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.
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.
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.
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.
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 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.