6.4 Acquisition Times and Instrument Overheads

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.

6.4.1 Guide Star Acquisition Times

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.

Table 6.2: Guide Star Acquisition Times

Type of Acquisition

Time [min.]

Use

Guide star acquisition

6

First orbit of every visit. Applies also to Snapshot observations.

Guide star re-acquisition

6

All orbits of a multi-orbit visit, except the first orbit.

May not be needed for CVZ observations (see text).

Type of Acquisition	Time [min.]	Use
Guide star acquisition	6	First orbit of every visit. Applies also to Snapshot observations.
Guide star re-acquisition	6	All orbits of a multi-orbit visit, except the first orbit. May not be needed for CVZ observations (see text).

6.4.2 Target Acquisition Times

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).

Table 6.3: Target Acquisition Times

SI

Type of Acquisition

Time [min.]

Notes

ACS

ACQ

3.5

Used to position a target behind the HRC coronagraphic spot. For faint targets, add 2 times the acquisition exposure time.

NICMOS

ACQ

3.0

Used to position a target behind the NIC2 coronagraphic hole.

Any

Interactive

30

Used for real-time interactions with the telescope in very exceptional circumstances.

SI	Type of Acquisition	Time [min.]	Notes
ACS	ACQ	3.5	Used to position a target behind the HRC coronagraphic spot. For faint targets, add 2 times the acquisition exposure time.
NICMOS	ACQ	3.0	Used to position a target behind the NIC2 coronagraphic hole.
Any	Interactive	30	Used for real-time interactions with the telescope in very exceptional circumstances.

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.

6.4.3 Instrument Overhead Times

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

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).

Table 6.4: ACS Exposure Overheads

SI Mode

Time [min.]

WFC

Time [min.]

HRC

Time [min.]

SBC

Notes

IMAGING/
SPECTROSCOPIC

4.0

2.5

1.7

A single exposure or the first exposure in a series of identical exposures.

IMAGING/
SPECTROSCOPIC

2.5

1.0

0.9

Subsequent exposures in an identical series of exposures.

IMAGING/
SPECTROSCOPIC

5.7

0

0

Additional overhead for subsequent exposures (except the last) in an identical series of exposures if the exposure time is less than 6 minutes.

SPECTROSCOPIC

N/A

8.5

N/A

Automatically executed (if AUTOIMAGE =YES) imaging exposure for prism spectroscopy (provides the image to co-locate the targets and their spectra; see the ACS Instrument Handbook for details).

SPECTROSCOPIC

7

5.5

N/A

Automatically executed (if AUTOIMAGE =YES) imaging exposure for grism spectroscopy (provides the image to co-locate the targets and their spectra; see the ACS Instrument Handbook for details).

SI Mode	Time [min.] WFC	Time [min.] HRC	Time [min.] SBC	Notes
IMAGING/ SPECTROSCOPIC	4.0	2.5	1.7	A single exposure or the first exposure in a series of identical exposures.
IMAGING/ SPECTROSCOPIC	2.5	1.0	0.9	Subsequent exposures in an identical series of exposures.
IMAGING/ SPECTROSCOPIC	5.7	0	0	Additional overhead for subsequent exposures (except the last) in an identical series of exposures if the exposure time is less than 6 minutes.
SPECTROSCOPIC	N/A	8.5	N/A	Automatically executed (if AUTOIMAGE =YES) imaging exposure for prism spectroscopy (provides the image to co-locate the targets and their spectra; see the ACS Instrument Handbook for details).
SPECTROSCOPIC	7	5.5	N/A	Automatically executed (if AUTOIMAGE =YES) imaging exposure for grism spectroscopy (provides the image to co-locate the targets and their spectra; see the ACS Instrument Handbook for details).

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.

Table 6.5: ACS Miscellaneous Overheads

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.

Type	Time [min.]
Overhead for switching from HRC to SBC in an orbit1	12.0

FGS

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).

Table 6.6: FGS Exposure Overheads

SI Mode

Time [min.]

Notes

POS

1

if target magnitude V < 14

POS

2

if target magnitude 14 < V < 15

POS

3

if target magnitude 15 < V < 16

POS

4

if target magnitude 16 < V < 16.5

POS

8

if target magnitude V > 16.5

TRANS

1

target acquisition (independent of target magnitude)

TRANS

0.2

overhead per scan (independent of target magnitude)

SI Mode	Time [min.]	Notes
POS	1	if target magnitude V < 14
POS	2	if target magnitude 14 < V < 15
POS	3	if target magnitude 15 < V < 16
POS	4	if target magnitude 16 < V < 16.5
POS	8	if target magnitude V > 16.5
TRANS	1	target acquisition (independent of target magnitude)
TRANS	0.2	overhead per scan (independent of target magnitude)

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

NICMOS

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

Table 6.10: NICMOS Miscellaneous Overheads

Type

Time

[min.]

Instrument set-up at the beginning of an orbit

0.3

Filter change in the same camera

0.3

Overhead for switching from NIC1 to NIC2, or vice versa, in an orbit

1.4

Overhead for switching from NIC1 to NIC3 or vice versa, in an orbit

9.7

Overhead for switching from NIC2 to NIC3 or vice versa, in an orbit

4.8

Type	Time [min.]
Instrument set-up at the beginning of an orbit	0.3
Filter change in the same camera	0.3
Overhead for switching from NIC1 to NIC2, or vice versa, in an orbit	1.4
Overhead for switching from NIC1 to NIC3 or vice versa, in an orbit	9.7
Overhead for switching from NIC2 to NIC3 or vice versa, in an orbit	4.8

WFPC2

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.

Table 6.11: WFPC2 Exposure Overheads

Mode

Time [min.]

Notes

IMAGING

3

Exposure without CR-SPLIT

IMAGING

5

CR-SPLIT exposure (i.e., two separate exposures and readouts)

IMAGING

2

Additional overhead for each exposure with the LRF (required for telescope repositioning).

Mode	Time [min.]	Notes
IMAGING	3	Exposure without CR-SPLIT
IMAGING	5	CR-SPLIT exposure (i.e., two separate exposures and readouts)
IMAGING	2	Additional overhead for each exposure with the LRF (required for telescope repositioning).

Type	Time [min.]
"Efficiency" overhead, per orbit	1

6.4.4 Telescope Repositioning Overhead Times

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.