8.2 ACS Exposure Overheads

---

Please check for updates on the ACS Web site.

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.		10.7

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.

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.

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.

For operational reasons related to power constraints and SAA avoidance, respectively, the HRC and SBC cannot be powered up simultaneously, and once on, the SBC must remain on for a minimum of 2 hours. These constraints affect programs using both cameras as follows. (1) Although not recommended, both may be used within a single orbit, but then the HRC observations must always be done first. Moreover, there is a 12 minute overhead penalty for the reconfiguration. If the buffer is full or will be before the end of the visibility period, then an additional 6 minute overhead is incurred for the switch. (2) Even if the two cameras are used in separate, consecutive orbits, either the HRC must be scheduled first, or the SBC must be used for a minimum of two orbits before the HRC is activated.

8.2.1 Subarrays

At the end of each exposure, data are read out into ACS’s internal buffer memory where they are stored until they are dumped into HST’s solid state data recorder. The ACS internal buffer memory holds 34 MB or the equivalent of 1 full WFC frame, or 16 HRC or SBC frames. Thus, after observing a full WFC frame, the internal buffer memory must be dumped before the next exposure can be taken. The buffer dump takes 349 seconds and may not occur while ACS is being actively commanded. Of this time, 339 seconds (for one WFC image) or 346 seconds (for 16 HRC images) is spent dumping the image. The buffer dump cannot be done during the next exposure if the latter is shorter than 339 seconds. If, however, the next exposure is less than 339 seconds the buffer dump will create an extra 5.8 minutes of overhead.

If your science program is such that a smaller FOV can be used, then one way of possibly reducing the frequency and hence overheads associated with buffer dumps is to use WFC subarrays. With subarrays, only the selected region of the detector is read out at a normal speed and stored in the buffer, and a larger number of frames can be stored before requiring a dump. Using subarrays not only reduces the amount of time spent dumping the buffer but in some cases may reduce the readout time. See Chapter 7 for a discussion of some of the limitations of subarrays. If the user elects to define a subarray of arbitrary size and location, allowed on an available-but-unsupported basis, then matching bias frames will not be automatically provided by STScI. Any bias frames specified by the user will typically be scheduled during the following occultation (i.e., they do not add to the overheads during visibility time). Dark frames and flat fields will be extracted from full frame images. In some special cases where a general subarray is cleverly defined so as to include a physical overscan region, no separate bias frames are needed.