STIS Instrument Handbook for Cycle 24

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9.2	STIS Exposure Overheads

Our current estimates of the overheads on STIS exposures are summarized in Table 9.1 and Table 9.2. All numbers given are approximate and rounded up to the nearest half minute; they do not differentiate in detail the overheads for different STIS modes and configurations. These overhead times are to be used (in conjunction with the actual exposure times and the HST Primer) to estimate the total number of orbits for your proposal. After your HST proposal is accepted, you will be asked to submit a Phase II proposal to support scheduling of your approved observations. At that time you will be presented with actual, up-to-date overheads by the scheduling software. Allowing sufficient time for overheads in your Phase I proposal is important; additional time to cover unplanned overheads will not be granted later.

The following list presents important points for each type of overhead:

•	Generic (Observatory Level) Overheads:

-	The first time you acquire an object you must include the overhead for the guide-star acquisition (6 minutes).

-	In subsequent contiguous orbits you must include the overhead for the guide-star reacquisition (5 minutes). If you are observing in the Continuous Viewing Zone (see the HST Primer), no guide-star reacquisitions are required.

-

Time needs to be allowed for each deliberate movement of the telescope; e.g., if you are performing a target acquisition exposure on a nearby star and then offsetting to your target, or if you are taking a series of exposures in which you move the target relative to the slit, you must allow time for the moves (20 seconds to 60 seconds depending on length of slew, see Table 9.1 and Table 9.2).

-	Remember the policy with regard to MAMA and CCD observations in the same visit (see Chapter 2).

•	STIS Onboard Target Acquisition Overheads:

-

All STIS spectroscopic exposures that use a slit (long slit or echelle slit) will need to include a target acquisition exposure to place the target in the slit (see Section 8.2). As discussed in Section 8.3, for the narrower slits you may also need to perform a peakup exposure to center the target in the slit.

-	An onboard target acquisition needs to be done only once in a series of continuous orbits (i.e., once per visit).

-

The drift rate induced by the Observatory is less than 10 milliarcsec/hour. Thermal drifts internal to STIS at the slit plane are less still. We recommend that for long series of exposures taken through slits which are less than or equal to 0.1 arcsecond in either dimension, a peakup be performed every 4-5 orbits. This procedure will ensure that drifts do not cause the target to move out of the slit. (See also Section 11.2 and Section 8.1.4.)

•	Scientific Exposures:

-

The overhead times are dominated by the time to move the grating wheel (MSM), which is ~3.0 minutes per move, worst case, and the readout time (CCD). Again, we stress that in Phase II the overheads will frequently be less, but it is important to plan Phase I using the conservative overheads given in Table 9.2 to ensure that you will have adequate time for your scientific goals.

•	CCD and MAMA Spectroscopic Exposures and Wavecals:

-

The quoted overheads on the first spectroscopic exposure in a visit, or a spectroscopic exposure within a visit containing a change of grating or grating tilt, allow for the taking of a single automatic wavecal exposure to permit post-observation determination of the zero point of the wavelength (and spatial) scales. If you plan a series of exposures at a given grating setting which extends over 40 minutes in exposure time, then you need to include time for an additional automatic wavecal for each 40-minute period. However for certain types of observing programs, careful crafting of the Phase II proposal can force the additional automatic wavecals associated with long observations (greater than ~40 minutes) into occultation (see Section 11.2.1).

•	Moving Targets:

-	Additional overheads will be incurred for observations of solar system targets. The moving target overheads (~1 minute in duration) are dependent upon current slew rates and are updated as necessary.

Table 9.1: Scientific Exposure Overheads: General, Acquisition, and Peakup

Action	Overhead
Generic (Observatory Level)
Guide-Star acquisition	Initial acquisition overhead = 6 minutes. Reacquisitions on subsequent orbits = 4 minutes per orbit.
Spacecraft POS-TARG moves	1 minute for offsets < 1 arcminute and > 10 arcseconds. 0.5 minute for offsets between 1-10 arcseconds. 10 seconds for offsets < 1 arcsecond in size.
Acquisitions and Peakups
Target acquisition (to place target in STIS aperture); see also Chapter 8.	For V ≤ 21 point sources, 6 minutes. For diffuse acqs, add 0.2 â checkbox2 seconds to the nominal 6 minutes.
Acquisition peakups; see also Chapter 8.	For V ≤ 21, 6 minutes for one peakup; note that a second peakup is required for the 0.1X0.03 aperture; dispersed light peakups may require an additional 3 minutes overhead for change in optical element. More generally, see Table 8.5.
End of CVZ Visit
End of CVZ visit	Final STIS buffer dump = 3 minutes. Return mechanisms to normal = 4.5 minutes. For non-CVZ observations, these activities occur during the final occultation period, with no impact on target visibility.

 

Table 9.2: STIS Scientific Exposure Overheads: Imaging and Spectroscopy 

Action	Overhead
	Imaging		Spectroscopy
	CCD (minutes)	MAMA (minutes)	CCD (minutes)	MAMA (minutes)
First scientific exposure	4.5	5.0	5.01	8.02
Identical exposure in series (within an orbit)	1.03	0.5	1.0	0.5
Exposure in series with grating move only	3.0	3.0	5.04	8.05
Move of aperture wheel: change of slit or aperture, change of filter	1.0	1.0	1.0	1.0
Additional automatic wavecal for series of identical exposures extending more than ~40 minutes	n/a	n/a	1.5	4.0
Overhead for data management for exposures > 3 minutes in duration	0	0	0	0
Overhead for data management, for a series of full-frame exposures, each SHORTER than 3 minutes in duration (see “CCD Subarrays” on page 223 for subarray rules)	3 minutes every 7 exposures	2 minutes every exposure	3 minutes every 7 exposures	2 minutes every exposure
Additional Calibration Exposures: Extra GO Wavecals6 and Fringe Flats
MAMA wavecal exposure	4.0 minutes
CCD wavecal exposure	1.5 minutes
CCD fringe flat exposure	2 minutes
Slitless Spectroscopy (image of field, image of slit, wavecal)	9 minutes

1

Includes auto wavecal. At Phase II, wavecals occurring at the beginning or end of an orbit will be pushed into occultation, providing increased time for scientific observing.

2

Includes auto wavecal. At Phase II, wavecals occurring at the beginning or end of an orbit will be pushed into occultation, providing increased time for scientific observing.

3

For CR-SPLIT=n, each exposure has a 1 minute overhead, so there will be (n – 1) minutes of extra overhead. If small CCD subarrays are used, the overhead per exposure decreases to 20 seconds.

4

Includes auto wavecal. At Phase II, wavecals occurring at the beginning or end of an orbit will be pushed into occultation, providing increased time for scientific observing.

5

Includes auto wavecal. At Phase II, wavecals occurring at the beginning or end of an orbit will be pushed into occultation, providing increased time for scientific observing.

6

Use these only for additional wavecal exposures beyond those taken automatically.