You cannot use the entire target visibility time for actual science exposures, 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.
Table 6.2 summarizes the times required for guide star acquisitions. A normal guide star acquisition, required in the first orbit of every visit, takes 6 minutes. At the beginning of subsequent orbits in a multi-orbit visit, the required guide star re-acquisition takes 6 minutes. For CVZ observations guide star re-acquisitions are not required but, if an observation extends into SAA-impacted orbits (see Section 2.3.2), 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 SI used and pointing requirements. See Section 5.2 for a basic overview of target acquisitions. Consult the
HST Instrument Handbooks (see Section 1.2) to determine whether a target acquisition is required for your particular observations, and which acquisition type is most appropriate. Then use Table 6.3 to determine the time that you need to budget for this.
Most normal imaging observations with ACS, NICMOS and WFPC2 do not require a target acquisition (assuming that the coordinates delivered by the observer in Phase II have sufficient accuracy of 1"-2"). However, for coronagraphic imaging with ACS/HRC or NICMOS/NIC2, you will need to perform a target acquisition to place the target behind the coronagraphic hole or feature. For ACS/HRC and NICMOS/NIC2, modes called ACQ are available. Note that the acquisition algorithms work differently for the different instruments, even if the modes have the same names.
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).
In exceptional cases you may require a real-time interaction with the telescope to perform a target acquisition (see Section 5.2.2). You will then first obtain an image which you should treat as a normal science exposure. Then add 30 minutes for the real-time contact (which may overlap with the occultation interval at the end of an orbit).
Generally, a target acquisition does not need to be repeated for separate orbits of a multi-orbit visit.
A target acquisition, if necessary, usually should be inserted in each visit. However, programs with multiple visits to the same target within a six-week period (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 your program, then you should include the full target acquisition sequence only in the initial visit; the subsequent visits will not need a full target acquisition. However, they will require a Small Angle Maneuver (SAM) (see Section 6.4.4) for the offset maneuver, and they usually require the final peakup stage used in the original acquisition. Please contact the
STScI Help Desk (see Section 1.3) if you feel your program can benefit from this capability.
There are a variety of instrument overheads associated with science exposures. Tables 6.4 to 6.12 summarize for each instrument how much time you need to budget for these overheads, depending on the observing strategy.
ACS overheads are listed in Tables 6.4 and 6.5.
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 you are unsure whether the shorter overhead time is appropriate, then use the longer overhead time (to avoid a possible orbit allocation shortfall later).
Note that if AUTOIMAGE=NO is invoked and a different direct image is specified for the WFC or HRC 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.
Type |
Time [min.] |
---|---|
Overhead for switching from HRC to SBC in an orbit1 |
12.0 |
1There is an additional 6 minutes overhead if the buffer is full or will be before the end of visibility. Switching from SBC to HRC within an orbit is not allowed. See the ACS Instrument Handbook for further details. |
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. In Table 6.8 we list the recommended number of scans as a function of target magnitude. The recommended exposure time is 40 seconds per scan (excluding overheads).
Type |
Time [min.] |
---|---|
Instrument Setup, per orbit |
4 |
Instrument Shutdown, per orbit |
3 |
V-magnitude |
# scans |
---|---|
8-12 |
10 |
13-14 |
20 |
15 |
30 |
16 |
60 |
A large number of different overheads exist for NICMOS observations, as listed in Tables 6.9 and 6.10, and discussed in detail (with examples) in Chapter 10 of the NICMOS Instrument Handbook.
The overhead for the MULTIACCUM mode (the readout mode that proposers are encouraged to use whenever possible) is fixed. The overhead on the ACCUM mode is a function of the number of reads, NREAD, obtained at the beginning (and at the end) of an exposure. The range of allowed NREADs is 1 (default) to 25.
SI Mode |
Time |
Notes |
---|---|---|
IMAGING/ SPECTROSCOPIC |
4 sec |
MULTIACCUM exposures |
IMAGING/ SPECTROSCOPIC |
7 + (NREAD x 0.6) sec |
ACCUM exposures; NREAD=1-25 |
WFPC2 overheads are listed in Tables 6.11 and 6.12.
Exposures are usually split in two (CR-SPLIT) to allow for cosmic ray rejection (this is the default for exposure times longer than 10 minutes). If an exposure is CR-SPLIT, you should count it as a single exposure with a single (5 minute) overhead. For exposures that are not CR-SPLIT (the default for exposure times equal to or shorter than 10 minutes), use the 'without CR-SPLIT' overhead time.
An 'efficiency' overhead of 1 minute should be added to each orbit of WFPC2 imaging, which allows for scheduling flexibility during SAA-impacted HST orbits.
Small Angle Maneuvers (SAMs) are changes in telescope pointing of less than 2 arcmin. Table 6.13 lists the overhead times for SAMs.
Step-size |
SAM time [seconds] |
---|---|
0" < step-size < 1.25" |
20 |
1.25" < step-size < 10" |
30 |
10" < step-size < 28" |
40 |
28" < step-size <60" |
50 |
60" < step-size < 2' |
65 |
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 (see 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 times the overhead time for a single SAM (see Table 6.13) whose size matches the pattern spacing.
In general, undithered observations with the ACS CCD detectors will not be approved without strong justification that such is REQUIRED for the scientific objectives. Otherwise, hot pixels and other detector artifacts will compromise the program and the archival value of the data. Further details about the options and advantages of ACS patterns can be found in the
ACS Instrument Handbook, the Phase II Proposal Instructions, and the ACS Dither Web page
.
Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |