SEARCH will require about seven minutes of overhead, while an ACQ/IMAGE requires three minutes. The combination of ACQ/PEAKXD and ACQ/PEAKD will also require seven minutes. The first science exposure in a visit requires five minutes, while subsequent identical exposures incur two minutes of overhead. An additional one minute is needed for each instrument change between exposures, except that incrementing the FP-POS at the same CENWAVE setting only requires three seconds if the ordering of the FP-POS positions is done correctly. COS exposure overheads are listed in the table below.

Table of COS Exposure Overheads

¹For FUV spectroscopy at Lifetime Position 6, add 2 minutes for the first spectroscopic exposure in an orbit and 4 minutes for each additional spectroscopic exposure in the same orbit.

FGS

FGS overheads are listed. The total TRANS mode overhead consists of an acquisition overhead plus an overhead per scan. Hence, the total overhead depends on the number of scans obtained during a target orbital visibility period. Also listed are the recommended number of scans as a function of target magnitude. The recommended exposure time is 40 seconds per scan (excluding overheads).

Table of FGS Exposure Overheads

Table of FGS Miscellaneous Overheads

Table of recommended number of FGS TRANS mode scans 

STIS

STIS overheads are listed. The overhead per exposure is shorter if the exposure is the same as the previous exposure (“no change”); this means that the exposures use the same aperture, grating, and central wavelength, but the exposure times need not be the same. If there is uncertainty over whether the shorter overhead time is appropriate, then use the longer overhead time.

Table of STIS Exposure Overheads

¹For medium resolution modes G140M, G230M, and G230MB, there are some wavelength-slit combinations that require longer AUTOWAVECAL exposure times. For each set of exposures totaling more than 2300 seconds at the same grating position for mode G230MB, an overhead of 4 minutes should be budgeted. For each set of exposures totaling more than 2300 seconds at the same grating position for modes G140M and G230M, an overhead of 6 minutes should be budgeted.

WFC3

WFC3 exposure overheads are listed in the table below

Telescope Repositioning Overhead Times

Small Angle Maneuvers (SAMs) are changes in telescope pointing of less than two arcminutes. 

A “Reuse Target Offset” visit will require a SAM to be scheduled at the start of the first orbit. To allow for the offset adjustment, the SAM should be assumed to have a duration of 30 seconds.

Patterns described in HST Primer: Observing Considerations perform a series of SAMs. The timing and subsequent overheads depend on the size of the pattern. However, a simple estimate for the overhead time associated with a pattern is obtained by multiplying the number of points minus 1 (one) times the overhead time for a single SAM whose size matches the pattern spacing.

Next: HST Primer: Orbit Calculation Examples