6.4 Acquisition Times and Instrument Overheads

6.4.1 Guide Star Acquisition Times

6.4.2 Target Acquisition Times

6.4.3 Instrument Overhead Times

6.4.4 Telescope Repositioning Overhead Times

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

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 4 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.

Table 6.2: Guide Star Acquisition Times

Type of acquisition	Acquisition time (minutes)	Use
Guide star acquisition	6	First orbit of every visit. Applies also to Snapshot observations.
Guide star re-acquisition	4	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 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.

The most common use of target acquisition is 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"-2"). As 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 you will need to perform a target acquisition 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

Scientific Instrument	Type of acquisition	Acquisition time (minutes)	Notes
COS	ACQ/IMAGE	3	Typical acquisition in NUV integrated light (recommended). Total time required is 2 min + (2 x exposure time.)
COS	ACQ/SEARCH ACQ/PEAKXD ACQ/PEAKD	15	Typical precision acquisition in dispersed light (for targets with either NUV or FUV). Total time required for full sequence is ~10 min. plus (15 x exposure time.)
STIS	ACQ	6	Used for STIS spectroscopy or coronagraphy. For faint targets (V > 20), add 4 times the acquisition exposure time determined by the Target Acquisition ETC.
STIS	ACQ/PEAK	6	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.
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. However, we recommend that observers planning multi-orbit observations in 0.1" or smaller STIS slits insert a target peakup maneuver every 4 orbits (see Section 3.2.1).

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 1.4) 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 6.9 summarize for each instrument how much time you need to budget for these 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. In general, undithered observations with the ACS CCD and WFC3 detectors will not be approved without strong justification that such is 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 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	Overhead time WFC (minutes)	Overhead time SBC (minutes)	Notes
IMAGING/ SPECTROSCOPIC	4.0	1.7	A single exposure or the first exposure in a series of identical exposures.
IMAGING/ SPECTROSCOPIC	2.5	0.9	Subsequent exposures in an identical series of exposures.
IMAGING/ SPECTROSCOPIC	5.7	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	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	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 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 7 minutes of overhead, while an ACQ/Image requires 3 minutes. The combination of ACQ/PEAKXD and ACQ/PEAKD will also require 7 minutes. The first science exposure in a visit requires 5 minutes, while subsequent identical exposures incur 2 minutes of overhead. An additional 1 minute is needed for each instrument change between exposures, except that incrementing the FP-POS at the same CENWAVE setting only requires 3 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

Acquisition or exposure	Overhead time (minutes)	Notes
Guide-star acquisition	6	First orbit of each visit
Guide-star reacquisition	4	Each subsequent visit
ACQ/IMAGE	3	Typical imaging acquisition
ACQ/SEARCH	7	Imaging or dispersed, FUV or NUV
ACQ/PEAKXD+ACQ/PEAKD	7	Typical dispersed peakups, FUV or NUV
Science (imaging or dispersed)	5 2 1	First exposure in a series Each subsequent exposure Change of instrument configuration (except to increment FP-POS)

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 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	Overhead time (minutes)	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)

Table 6.7: FGS Miscellaneous Overheads

Type	Time [min.]
Instrument setup, per orbit	4
Instrument shutdown, per orbit	3

Table 6.8: Recommended number of FGS TRANS mode scans

Magnitude (V-band)	Number of scans
8-12	10
13-14	20
15	30
16	60

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 you are unsure whether the shorter overhead time is appropriate, then use the longer overhead time.

Table 6.9: STIS Exposure Overheads

Configuration/Mode	Overhead time (minutes)	Notes
CCD IMAGING	5	Overhead per exposure.
CCD SPECTROSCOPIC	5	Overhead per exposure.
CCD SPECTROSCOPIC	2 (4)1	Overhead for a series of identical exposures extending more than ~40 min
CCD IMAGING/SPECTROSCOPIC	1	Overhead per exposure, if no change from the previous exposure.
MAMA IMAGING (FUV or NUV)	5	Overhead per exposure.
MAMA IMAGING (FUV or NUV)	1	Overhead per exposure, if no change from the previous exposure.
MAMA SPECTROSCOPIC (FUV or NUV)	8	Overhead per exposure.
MAMA SPECTROSCOPIC (FUV or NUV)	4 (6)1	Overhead for a series of identical exposures extending more than ~40 min
MAMA SPECTROSCOPIC (FUV or NUV)	1	Overhead per exposure, if no change from the previous exposure.

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1

For the 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

Action	Overhead time (minutes)
Reconfiguration between UVIS and IR channels during a single orbit	1.0
Change quad filter (UVIS only)	1.0
UVIS ACCUM Mode
Single exposure or first exposure in a set of identical exposures (e.g., the first sub-exposure of a CR-SPLIT set)	2.6
Subsequent exposures in set of identical exposures (e.g., subsequent exposures in a CR-SPLIT set), per exposure	2.1
Buffer dump if exposure is not last one in an orbit, or if next exposure is less than 339 seconds	5.8
IR MULTIACCUM Mode
Each exposure	1.0
Buffer dump if 16-read exposure is not last one in an orbit, or if next exposure is less than 346 seconds	5.8

6.4.4

Telescope Repositioning Overhead Times

Small Angle Maneuvers (SAMs) are changes in telescope pointing of less than 2 arcmin. Table 6.11 lists the overhead times for SAMs.

Table 6.11: Small Angle Maneuver Time

Step-size interval	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 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.