|WFC3 Instrument Handbook for Cycle 24|
10.3 10.3.1 Exposure OverheadsThe instrument-specific overhead times for WFC3 exposures are dominated by the time to move the filter wheels, to read out the detector, and especially to transfer and store the data. Although in a Phase II proposal the overheads computed with APT may prove to be smaller than the values given in this section, it is nevertheless important to plan your Phase I proposal using the conservative values given here in order to ensure the award of time adequate to attain your scientific goals.Several kinds of overhead activities are associated with each exposure, and the specific activities depend on whether the exposure is a new one, or part of a series of identical ones. Identical exposures are defined as second and subsequent exposures on the same target, with the same filter.For UVIS ACCUM exposures (i.e., CCD exposures), identical exposures are generated if the observer does any of the following: (1) specifies a CR-SPLIT number greater than 1; (2) does not specify the CR-SPLIT Optional Parameter, in which case it defaults to CR-SPLIT=2 for all exposures regardless of exposure time; (3) specifies the Special Requirement PATTERN (in order to dither, or to mosaic, a set of images); or (4) specifies a Number of Iterations greater than 1.For IR MULTIACCUM exposures, CR-splitting is not used, and only items (3) and (4) in the preceding list apply. Furthermore, unless otherwise specified, a full 16-read (i.e., 15-sample, specified with NSAMP=15) sequence will be obtained for each IR MULTIACCUM exposure. The full set of samples (i.e., images) is considered to be one exposure.At the end of each UVIS or IR exposure, images are read into WFC3's internal buffer memory, where they are stored until they are transferred to HST's solid-state data recorder. The time needed to read a UVIS CCD image is 98 seconds. The time needed for a single read of an IR image is 3 seconds, leading to a total of 48 seconds for a full 16-read exposure. These times are included in the overhead times for the first and subsequent exposures presented in Table 10.2 below.The WFC3 buffer provides temporary storage of images read from the WFC3 detectors before they are dumped through the HST science data formatter (SDF) to the solid state recorder (SSR). The buffer can be dumped either between exposures (a “serial” dump), or during an exposure (a “parallel” dump), but cannot overlap any commands executed in WFC3, including the commands at the beginning or at the end of an exposure. The buffer may be dumped during pointing maneuvers, but not during acquisition of guide stars. The buffer may be dumped during target occultation, which does not deduct from the target visibility time. Switching channels (IR and UVIS) does not require dumping the buffer. Observers will generally prefer to use parallel dumps, in order to more fully utilize the time when a target is visible for science exposures. Although buffer dumps are typically forced by science data volume, a buffer dump will also be forced whenever the buffer holds 304 image headers, regardless of the size of the images themselves. The 304-file limit is unlikely to be reached under typical conditions.The rules for dumping the buffer in parallel with UVIS exposures differ in some respects from those for dumping in parallel with IR exposures. The two channels are considered separately in the following paragraphs.The buffer can hold up to two full-frame UVIS images. A single full-frame image can be dumped in parallel with a UVIS exposure greater than 347 seconds, and two full-frame images can be dumped in parallel with a UVIS exposure greater than 663 seconds. When the buffer is dumped, all stored images must be dumped. Consequently, a sequence of 348-second (or longer) exposures will incur no overhead for dumping the buffer. Whether a sequence comprised of short exposures (less than 348 seconds) and long exposures (greater than 347 seconds) will require serial buffer dumps will depend upon the order of the long and short exposures and the duration of the long exposures. Dumping the buffer during a sequence of short and long exposures will be more efficient if the long exposures are 664 seconds (or longer). For example, an orbit with exposures with exposure times in the sequence 10-348-10-664-10-664-10 will incur no serial dump penalty. The observer will plan such sequences with APT in Phase II. Sequences of full-frame, un-binned exposures less than 348 seconds will require the overhead of serial buffer dumps. For short exposures, using subarrays or binning may be advantageous in order to reduce the overhead of serial buffer dumps. The time to dump a subarray or binned exposure scales approximately with the number of pixels stored in the buffer.The buffer can hold up to two full-frame, 15-sample IR images. To dump one such image in parallel with an IR exposure requires that exposure to be longer than 348 seconds. To dump two such images requires an exposure longer than 646 seconds. The rules for dumping IR exposures are somewhat more efficient than those for dumping UVIS exposures. For purposes of dumping IR exposures, each sample is treated individually; all samples in the buffer are not required to be dumped together; and samples can be dumped during the non-destructive read of the FPA. Sequences of full-frame, 15-sample exposures shorter than 349 seconds will require serial dumps after the second and subsequent exposures. Sequences of longer-exposure (i.e., greater than 348 seconds), full-frame, 15-sample exposures will incur no overhead for dumping the buffer. Sequences comprised of short (less than 349 seconds) full-frame, 15-sample exposures and long exposures (greater than 348 seconds) may incur overhead for serial dumps, depending upon the sequence of exposures and the duration of the long exposures. The observer will plan such sequences with APT in Phase II. The time to dump an n-sample, full-frame exposure is approximately 39 + 19 × (n + 1) seconds. Subarrays may also be used to reduce the overhead of serial buffer dumps.Both the UVIS and IR channels may be used during a single orbit, although not simultaneously. The time required to reconfigure between the two channels is 1 minute. If the buffer is full when switching channels, then time must also be taken to dump it before the exposure can begin with the other channel. Because the centers of the fields of view of the UVIS and IR channels are the same, acquisition of new guide stars is not required when changing channels to observe the same target.The overhead for each exposure includes an allowance for the time required to position the filter or grism; however, selecting a UVIS quad filter requires an additional 1 minute of overhead to re-position the telescope, as indicated in Table 10.1.Table 10.2 summarizes all of the instrument overheads described in this subsection.Table 10.2: WFC3 Instrument Overhead Times.
If your science program is such that a field of view smaller than the full detector size is adequate and you require many short exposures, then one way to reduce the frequency of buffer dumps, and hence their associated overheads, is to use a WFC3 subarray. Subarrays are described for the UVIS channel in Section 6.4.4, and for the IR channel in Section 7.4.4. When subarrays are used, only a small region of the detector is read out and stored in WFC3’s buffer. The reduced data volume permits a larger number of exposures to be stored in the buffer before the memory fills and it becomes necessary to transfer them to the telescope’s solid-state recorder. Use of subarrays reduces the amount of time spent dumping the buffer, and also usually reduces detector readout time. (Note, however, that the full-quadrant UVIS 2K2 and UVIS-QUAD-SUB apertures have somewhat longer readout times than the full- detector apertures because of the way that the readout is performed.) A dump is still required if the 304-file limit is reached before buffer memory is filled.The available subarrays are specified for the UVIS and IR channels by apertures with “SUB” in their names. See Sections 6.4.4 and 7.4.4 further details.The areas (ASA) of the supported UVIS subarrays are 1/4, 1/16, or 1/64 of the area (AFF) of a full-frame image. The areas of the IR subarrays are 1/4, 1/16, 1/64, or 1/256 of the area of a full-frame image. The number of subarray exposures that may be stored in the buffer, limited by image data volume, is n = 2 (AFF/ASA). For example, eight 1/4-area exposures may be stored in the buffer, which would allow eight 4-minute exposures to be taken and stored before having to dump the buffer. If the exposures were full-frame, the buffer would have to be dumped after each pair of observations, thus leading to very low observing efficiency.The 304-file limit must also be considered in optimizing buffer dumps. For UVIS exposures, the limit will almost never be encountered. For IR exposures, each read (not each exposure) counts against the limit. The number of IR exposures that can be stored before a buffer dump is forced is therefore n = 304/(NSAMP +1), or 19 exposures for NSAMP = 15.In the IR channel, certain combinations of subarrays and sample sequences give rise to images containing a sudden low-level jump in the overall background level of the image (see Section 7.4.4).Data volume and overhead time can also be reduced for UVIS images by using on- chip binning of adjacent pixels, as described in Section 6.4.4. By using 2×2 pixel binning, the data volume is reduced by a factor of 4, although the readout time is only reduced by about a factor of 2 to 50 sec. For 3×3 pixel binning it is reduced by a factor of 9, and the readout time by a factor of 4 to 23 s. IR readouts cannot be binned, but data volume may be reduced by taking less than the default 15 samples during an exposure.