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 re-acquisition takes four minutes. For CVZ observations, guide star re-acquisitions are not required but, if an observation extends into SAA-impacted orbits (see
Section 2.2.2), then guide star re-acquisitions will be necessary for those orbits.
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.
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).
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.
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 subpixel 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 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).
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.
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 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).
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.
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.
