|Space Telescope Science Institute|
|ACS Instrument Handbook|
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.
- 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 Tables 8.1 and 8.2.Table 8.1: Science Exposure Overheads: General
- The overhead times are dominated by the time required 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 lower, 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.
- 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 exposures with identical observing modes are automatically generated if the observer specifies:
• Phase 2 proposal optional parameter CR-SPLIT with a value greater than or equal to 2. (The default value is CR-SPLIT=2 if the Phase 2 exposure log sheet field Number_of_Iterations is at its default value of 1.)
• Phase 2 exposure log sheet field Number_of_Iterations is greater than or equal to 2 (where CR-SPLIT must be set to “NO”.)
• Phase 2 special requirement PATTERN is used to execute a dither pattern. In this instance, overheads will also include slew overheads.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 onboard 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.
Sequences of many short 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.8.2.1 SubarraysAt 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 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 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.