Space Telescope Science Institute
Call for Proposals and HST Primer
help@stsci.edu
Table of Contents Previous Next Print


Hubble Space Telescope Primer for Cycle 22 > Chapter 6: Orbit Calculation for a Phase I Proposal > 6.4 Acquisition Times and Instrument Overheads

6.4 Acquisition Times and Instrument Overheads
ACS
COS
FGS
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.
6.4.1 Guide Star Acquisition Times
A normal guide star acquisition, required in the first orbit of every visit, takes six minutes. At the start of subsequent orbits in a multi-orbit visit, the required guide star reacquisition takes four minutes. For CVZ observations, guide star reacquisitions are not required but, if an observation extends into SAA-impacted orbits (see Section 2.2.2), then guide star reacquisitions will be necessary for those orbits.
Table 6.2: Guide Star Acquisition Times
6.4.2 Target Acquisition Times
A target acquisition may be required after the guide star acquisition, depending on the science instrument (SI) used and pointing requirements. See Section 5.2 for a basic overview of target acquisitions. Consult the HST Instrument Handbooks to determine whether a target acquisition is required for particular types of observations, and which acquisition type is most appropriate. Table 6.3 can be used to determine the time budget for these activities.
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.
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 (see Table 6.6).
Table 6.3: Target Acquisition Times
Used for STIS spectroscopic observations in apertures <= 0.1" in size, or for any STIS spectroscopic observation that requires the highest possible absolute precision in the zeropoint of the wavelength scale.This type of target acquisition always follows an ACQ. For faint targets (V > 20), add 4 times the acquisition exposure time determined by the Target Acquisition ETC.
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 “FGS - Dual Guide Star Acquisitions”).
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 (see Section 5.2.2). 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 (see Section 6.4.4) for the offset maneuver, usually followed by the final peak-up stage used in the original acquisition. Please contact the STScI Help Desk (see Section 1.3.8) if this capability would benefit your program.
6.4.3 Instrument Overhead Times
There are a variety of instrument overheads associated with science exposures. Tables 6.4 to 6.9 summarizes, 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 (see Section 5.2.)
ACS
ACS exposure overheads are listed in Table 6.4. 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 6.4: 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 overhead are listed in Table 6.5
Table 6.5: COS Exposure Overheads
FGS
FGS overheads are listed in Tables 6.6 and 6.7. 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. Table 6.8 lists the recommended number of scans as a function of target magnitude. The recommended exposure time is 40 seconds per scan (excluding overheads).
Table 6.6: FGS Exposure Overheads
Table 6.7: FGS Miscellaneous Overheads
Table 6.8: Recommended number of FGS TRANS mode scans
STIS
STIS overheads are listed in Table 6.9. 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 6.9: STIS Exposure Overheads
2 (4)1
1
For 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 Table 6.10.
Table 6.10: WFC3 Instrument Overhead Times
6.4.4 Telescope Repositioning Overhead Times
Small Angle Maneuvers (SAMs) are changes in telescope pointing of less than two arcminutes. Table 6.11 lists the overhead times for SAMs.
Table 6.11: Small Angle Maneuver Time
A “Reuse Target Offset” visit (see Section 5.2.2 and Section 6.4.2) 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 Section 5.4 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 (see Table 6.11) whose size matches the pattern spacing.

Hubble Space Telescope Primer for Cycle 22 > Chapter 6: Orbit Calculation for a Phase I Proposal > 6.4 Acquisition Times and Instrument Overheads

Table of Contents Previous Next Print