14.3 Mode = MULTIACCUM
Config = WFC3/IR

MULTIACCUM is the only observing mode for the IR channel. An exposure in MULTIACCUM mode begins with an array reset followed by an initial readout. Next, one or more nondestructive readouts are obtained at user-selectable times. All of the readouts, including the initial readout, are recorded onboard and returned to the ground for analysis. The difference between each successive pair of reads is the image data accumulated between reads.

There are two major advantages of this approach. First, the multiple readouts provide a way to record what is happening in a pixel before it saturates, increasing dynamic range. Second, the multiple readouts can be compared to remove cosmic ray effects. See the WFC3 Instrument Handbook for more information.

14.3.1 Aperture or FOV

The specification of the FOV controls where the target is placed. The target will be placed at the default location within the specified FOV, unless the POSition TARGet exposure-level Special Requirement is used to specify an offset.

Valid apertures are defined in Table 14.3 below. The IR aperture is designed for placing targets at the "optimum center" of the detector. The default location within this aperture will be routinely adjusted by STScI to reflect any changes in detector performance. This aperture is appropriate for targets that are small compared to the scale size of defects in the chips.

The IR-FIX aperture defines the geometric center of the detector and will remain FIXED in aperture coordinates. This location will not be adjusted for changes in detector characteristics, and should be used to specify the location of the target relative to the detector. This geometric center aperture is appropriate for pointings designed to position an extended scene within the WFC3 FOV.

The IR aperture is required for exposures using the G102 or G141 spectral elements. In this case the STScI ground system will substitute a special aperture which has approximately the same pointing but is optimized for use with the selected grism. If an undispersed image exposure is taken in conjunction with the grism exposure, it is recommended that the appropriate grism reference aperture (G102-REF or G141-REF) be used on the undispersed image exposure to provide an optimal pointing offset between the two exposures.

IR subarrays are specified by selecting the appropriate aperture. The IRSUBnn or IRSUBnn-FIX apertures will result in the use of the subarray readout mode of the IR detector with the size of the subarray being that indicated by the aperture name. The use of the subarray readout mode will result in different sample times than listed in Table 14.4; see the discussion under the SAMP-SEQ optional parameter for more details. The subarray readouts will have a border of five reference pixels added around the edge of the subarray used for imaging, making the total data sizes 1024x1024 (full-frame), 74x74, 138x138, 266x266, and 522x522 pixels.

The IRSUBnn apertures will place the target at the "optimum center" of the corresponding subarray; note that these positions may, in principle, be different for the different subarrays. The default position of each of these apertures will be updated by STScI to reflect changes in instrument performance. These apertures are appropriate for targets which are small compared to the scale size of defects on the appropriate for targets which are small compared to the scale size of defects on the chips.

The IRSUBnn-FIX apertures defines the geometric center of the subarray and will remain fixed in aperture coordinates. These locations will not be adjusted for changes in detector performance.chips.

14.3.2 Spectral Elements

See Table 14.6: Spectral Elements for use with WFC3/IR Configurations.

14.3.3 Wavelength

This parameter should be left blank.

14.3.4 Optional Parameters

SAMPSEQ
=RAPID, SPARS10, SPARS25, SPARS50, SPARS100, SPARS200, STEP25, STEP50, STEP100, STEP200, STEP400, MIF600, MIF900, MIF1200, MIF1500

A required parameter specifying the name of a predefined sequence of times from the start of the exposure at which the nondestructive readouts (samples) are performed. The number of readouts (up to 15, plus one for the initial readout) taken for each exposure is controlled by the NSAMP parameter (see below).

Table 14.4 gives the sample times (defined as the time from the start of the initial readout to the start of a given readout) for each sequence and image size. Four different types of sequences are provided. The RAPID sequence provides linear sampling as fast as possible (limited by the readout time for the selected image size) and is intended for bright targets that could saturate in the other sample sequences.

Sequences STEP25, STEP50, STEP100, STEP200, and STEP400 begin with four rapid samples (five readouts), switch to logarithmic spacing up to the given number of seconds (25-400), and then continue with linear spacing for the remainder of the sequence with adjacent steps separated by 25-400 seconds depending on the selected sequence. These sequences are intended to compensate for any nonlinearities near the start of the exposure and to provide increased dynamic range for images that contain both faint and bright targets.

Sequences SPARS10, SPARS25, SPARS50, SPARS100, and SPARS200 begin with one rapid sample (two readouts) then provide linear spacing to allow observers to "read up the ramp" at evenly spaced intervals. The variety of sampling intervals allows this basic strategy to be applied over a wide range in target flux.

Sequences MIF600, MIF900, MIF1200, and MIF1500 provide pseudo-Fowler sampling (described in Chapter 7 of the WFC3 Instrument Handbook) of the signal, using total exposure times of 600 seconds through 1500 seconds in 300 second increments. They start with a series of rapid readouts (7 reads, but the initial read is to be ignored in processing due to potential reset anomalies), followed by three widely spaced readouts, and finish with six rapid readouts at the end of the exposure. The six initial useful readouts and the six final readouts are to be averaged then differenced to form the final difference image. The middle reads are provided for cosmic ray and saturation detection. These sequences should always be specified with NSAMP = 15.

The different sequences are designed to efficiently fill the target visibility period with one, two, or several exposures. See the WFC3 Instrument Handbook for recommendations on which sequences to use in different situations.

NSAMP
=1-15

A required parameter specifying the number of samples in a predefined sequence that should actually be taken, not counting the initial readout. Table 14.4 defines 15 sample times for each sequence. If an NSAMP value smaller than 15 is used, samples will be taken at only the first NSAMP times from this table.

The total number of readouts will be NSAMP plus one for the initial readout, giving a maximum of 16 readouts for a single execution of a MULTIACCUM exposure. Each readout will be recorded and will appear in the final data set.

14.3.5 Number of Iterations

Enter the number of times this exposure should be iterated, and the duration in seconds of each iteration. This option should be used in observational situations when two or more identical exposures should be taken of the same field. If the Number_Of_Iterations is n, the exposure will be iterated n times.

14.3.6 Time Per Exposure

Time_Per_Exposure must be DEF in this Mode. The exposure time is unnecessary, because it is specified by SAMPSEQ and NSAMP.

Table 14.4 shows the sequence of 15 sample times corresponding to the different SAMP-SEQ values, in seconds from the start of the initial readout to the start of the readout for the given sample. These values are given to the nearest millisecond

SAMP-SEQ=SPARS10 

	Image Size
Sample Number	1024x1024	512x512	256x256	128x128	64x64
1	2.932	0.853	0.278	0.113	0.061
2	12.933	8.775	7.624	7.294	7.190
3	22.934	16.696	14.971	14.475	14.320
4	32.935	24.618	22.317	21.657	21.449
5	42.936	32.540	29.664	28.838	28.579
6	52.937	40.462	37.010	36.020	35.708
7	62.938	48.383	44.357	43.201	42.837
8	72.939	56.305	51.703	50.382	49.967
9	82.940	64.227	59.050	57.564	57.096
10	92.941	72.148	66.396	64.745	64.226
11	102.942	80.070	73.743	71.926	71.355
12	112.943	87.992	81.089	79.108	78.485
13	122.944	95.913	88.436	86.289	85.614
14	132.945	103.835	95.782	93.471	92.744
15	142.946	111.757	103.129	100.652	99.873

SAMP-SEQ= SPARS25 

	Image Size
Sample Number	1024x1024	512x512	256x256	128x128	64x64
1	2.932	0.853	0.278	0.113	0.061
2	27.933	23.774	22.624	22.293	22.190
3	52.933	46.696	44.970	44.475	44.319
4	77.934	69.617	67.316	66.655	66.448
5	102.934	92.538	89.662	88.836	88.577
6	127.935	115.459	112.008	111.017	110.706
7	152.935	138.381	134.354	133.198	132.835
8	177.936	161.302	156.700	155.379	154.964
9	202.936	184.223	179.046	177.560	177.093
10	227.937	207.144	201.392	199.741	199.222
11	252.937	230.066	223.738	221.922	221.351
12	277.938	252.987	246.084	244.103	243.480
13	302.938	275.908	268.430	266.284	265.609
14	327.939	298.829	290.776	288.465	287.738
15	352.940	321.751	313.122	310.646	309.867

SAMP-SEQ=SPARS50 

	Image Size
Sample Number	1024x1024	512x512	256x256	128x128	64x64
1	2.932	0.853	0.278	0.113	0.061
2	52.933	48.774	47.624	47.294	47.190
3	102.933	96.695	94.970	94.474	94.319
4	152.934	144.617	142.316	141.655	141.448
5	202.934	192.538	189.662	188.836	188,577
6	252.935	240.459	237.008	236.017	235.705
7	302.935	288.380	284.354	283.198	282.834
8	352.935	336.301	331.700	330.379	329.963
9	402.936	384.223	379.046	377.560	377.092
10	452.936	432.144	426.392	424.740	424.221
11	502.937	480.065	473.738	471.921	471.350
12	552.937	527.986	521.084	519.102	518.479
13	602.938	575.907	568.430	566.283	565.608
14	652.938	623.828	615.775	613.464	612.737
15	702.939	671.750	663.121	660.645	659.866

SAMP-SEQ=SPARS100 

	Image Size
Sample Number	1024x1024	512x512	256x256	128x128	64x64
1	2.932	0.853	0.278	0.113	0.061
2	102.933	98.774	97.624	97.293	97.190
3	202.933	196.695	194.970	194.474	194.318
4	302.933	294.616	292.315	291.655	291.447
5	402.934	392.537	389.661	388.836	388.576
6	502.934	490.458	487.007	486.016	485.705
7	602.934	588.379	584.353	583.197	582.884
8	702.935	686.300	681.699	680.378	679.962
9	802.935	784.221	779.045	777.559	777.091
10	902.935	882.143	876.390	874.739	874.220
11	1002.936	980.064	973.736	971.920	971.349
12	1102.936	1077.985	1071.082	1069.101	1068.478
13	1202.936	1175.906	1168.428	1166.282	1165.606
14	1302.936	1273.827	1265.774	1263.462	1262.735
15	1402.937	1371.748	1363.120	1360.643	1359.864

SAMP-SEQ=SPARS200 

	Image Size
Sample Number	1024x1024	512x512	256x256	128x128	64x64
1	2.932	0.853	0.278	0.113	0.061
2	202.932	198.774	197.623	197.293	197.189
3	402.932	396.695	394.969	394.474	394.318
4	602.932	594.615	592.315	591.654	591.446
5	802.933	792.536	789.660	788.835	788.575
6	1002.933	990.457	987.006	986.015	985.704
7	1202.933	1188.378	1184.351	1183.196	1182.832
8	1402.933	1386.299	1381.697	1380.376	1379.961
9	1602.933	1584.219	1579.043	1577.557	1577.089
10	1802.933	1782.140	1776.388	1774.737	1774.218
11	2002.933	1980.061	1973.734	1971.918	1971.346
12	2202.933	2177.982	2171.079	2169.098	2168.475
13	2402.933	2375.903	2368.425	2366.278	2365.603
14	2602.933	2573.823	2565.770	2563.459	2562.732
15	2802.933	2771.774	2763.116	2760.639	2759.860

SAMP-SEQ=STEP25 

	Image Size
Sample Number	1024x1024	512x512	256x256	128x128	64x64
1	2.932	0.853	0.278	0.113	0.061
2	5.865	1.706	0.556	0.225	0.122
3	8.797	2.559	0.833	0.338	0.182
4	11.729	3.412	1.111	0.451	0.243
5	24.230	13.833	10.957	10.132	9.872
6	49.230	36.755	33.303	32.313	32.001
7	74.231	59.676	55.649	54.494	54.130
8	99.231	82.597	77.995	76.675	76.259
9	124.232	105.518	100.341	98.855	98.388
10	149.232	128.440	122.688	121.036	120.517
11	174.233	151.361	145.034	143.217	142.646
12	199.233	174.282	167.380	165.398	164.775
13	224.234	197.203	189.726	187.579	186.904
14	249.234	220.125	212.072	209.760	209.033
15	274.235	243.046	234.418	231.941	231.162

SAMP-SEQ=STEP50 

	Image Size
Sample Number	1024x1024	512x512	256x256	128x128	64x64
1	2.932	0.853	0.278	0.113	0.061
2	5.865	1.706	0.556	0.225	0.122
3	8.797	2.559	0.833	0.338	0.182
4	11.729	3.412	1.111	0.451	0.243
5	24.230	13.833	10.957	10.132	9.872
6	49.230	36.755	33.303	32.313	32.001
7	99.231	84.676	80.649	79.494	79.130
8	149.231	132.597	127.995	126.674	126.259
9	199.232	180.518	175.341	173.855	173.388
10	249.232	228.439	222.687	221.036	220.517
11	299.232	276.361	270.033	268.217	267.646
12	349.233	324.282	317.379	315.398	314.775
13	399.233	372.203	364.725	362.579	361.904
14	449.234	420.124	412.071	409.760	409.033
15	499.234	468.045	459.417	456.940	456.162

SAMP-SEQ=STEP100 

	Image Size
Sample Number	1024x1024	512x512	256x256	128x128	64x64
1	2.932	0.853	0.278	0.113	0.061
2	5.865	1.706	0.556	0.225	0.122
3	8.797	2.559	0.833	0.338	0.182
4	11.729	3.412	1.111	0.451	0.243
5	24.230	13.833	10.957	10.132	9.872
6	49.230	36.755	33.303	32.313	32.001
7	99.231	84.676	80.649	79.494	79.130
8	199.231	182.597	177.995	176.674	176.259
9	299.231	280.518	275.341	273.855	273.388
10	399.232	378.439	372.687	371.036	370.516
11	499.232	476.360	470.033	468.217	467.645
12	599.232	574.281	567.379	565.397	564.774
13	699.233	672.202	664.724	662.578	661.903
14	799.233	770.123	762.070	759.759	759.032
15	899.233	868.044	859.416	856.939	856.161

SAMP-SEQ=STEP200 

	Image Size
Sample Number	1024x1024	512x512	256x256	128x128	64x64
1	2.932	0.853	0.278	0.113	0.061
2	5.865	1.706	0.556	0.225	0.122
3	8.797	2.559	0.833	0.338	0.182
4	11.729	3.412	1.111	0.451	0.243
5	24.230	13.833	10.957	10.132	9.872
6	49.230	36.755	33.303	32.313	32.001
7	99.231	84.676	80.649	79.494	79.130
8	199.231	182.597	177.995	176.674	176.259
9	399.231	380.518	375.341	373.855	373.387
10	599.231	578.438	572.686	571.035	570.516
11	799.231	776.359	770.032	768.216	767.645
12	999.231	974.280	967.378	965.396	964.773
13	1199.231	1172.201	1164.723	1162.577	1161.902
14	1399.231	1370.122	1362.069	1359.757	1359.030
15	1599.231	1568.043	1559.414	1556.938	1556.159

SAMP-SEQ=STEP400 

	Image Size
Sample Number	1024x1024	512x512	256x256	128x128	64x64
1	2.932	0.853	0.278	0.113	0.061
2	5.865	1.706	0.556	0.225	0.122
3	8.797	2.559	0.833	0.338	0.182
4	11.729	3.412	1.111	0.451	0.243
5	24.230	13.833	10.957	10.132	9.872
6	49.230	36.755	33.303	32.313	32.001
7	99.231	84.676	80.649	79.494	79.130
8	199.231	182.597	177.995	176.674	176.259
9	399.231	380.518	375.341	373.855	373.387
10	799.232	778.439	772.687	771.036	770.516
11	1199.232	1176.360	1170.033	1168.217	1167.646
12	1599.233	1574.282	1567.379	1565.398	1564.775
13	1999.233	1972.203	1964.725	1962.579	1961.904
14	2399.234	2370.124	2362.071	2359.760	2359.033
15	2799.235	2768.046	2759.417	2756.941	2756.162

SAMP-SEQ=MIF600 

	Image Size
Sample Number	1024x1024	512x512	256x256	128x128	64x64
1	2.932	0.853	0.278	0.113	0.061
2	5.865	1.706	0.556	0.225	0.122
3	8.797	2.559	0.833	0.338	0.182
4	11.729	3.412	1.111	0.451	0.243
5	14.661	4.265	1.389	0.564	0.304
6	17.594	5.118	1.667	0.676	0.365
7	159.531	144.976	140.950	139.794	139.430
8	301.468	284.834	280.232	278.911	278.496
9	443.405	424.692	419.515	418.029	417.562
10	585.343	564.550	558.798	557.147	556.627
11	588.275	565.403	559.076	557.259	556.688
12	591.207	566.256	559.353	557.372	556.749
13	594.139	567.109	559.631	557.485	556.810
14	597.072	567.962	559.909	557.598	556.871
15	600.004	568.815	560.187	557.710	556.931

SAMP-SEQ=MIF900 

	Image Size
Sample Number	1024x1024	512x512	256x256	128x128	64x64
1	2.932	0.853	0.278	0.113	0.061
2	5.865	1.706	0.556	0.225	0.122
3	8.797	2.559	0.833	0.338	0.182
4	11.729	3.412	1.111	0.451	0.243
5	14.661	4.265	1.389	0.564	0.304
6	17.594	5.118	1.667	0.676	0.365
7	234.531	219.976	215.949	214.794	214.430
8	451.468	434.834	430.232	428.911	428.496
9	668.405	649.691	644.515	643.029	642.561
10	885.342	864.549	858.797	857.146	856.627
11	888.274	865.402	859.075	857.259	856.687
12	891.206	866.255	859.353	857.371	856.748
13	894.139	867.108	859.631	857.484	856.809
14	897.071	867.961	859.908	857.597	856.870
15	900.003	868.814	860.186	857.710	856.931

SAMP-SEQ=MIF1200 

	Image Size
Sample Number	1024x1024	512x512	256x256	128x128	64x64
1	2.932	0.853	0.278	0.113	0.061
2	5.865	1.706	0.556	0.225	0.122
3	8.797	2.559	0.833	0.338	0.182
4	11.729	3.412	1.111	0.451	0.243
5	14.661	4.265	1.389	0.564	0.304
6	17.594	5.118	1.667	0.676	0.365
7	309.531	294.976	290.949	289.793	289.430
8	601.467	584.833	580.232	578.911	578.495
9	893.404	874.691	869.514	868.028	867.561
10	1185.341	1164.548	1158.796	1157.145	1156.626
11	1188.273	1165.401	1159.074	1157.258	1156.687
12	1191.206	1166.255	1159.352	1157.371	1156.747
13	1194.138	1167.108	1159.630	1157.483	1156.808
14	1197.070	1167.961	1159.908	1157.596	1156.869
15	1200.003	1168.814	1160.185	1157.709	1156.930

SAMP-SEQ=MIF1500 

	Image Size
Sample Number	1024x1024	512x512	256x256	128x128	64x64
1	2.932	0.853	0.278	0.113	0.061
2	5.865	1.706	0.556	0.225	0.122
3	8.797	2.559	0.833	0.338	0.182
4	11.729	3.412	1.111	0.451	0.243
5	14.661	4.265	1.389	0.564	0.304
6	17.594	5.118	1.667	0.676	0.365
7	384.530	369.976	365.949	364.793	364.430
8	751.467	734.833	730.231	728.910	728.495
9	1118.404	1099.690	1094.513	1093.027	1092.560
10	1485.340	1464.548	1458.796	1457.144	1456.625
11	1488.273	1465.401	1459.073	1457.257	1456.686
12	1491.205	1466.254	1459.351	1457.370	1456.747
13	1494.137	1467.107	1459.629	1457.483	1456.807
14	1497.069	1467.960	1459.907	1457.595	1456.868
15	1500.002	1468.813	1460.185	1457.708	1456.929