Chapter 8:
Overheads and Orbit-Time Determination

In this chapter. . .

8.1 Overview / 167 8.2 ACS Exposure Overheads / 168 8.3 Orbit Use Determination Examples / 172

8.1 Overview

After you establish a set of scientific exposures, as well as any additional target acquisition or calibration exposures required for your program, you are ready to determine the total number of orbits to request. Generally, this straightforward exercise involves compiling the overheads for individual exposures, packing exposure plus overhead time into individual orbits, and tallying up the results to determine the total number of orbits needed. It may be an iterative process as you refine your exposures to better use the orbit visibility times.

The Phase I Call for Proposals (http://www.stsci.edu/hst/hst/proposing/documents/cp/cp_cover.html) includes proposal instructions that provide information on the Observatory policies and practices with respect to orbit time requests. The HST Primer (http://www.stsci.edu/hst/proposing/docs/proposing/documents/cp/primer.pdf) 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 a Phase I Proposal.

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.

8.3 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.

8.3.1 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 seconds 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 8.3: Orbit calculation for example 1.

Action	Time (minutes)	Explanation
Orbit 1
Initial guide-star acquisition	6	Needed at start of observation of a new target
WFC overhead for the first exposure	4.0	Includes filter change, camera set-up, and readout
First science exposure	20.0
WFC overhead for the subsequent science exposure in the series	2.5	Includes readout
The next science exposure in the series	20.0
Total time for science exposures	40.0
Total used time in the orbit	52.6

Thus, the two WFC exposures totaling 2400 seconds 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.

8.3.2 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-SPLIT exposures. The ETC has shown that at the minimally accepted signal-to-noise ratio the exposure time must be 540 seconds for each of the filters, so each of the CR-SPLITs must be at least 270 seconds long. For the target declination, we find that the visibility time is 55 minutes. The time budget for the orbit is then as follows:

Table 8.4: Orbit calculation for example 2.

Action	Time (minutes)	Explanation
Orbit 1
Initial guide-star acquisition	6	Needed at start of observation of a new target
WFC overhead for the first exposures in two series	2 ¥ 4 = 8	Includes filter change, camera set-up, and readout
WFC overhead for subsequent exposures in each of the two series	2 ¥ 2.5 = 5	Includes readout
Additional overhead for all but the last exposures in the orbit	5.8 ¥ 3= ~17	Needed to dump the buffer because the next exposure is too short (> 339 seconds) to accommodate the dump time.
Science exposures	4 ¥ 4.5 = 18
Total time for science exposures	18
Total used time in the orbit	54

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 seconds, we avoid these additional overheads. This would free ~17 minutes of the orbit time for science, which allows us to almost double the science exposure time (35 minutes instead of 18 minutes) 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 < 339 seconds) exposures would save 176 seconds in readout time and 1047 seconds in dump time. This frees up ~20 minutes of orbit time to be used for science.

8.3.3 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 minutes. in F606W and 30 minutes. in F250W. From the orbit visibility table (see the HST Primer) we find that at the target declination of 15× the target visibility time is 52 minutes. With CR-SPLIT=2, we thus have to accommodate in that period 35 minutes. of four science exposures grouped in two series. The orbit calculation goes like this:

Table 8.5: Orbit calculation for example 3.

Action	Time (minutes)	Explanation
Orbit 1
Initial guide-star acquisition	6	Needed at start of observation of a new target
Target acquisition	3.5 + (2 ¥ 0.1) = 3.7	Point-source acquisition on target
HRC overhead for the first exposures in the series	2 ¥ 2.5 = 5	Includes filter change, camera set-up, and readout
HRC overhead for the subsequent exposures in the series	2 ¥ 1 = 2	Includes readout
Science exposures in F606W	2 ¥ 2.5 = 5
Science exposures in F250W	2 ¥ 15 = 30
Total time for science exposures	35
Total used time in the orbit	~52

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 minutes of unused time, we can adjust our exposure times to make full use of the available time.

8.3.4 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 arcseconds/pixel, this requires that the offsets in the dithering pattern are larger than 2.5 arcseconds. Each offset will then take 0.5 minutes 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 minutes. The visibility time at our target declination is 58 minutes. In this observation we do not want to rely on cosmic ray removal provided by the dithering data reduction package, and set CR-SPLIT=2 to 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:

Table 8.6: Orbit calculation for example 4.

Action	Time (minutes)	Explanation
Orbit 1
Initial guide-star acquisition	6	Needed at start of observation of a new target
WFC overhead for the first exposures in the series	4	Includes filter change, camera set-up, and readout
WFC overhead for the subsequent 3 exposures in the series	3 ¥ 2.5 = 7.5	Includes readout
Spacecraft slew	0.5	To offset from the first to the second pointing
Four science exposures	4 ¥ 10 = 40	Exposures at the first two pointings in the dither pattern
Total time for science exposures	40
Total used time in the orbit	58
Orbit 2
Guide-star re-acquisition	~6	Needed at start of a new orbit to observe the same target
WFC overhead for the remaining exposures in the series	4 ¥ 2.5 = 10	Includes readout
Spacecraft slews	2 ¥  0.5 = 1	To offset to the third and fourth pointings
Four science exposures	4 ¥ 10 = 40	Exposures at the second two pointings in the dither pattern
Total time for science exposures	40
Total used time in the orbit	57

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 should 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.

8.3.5 Sample Orbit Calculation 5:

This example illustrates the orbit calculation for a simple 30 minutes WFC grism spectroscopic observation broken down by CR-SPLIT=2 into a series of two exposures.

Table 8.7: Orbit calculation for example 5.

Action	Time (minutes)	Explanation
Orbit 1
Initial guide-star acquisition	6	Needed at start of observation of a new target
Predefined imaging exposure for grism spectroscopy	7	Needed to co-locate the targets and their spectra in the FOV
WFC overhead for the first science exposure in the series	4.0	Includes filter change, camera set-up, and readout
WFC overhead for the subsequent science exposure in the series	2.5	Includes readout
Two science exposures	2 ¥ 15.0 = 30
Total science exposure time	30.0
Total used time in the orbit	49.5

Unlike similar imaging exposures, here we have to take into account an additional imaging exposure before the sequence of spectroscopic exposures, which takes 10 minutes. off the available orbit time.