8.2 ACS Exposure Overheads
Exposure overheads are summarized in Table 8.1 and Table 8.2. All numbers given are approximate; they do not make detailed differentiations between overheads for different ACS modes and configurations. These overhead times are to be used (in conjunction with the actual exposure times and the instructions in 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 APT
scheduling software. Allowing sufficient time for overhead in your Phase I proposal is important; additional time to cover unplanned overhead 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 overhead for the guide-star acquisition (6 minutes).
- In subsequent contiguous orbits you must include the overhead for the guide-star reacquisition (6 minutes); if you are observing in the Continuous Viewing Zone (see the
Phase I Proposal Instructions
), no guide-star reacquisitions are required.
- Allocate some time 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 on the detector, you must allow time for the moves (20 seconds to 60 seconds, depending on the size of the slew (see Table 8.1 and Table 8.2).
Table 8.1: Science exposure overheads: general.
Action |
Overhead |
Generic (observatory level) |
Guide-star acquisition |
Initial acquisition overhead = 6 minutes. Reacquisitions on subsequent orbits = 6 minutes per orbit. |
Spacecraft moves |
For offsets less than 1.5 arcminutes and more than 10 arcseconds = 1 minute. For offsets between 10 arcseconds and 1 arcseconds = 0.5 minutes. For offsets less than 1 arcseconds in size = 20 seconds. |
Table 8.2: ACS science exposure overhead times (minutes).
Exposure type |
WFC |
HRC |
SBC |
Mode: ACCUM |
Single exposure or the first exposure in a series of identical exposures. |
4.0 |
2.5 |
1.7 |
Subsequent identical exposures in series (within an orbit). |
2.5 |
1.0 |
0.7 |
Additional overhead for each serial buffer dump (added when WFC exposures are less than 339 seconds long, or the buffer fills with short HRC or SBC exposures). |
5.8 |
5.8 |
5.8 |
Predefined imaging exposure for prism spectroscopy. |
N/A |
8.5 |
N/A |
Predefined imaging exposure for grism spectroscopy. |
7 |
5.5 |
N/A |
Mode:ACQ |
For the specified acquisition exposure time, tacq, the total acquisition time is: |
N/A |
3.5 + (2 ×tacq) |
N/A |
Additional overheads |
Additional overhead if switching over from HRC to SBC within an orbit. |
|
11.0 |
|
Additional overhead if switching over from SBC to HRC within an orbit. |
|
8.0 |
|
- Onboard Target-Acquisition Overheads:
- On board target acquisitions only need to be done to place the target under one of the coronagraphic spots.
- An on board target acquisition needs to be done only once for a series of observations in contiguous orbits (i.e., once per visit).
- The drift rate induced by the Observatory is less than 10 milliarcseconds per hour. Thermal drifts internal to ACS are even less.
- Scientific Exposures:
- The overhead times are dominated by the time to move the filter wheel, the CCD readout time, and any necessary serial buffer dumps. Again, it should be stressed 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 8.2 to ensure adequate time for the proposal's scientific goals.
- Spectroscopy:
- Each CCD spectroscopic observation is preceded by an imaging exposure used for calibration, with exposure times of 3 and 6 minutes, respectively, for grism and prism observations. SBC prism exposures are not preceded by an automatic calibration exposure. Technically this is an individual single exposure requiring all regular science exposure overheads. For the observer, however, it represents an additional overhead in the observation time budget, so it has been included in the table of instrument overhead times for science exposures. However, if the observing program is already taking an appropriate broadband image, the automatic imaging and associated overheads preceding the spectroscopic grism or prism observations can be avoided by invoking the Optional Parameter AUTOIMAGE=NO during the Phase II preparations. More details can be found in the
Phase II Proposal Instructions
.
Note that identical exposures are generated automatically if the observer specifies the proposal optional parameters CR-SPLIT (for n > 1), or PATTERN,
or if Number_of_Iterations > 1. If it is not specified, CR-SPLIT defaults to n = 2. In general, identical exposures are defined here as exposures of the same target, with the same detector and filter(s). For identical exposures in PATTERNS,
this also involves slews and therefore slew overheads.
For ACQ mode, the overhead includes double the specified exposure time. The reason for having two acquisition images is to eliminate possible image defects which can interfere with target acquisition. The flight software ensures that two images are taken, so the user does not need to specify that in the proposal.
The overhead time for serial buffer dumps arises in certain cases from the overheads associated with the onboard data management and switching over the cameras. The on-board buffer memory can hold no more than one WFC image. The next WFC image can be placed into the buffer only after the buffer has dumped the previous image, which takes 349 seconds.
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If the next exposure time is longer than 339 seconds (for WFC) or 346 seconds (for HRC; 16 HRC images may be taken before a buffer dump is triggered), the buffer dump will occur during that exposure, and no overhead is imposed. However, if the next exposure time is shorter than 339 seconds (WFC) or 346 seconds (HRC), then the dump must occur between the two exposures.
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Sequences of many short HRC or SBC exposures can also lead to serial dumps when the buffer becomes full. In this case the buffer dump time becomes an overhead to be included into the orbit time budget. This overhead can severely constrain the number of short exposures one can squeeze into an orbit. Subarrays can be used to lower the data volume for some applications.
A serious penalty is incurred by toggling between SBC and HRC within an orbit. The time to switch over from SBC to HRC is 480 seconds. The opposite switch takes 650 seconds. In both cases, if the buffer has been filled, there will be an additional overhead of 6 minutes for the dump; but if the buffer is not filled during the orbit, the dump will occur at the end of visibility and can be pushed into occultation. Thus, whenever possible, one should plan to use HRC and SBC in different orbits.
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