7.7	IR Exposure and Readout

7.7.1 Exposure Time

7.7.2 MULTIACCUM Mode

7.7.3 MULTIACCUM Timing Sequences: Full Array Apertures

7.7.4 MULTIACCUM Timing Sequences: Subarray Apertures

The general operating modes of IR detectors have been described in Chapter 5. In this section we will detail the readout modes implemented in WFC3.

7.7.1 Exposure Time

Unlike the UVIS channel, the IR channel does not have a mechanical shutter. Integration times are thus determined purely electronically, by resetting the charge on the detector, and then accumulating signal until the exposure is completed. A second major difference from the UVIS channel is that the IR detector can be read out non-destructively as the exposure accumulates, as opposed to the single destructive readout at the end of a CCD exposure.

There is a set of pre-defined accumulation and readout sequences available to IR observers, which are used to set the total exposure time as described in the next subsection.

7.7.2 MULTIACCUM Mode

In IR observing it is possible, and desirable, to sample the signal multiple times as an exposure accumulates, and the MULTIACCUM mode accomplishes this. MULTIACCUM is the only observing mode available for the IR channel.

Multiple readouts offer three major advantages. First, the multiple reads provide a way to record a signal in a pixel before it saturates, thus effectively increasing the dynamic range of the final image. Second, the multiple reads can be analyzed to isolate and remove cosmic-ray events. Third, fitting to multiple reads provides a means for reducing the net effective read noise, which is relatively high for the IR detector.

The disadvantage of multiple readouts is that they are data-intensive. The HgCdTe detector array is 1024×1024 pixels, which is only about 1/16 by pixel number of the 4096×4102 UVIS array. However, since up to 16 IR readouts are used, the total data volume of a single IR exposure approaches that of a single UVIS frame. A maximum of 32 IR full array readouts can be stored in the instrument buffer, after which the content of the buffer must be dumped to the HST Solid State Recorder (SSR). A buffer dump of 16 full array reads takes about 5.8 minutes.

MULTIACCUM readout consists of the following sequence of events:

1.	Array reset: After a fast calibration of the Analog to Digital Converters, all pixels are set to the detector bias level, with two rapid reset cycles of the entire chip.

2.

Array read: The charge in each pixel is measured and stored in the on-board computer's memory. This is done as soon as practical after the second array reset in step 1. In effect, given the short delay and the time needed to read the array, a very short-exposure image is stored in memory. This is known as the zero read.

3.

Multiple integration-read cycles: The detector integrates for a certain amount of time and then the charge in each pixel is read out. This step can be repeated up to a total of 15 times following the zero read during the exposure. All frames are individually stored in the on-board computer memory. Note that it takes a finite time (2.93 sec) to read the full array, so there is a time delay between reading the first and last pixel. Because this delay is constant for each read, it cancels out in difference images.

4.	Return to idle mode: The detector returns to idle mode, where it is continuously flushed in order to prevent charge build-up and to limit the formation of residual images.

All sequences start with the same “reset, reset, read, read” sequence, where the two reads are done as quickly as possible. This “double reset read” was originally implemented because the very first read after the reset may show an offset that does not reproduce in the following reads.

7.7.3 MULTIACCUM Timing Sequences: Full Array Apertures

RAPID Sequence

SPARS Sequences

STEP Sequences

There are 11 pre-defined sample sequences, optimized to cover a wide range of observing situations, available for the full-frame IR apertures. (See Section 7.7.4 for a discussion of the sample sequences available for the IR subarray apertures. The same names are used for the sample sequences, but the times are different.) The maximum number of reads (following the zero read) during an exposure is 15, which are collected as the signal ramps up. It is possible to select less than 15 reads, thus cutting short the ramp and reducing the total exposure time. However, the timing of the individual reads within any of the 11 sequences cannot be adjusted by the user. This approach has been adopted because optimal calibration of IR detectors requires a dedicated set of reference files (e.g., dark frames) for each timing pattern.

In summary, a WFC3/IR exposure is fully specified by choosing:

•	one of the 11 available pre-defined timing sequences, and

•	the total number of samples (NSAMP, which must be no more than 15), which effectively determines the total exposure time

The 11 timing sequences for the IR channel are:

•	One RAPID sequence: the detector is sampled with the shortest possible time interval between reads.

•	Five linear (SPARS) sequences: the detector is sampled with uniform time intervals between reads, a so-called “linear sample up the ramp.” (“SPARS” is a contraction of the word “sparse.”)

•	Five rapid-log-linear (STEP) sequences: the detector is initially sampled with the shortest possible time interval between reads, then uses logarithmically spaced reads to transition to a sequence of uniform samples.

All 11 of the sequences above refer to readouts of the full 1024×1024 detector array. See Section 7.7.4 below for the timing sequences available for subarrays. Details of the sequences are in the following paragraphs. The timings of the individual reads are given in Table 7.8.

RAPID Sequence

The RAPID sequence provides linear sampling at the fastest possible speed. For the full array, this means one frame every 2.9 s, and the entire set of 16 reads completed in less than 44 s. The RAPID mode is mainly intended for the brightest targets. Due to the overheads imposed by buffer dumps (see Chapter 10), observations in this mode done continuously would have low observing efficiency.

SPARS Sequences

The SPARS sequences use evenly spaced time intervals between reads. The five available SPARS sequences are designated SPARS10, SPARS25, SPARS50, SPARS100 and SPARS200, corresponding to sampling intervals of approximately 10, 25, 50, 100, and 200 s, respectively.

The SPARS modes can be regarded as the most basic readout modes, and they are applicable to a wide range of target fluxes. They provide flexibility in integration time and are well suited to fill an orbital visibility period with several exposures.

STEP Sequences

The five available rapid-logarithmic-linear sequences are STEP25, STEP50, STEP100, STEP200, and STEP400. They begin with linear spacing (the same as the RAPID sequence), continue with logarithmic spacing up to the given number of seconds (e.g., 50 s for STEP50), and then conclude with linear spacing in increments of the given number of seconds for the remainder of the sequence.

The STEP mode is intended to provide a more uniform sampling across a wide range of stellar magnitudes, which is especially important for imaging fields containing both faint and bright targets. The faint targets require a long, linearly sampled integration, while the bright targets need to be sampled several times early in the exposure, before they saturate. Thus, the dynamic range of the final image is increased.

Figure 7.7: Example of STEP sequence with NSAMP=4. NSAMP+1 images are stored in the observer’s FITS image.

Table 7.8: Sample times of 1024×1024 MULTIACCUM readouts in seconds. The information in this table can also be found in Table 13.4 of the Phase II Proposal Instructions.

NSAMP	RAPID (sec)		SPARS (sec)						STEP (sec)
			SPARS10	SPARS25	SPARS50	SPARS100	SPARS200		STEP25	STEP50	STEP100	STEP200	STEP400
1	2.932		2.932	2.932	2.932	2.932	2.932		2.932	2.932	2.932	2.932	2.932
2	5.865		12.933	27.933	52.933	102.933	202.932		5.865	5.865	5.865	5.865	5.865
3	8.797		22.934	52.933	102.933	202.933	402.932		8.797	8.797	8.797	8.797	8.797
4	11.729		32.935	77.934	152.934	302.933	602.932		11.729	11.729	11.729	11.729	11.729
5	14.661		42.936	102.934	202.934	402.934	802.933		24.230	24.230	24.230	24.230	24.230
6	17.594		52.937	127.935	252.935	502.934	1002.933		49.230	49.230	49.230	49.230	49.230
7	20.526		62.938	152.935	302.935	602.934	1202.933		74.231	99.231	99.231	99.231	99.231
8	23.458		72.939	177.936	352.935	702.935	1402.933		99.231	149.231	199.231	199.231	199.231
9	26.391		82.940	202.936	402.936	802.935	1602.933		124.232	199.232	299.231	399.231	399.231
10	29.323		92.941	227.937	452.936	902.935	1802.933		149.232	249.232	399.232	599.231	799.232
11	32.255		102.942	252.937	502.937	1002.936	2002.933		174.233	299.232	499.232	799.231	1199.232
12	35.187		112.943	277.938	552.937	1102.936	2202.933		199.233	349.233	599.232	999.231	1599.233
13	38.120		122.944	302.938	602.938	1202.936	2402.933		224.234	399.233	699.233	1199.231	1999.233
14	41.052		132.945	327.939	652.938	1302.936	2602.933		249.234	449.234	799.233	1399.231	2399.234
15	43.984		142.946	352.940	702.939	1402.937	2802.933		274.235	499.234	899.233	1599.231	2799.235

7.7.4 MULTIACCUM Timing Sequences: Subarray Apertures

As described in Section 7.4.4, subarrays are available in order to reduce data volume and enable short exposure times, defined in sample sequences. For a given sample sequence name, the sample times are shorter for smaller subarrays. However, only certain combinations of subarrays and sample sequences are supported by STScI. Other MULTIACCUM sequences can be used in principle but are not supported, and additional calibration observations would have to be made by the observer. The supported combinations are presented in Table 7.9. The exposure times may be found in the Phase II Proposal Instructions, Chapter 13 (Wide Field Camera 3).

See Section 13.3.6 of the GO Phase II Proposal Instructions, Table 13.4, for the sample times associated with each combination of sample sequence and subarray size. Note that the sample times for a given sample sequence name are shorter for smaller subarrays.

Certain combinations of IR subarrays and sample sequences give rise to images containing a sudden low-level jump in the overall background level of the image. The cause of the artifact is under investigation (see Section 7.4.4).

Table 7.9: Supported subarray sample sequences.

Aperture	Sample Sequence
	RAPID	SPARS10	SPARS25	STEP25
IRSUB64	yes	no	no	no
IRSUB64-FIX	yes	no	no	no
IRSUB128	yes	yes	no	no
IRSUB128-FIX	yes	yes	no	no
IRSUB256	yes	yes	yes	no
IRSUB256-FIX	yes	yes	yes	no
IRSUB512	yes	no	yes	yes
IRSUB512-FIX	yes	no	yes	yes