Space Telescope Science Institute   8.1 Overview  8.2.1 Subarrays

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:

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.

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.


 8.1 Overview  8.2.1 Subarrays
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