Advanced Camera for Surveys Instrument Handbook for Cycle 14
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9.1 OverviewChapter 9:
Overheads and Orbit-Time Determination
9.2 ACS Exposure Overheads
9.2.1 Subarrays
9.3 Orbit Use Determination Examples
9.3.1 Sample Orbit Calculation 1:
9.3.2 Sample Orbit Calculation 2
9.3.3 Sample Orbit Calculation 3:
9.3.4 Sample Orbit Calculation 4:
9.3.5 Sample Orbit Calculation 5:
This Chapter describes the overheads associated with ACS observations and gives examples showing how to determine the number of orbits your program will require.
9.1 Overview
After you establish the set of scientific exposures and any additional target-acquisition or calibration exposures that you require for your program, you are ready to determine the total number of orbits to request. Generally, this is a straightforward exercise involving compilation of the overheads on the individual exposures, packing the exposure plus overhead time into individual orbits, and tallying up the results to determine your total orbit request. In some cases, it may be an iterative process as you refine your exposure requests to more fully exploit the orbits.
The Phase I Call for Proposals includes proposal instructions that provide information on the Observatory policies and practices with respect to orbit-time requests. The HST Primer provides specific advice on orbit determination. Below, we provide a summary of the ACS specific overheads and give several examples that illustrate how to calculate your orbit requirements for Phase I Proposals.
9.2 ACS Exposure Overheads
The overheads on exposures are summarized in Table 9.1 and Table 9.2. All numbers given are approximate; they do not differentiate in detail the 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 the overhead for the guide-star acquisition (6.1 min).
- In subsequent contiguous orbits you must include the overhead for the guide-star reacquisition (5.7 min); if you are observing in the Continuous Viewing Zone (see the Phase I Proposal Instructions), no guide-star reacquisitions are required.
- Time needs to be allowed 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 sec to 60 sec depending on the size of the slew, see Table 9.1 and Table 9.2).
- Onboard Target-Acquisition Overheads:
- On-board target acquisitions in general 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 in a series of contiguous orbits (i.e., once per visit).
- The drift rate induced by the Observatory is less than 10 milli-arcsec per hour. Thermal drifts internal to ACS are less still.
- Scientific Exposures:
- The overhead times are dominated by the time to move the filter wheel, the readout time (CCD), and any serial buffer dump that is necessary. Again, we stress 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 9.2 to ensure you will have adequate time for your scientific goals.
- Spectroscopy:
- Each CCD spectroscopic observation is preceded by an imaging exposure used for calibration, with the exposure time 3 and 6 min. for grism and prism observations, respectively. 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 it represents, however, an additional overhead to include in the observation time budget, so we have included it 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 available-but-unsupported mode AUTOIMAGE=NO during the phase II preparations. More details can be found in the Phase II Instructions Handbook.
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 at the same target and with the same detector and the same filter(s). PATTERNS also involve slews and therefore slew overheads.
For ACQ mode, the overhead includes double the specified exposure time. The reason is that it is necessary to have two acquisition images, hence two exposures, in order to eliminate possible image defects which can interfere with target acquisition. The flight software ensures that two images are taken, the user does not need to specify them in the proposal.
The overhead time for serial buffer dumps arises in certain cases from the overheads associated with the on-board 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 dumps the previous image, which takes 349 sec. If the next exposure time is longer than 339 sec (for WFC) or 346 (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 sec (WFC) or 346 sec (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 imposed for toggling between SBC and HRC within an orbit. The time to switch over from SBC to HRC is 480 sec. The opposite switch takes 650 sec, and in both cases there is an additional overhead of 6 min. associated with buffer dump. This consumes a significant portion of the available orbital time. Thus, whenever possible, one should plan to use HRC and SBC in different orbits.
9.2.1 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 Mbyte 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 sec and may not occur while ACS is being actively commanded. Of this time, 339 sec (for one WFC image) or 346 sec (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 sec. 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 a fraction of the detector is read out, a smaller volume of data per frame is 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, since only a portion of the detector is readout. We refer the user to Chapter 8 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.
9.3 Orbit Use Determination Examples
The easiest way to learn to compute total orbit time requests is to work through a few examples. Below we provide five different examples:
- Example 1 is a simple WFC image in one filter.
- Example 2 is a set of short WFC exposures that may require large overheads associated with buffer dumps.
- Example 3 is a one-orbit coronagraphic observation in two filters.
- Example 4 is a two-orbit observation using dithering.
- Example 5 is a one orbit WFC grism spectroscopic observation.
These examples represent fairly typical uses of ACS.
9.3.1 Sample Orbit Calculation 1:
Consider a target to be imaged with WFC in a given filter in one orbit. Using the Exposure Time Calculator (ETC), we find that we need 2400 sec of exposure time to reach the desired level of signal-to-noise ratio. Given that the observation must be split into a series of two exposures by
CR-SPLIT(CR-SPLIT=2), we map the overheads and the science exposure times onto the orbit as follows:
Table 9.3: Orbit Calculation for Example
Thus, the two WFC exposures totaling 2400 sec make full use of the typically available time in one orbit. The exposure times can be adjusted if the actual target visibility time differs from the derived total used time.
9.3.2 Sample Orbit Calculation 2
This example illustrates the impact of short WFC exposures on the useful time in the orbit. We have one orbit to observe a target with WFC in two filters, so the observation consists of two series, each with two identical
CR-SPLITexposures. The ETC has shown that at the minimally accepted signal-to-noise ratio the exposure time must be 540 sec for each of the filters, so each of theCR-SPLITsmust be at least 270 sec long. For the target declination, we find that the visibility time is 55 min. The time budget for the orbit is then as follows:
Table 9.4: Orbit Calculation for Example 2
Comparing with the previous example, we see that although with the adopted minimum exposure times we can squeeze the observation into one orbit, the efficiency of the orbit use is very low because of the large overheads associated with buffer dumps. However, if we increase each of the four exposure times so that they are larger than 339 sec, we avoid these additional overheads. This would free ~17 min. of the orbit time for science, which allows us to almost double the science exposure time (35 min. instead of 18 min.) and thus significantly improve signal-to-noise.
Similarly, a subarray can be used to readout only a fraction of the detector, allowing more frames to be stored in the buffer before requiring a dump. In this example, using four WFC1-1K subarrays for 4 short (t<339s) exposures would save 176s in readout time and 1047s in dump time. This frees up ~20 minutes of orbit time to be used for science.
9.3.3 Sample Orbit Calculation 3:
This example demonstrates the orbit calculation for a coronagraphic observation. We want to obtain coronagraphic images of a star in two filters, F250W and F606W. The ETC has shown that the exposure times adequate for our scientific goals are 5 min. in F606W and 30 min. in F250W. From the orbit visibility table (see the HST Primer) we find that at the target declination of 15 degrees the target visibility time is 52 min. With
CR-SPLIT=2, we thus have to accommodate in that period 35 min. of four science exposures grouped in two series. The orbit calculation goes like this:
Table 9.5: Orbit Calculation for Example 3
The derived total used time in the orbit shows that our target can indeed be imaged in the selected filters in one orbit. Since there remains 3 min. of unused time, we can adjust our exposure times to make full use of the available time.
9.3.4 Sample Orbit Calculation 4:
This example illustrates the orbit calculation for a WFC observation with the ACS box pattern, which implements imaging at four offset pointings. The goal of the observation is to obtain a dithered image of a field in such a way that would allow us to bridge the 50-pixel interchip gap between the WFC CCDs in the combined image. Given the WFC plate scale of 0.05 arcsec/pix, this requires that the offsets in the dithering pattern are larger than 2.5 arcsec. Each offset will then take 0.5 min. to move the spacecraft from one pointing in the pattern to another. We have determined that the exposure time necessary to reach the desired signal-to-noise ratio is 80 min. The visibility time at our target declination is 58 min. In this observation we do not want to rely on cosmic ray removal provided by the dithering data reduction package, and set
CR-SPLIT=2to be able to remove cosmic rays from the four individual images separately. As a result, the orbit calculation will involve a series of 8 exposures (two exposures at each of the four pointings in the dithering pattern) split across two orbits:
The total used time in the first orbit comes out a little bit larger than the visibility time. However, given the conservative nature of the adopted overhead times as well as a bit of flexibility in the adopted signal-to-noise ratio, the difference is not significant. It is to be remembered that the purpose of the above exercises is to estimate how many orbits to request for our science program rather than to exactly design the observation.
9.3.5 Sample Orbit Calculation 5:
This example illustrates the orbit calculation for a simple 30 min. WFC grism spectroscopic observation broken down by
CR-SPLIT=2into a series of two exposures.
Unlike similar imaging exposures, here we have to take into account an additional imaging exposure before the sequence of spectroscopic exposures, which takes 10 min. off the available orbit time.
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