Hubble Space Telescope Primer for Cycle 11

6.4 Acquisition Times and Instrument Overheads

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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.

If you need to know the overhead times associated with a planned set of observations with high precision, then please use the Phase II program preparation software RPS2, available from the Phase II Program Preparation Web Page.

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 5 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. If gyro-only guiding is used (see Section 3.2.3), then there is no overhead for guide-star acquisition.

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	5	All orbits of a multi-orbit visit, except the first orbit. May not be needed for CVZ observations (see text).
No guide star acquisition	0	Used for gyro-only guiding (see Section 3.2.3).

 

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 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.

The most common use of target acquisitions is for STIS spectroscopy. Two target acquisition strategies are provided: ACQ and ACQ/PEAK. Consult the STIS Instrument Handbook for details.

Most normal imaging observations with ACS, NICMOS, STIS 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 coronographic imaging with ACS/HRC, NICMOS/NIC2 or STIS, you will need to perform a target acquisition to place the target behind the coronographic hole or feature. For STIS, the same ACQ and ACQ/PEAK strategies are available as for spectroscopy, while 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 coronographic spot. For faint targets, add 2 times the acquisition exposure time.
NICMOS	ACQ	2.6	Used to position a target behind the NIC2 coronographic hole.
STIS	ACQ	6	Used for STIS spectroscopy or coronography. 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 spectroscopy or coronographic observations that require the highest precision. 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 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 6.13 summarize for each instrument how much time you need to budget for these overheads, depending on the observing strategy. See Appendix C of the Call for Proposals for the definitions of the Instrument Mode keywords listed in the Table.

ACS

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. This means that the exposures use the same aperture and spectral element, 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 (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/ SPECTRA	4.0	2.5	1.7	A single exposure or the first exposure in a series of identical exposures.
IMAGING/ SPECTRA	2.5	1.0	0.9	Subsequent exposures in an identical series of exposures.
IMAGING/ SPECTRA	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.
SPECTRA	10	8.5	7.7	Automatically executed imaging exposure for prism spectroscopy (provides the image to co-locate the targets and their spectra; see the ACS Instrument Handbook for details).
SPECTRA	7	5.5	4.7	Automatically executed imaging exposure for grism spectroscopy (provides the image to co-locate the targets and their spectra; see the ACS Instrument Handbook for details).

 
Table 6.5: ACS Miscellaneous Overheads

Type	Time [min.]
Overhead for switching from HRC to SBC in an orbit	17.0
Overhead for switching from SBC to HRC in an orbit	14.0

 

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 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)

 
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

V-magnitude	8-12	13-14	15	16
# scans	10	20	30	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. The two available readout modes, FAST and SLOW, are explained in detail in the NICMOS Instrument Handbook.

Table 6.9: NICMOS Exposure Overheads

SI Mode	Time	Notes
IMAGING/ SPECTRA	4 sec	MULTIACCUM exposures.
IMAGING/ SPECTRA	7 + (NREAD x 0.6) sec	ACCUM exposures with FAST readout; NREAD=1-25
IMAGING/ SPECTRA	10 + (NREAD x 3.3) sec	ACCUM exposures with SLOW readout; 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

 

STIS

STIS overheads are listed in Table 6.11.

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 6.11: STIS Exposure Overheads

Config/Mode	Time [min.]	Notes
CCD IMAGING/SPECTRA	5	Overhead per exposure.
CCD IMAGING/SPECTRA	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 SPECTRA (FUV or NUV)	8	Overhead per exposure.
MAMA SPECTRA (FUV or NUV)	1	Overhead per exposure, if no change from the previous exposure.

 

WFPC2

WFPC2 overheads are listed in Tables 6.12 and 6.13.

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 6.12: 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 because of telescope repositioning).

 
Table 6.13: WFPC2 Miscellaneous Overheads

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 6.14 lists the overhead times for SAMs.

Table 6.14: Small Angle Manuver Time

Step-size	SAM time
0" < step-size < 1.25"	20 seconds
1.25" < step-size < 10"	30 seconds
10" < step-size < 28"	40 seconds
28" < step-size <60"	50 seconds
60" < step-size < 2'	65 seconds

 

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 6.14) whose size matches the pattern spacing.

In recent years, many observers have been using dithering, or small spatial displacements, to allow for better removal of chip defects and the reconstruction of sub-pixel resolution. Successful proposers will be provided with "canned" dithering routines in Phase II, which avoid some of the tricky details involved in planning patterns. The dithering strategies are implemented as Convenience Patterns and the SAM overheads can thus be estimated as described above. Please consult the WFPC2 and STIS Instrument Handbooks for details on the advantages, disadvantages, and overheads associated with dithering.