Instrument Science Report NICMOS-017
Supported MULTIACCUM Sequences
John W. MacKenty and Luis Colina
November 21, 1996
ABSTRACT
In this ISR we define a specific set of MULTIACCUM mode exposure time sequences. The
selected sequences are flexible and cover the great majority of MULTIACCUM options
from linear to logarithmic stepping, and from short fast exposures to long exposures covering a full orbit. We also define a modification to the RPS2 Proposal Editor (PED) which
greatly simplifies the input of these MULTIACCUM exposure time sequences during phase
II proposal implementation. These MULTIACCUM sequences will be calibrated preferentially by STScI and their use will avoid the interpolation of the darks during the reduction
process. We strongly encourage observers to use these recommended sequences whenever
practical.
1. Introduction
Ground testing of NICMOS has led the NICMOS IDT to recommend that interpolation of
dark current calibration exposures be avoided whenever possible. While STScI has previously defined a specific set of ACCUM mode exposure times, this has not yet been done
for MULTIACCUM mode exposure times. Due to clocking constraints, NICMOS MULTIACCUM exposure times have values that need a lot of significant figures, but a large
number of times are possible. In order to simplify the phase II proposal implementation,
and to avoid any undesirable interpolation of dark current calibration exposures, we
present in this ISR a set of 16 MULTIACCUM sequences which should cover the great
majority of users’ observations.
2. Types of Sequences
In consultation with Marcia Rieke and Glenn Schneider of the NICMOS IDT, we have
defined the MULTIACCUM exposure time sequences listed in Table 1. As discussed in
Section 3, these sequences may be truncated by observers to achieve their desired maximum exposure time. Three flavors of sequence are defined:
1
•
Sequences with rapid readouts which obtain the densest temporal sampling operating
either a single camera (SCAMRR), or two or three cameras in parallel (MCAMRR).
•
Sequences with linear step sizes. These sequences are valid for single and parallel
observations with any of the cameras. The sequences each include 3 readouts during
the first second to permit the (possible) compensation for non-linear effects, and the
subsequent stepping is 1 second (STEP1), or 2 seconds (STEP2). Additional
sequences consist of 2 readouts during the first second, and a subsequent linear stepping of 64 seconds (SPARS64), or 256 seconds (SPARS256).
•
Sequences which combine logarithmic and linear step sizes. These sequences are valid
for single and parallel observations with any of the cameras. Each sequence includes 3
readouts during the first second to permit the (possible) compensation for non-linear
effects. Subsequent steppings are logarithmic up to 8 secs (STEP8), 16 secs (STEP16),
32 secs (STEP32), 64 secs (STEP64), 128 secs (STEP128), or 256 secs (STEP256)
and then linear in intervals of 8 secs (STEP8), 16 secs (STEP16), 32 secs (STEP32),
64 secs (STEP64), 128 secs (STEP128), or 256 secs (STEP256).
•
Sequences with 9 rapid readouts at both their start and end points to combine the readnoise reduction of the ACCUM MIF (i.e. ACCUM with Multiple Initial and Final
readouts, NREAD>1) strategy with the dynamic range benefits of multiple samples
during the integration. These sequences are valid for single and parallel observations
with any of the cameras. These sequences extend up to 512 seconds (MIF512), 1024
secs (MIF1024), 2048 secs (MIF2048), and 3072 seconds (MIF3072).
Table 1: MULTIACCUM Exposure Time Sequences
Sequence
Name
Type
SCAMRR
Single Camera Rapid Readout
MCAMRR
STEP1
Multiple Cameras Rapid Readout
Linear 1 seconds step
SAMP-TIME01 to SAMP-TIME25 (seconds)
0.203
0.406
0.609
0.812
1.015
1.218
1.421
1.624
1.827
2.030
2.233
2.436
2.639
2.842
3.045
3.248
3.451
3.654
3.857
4.060
4.263
4.466
4.669
4.872
5.075
0.303
0.606
0.909
1.212
1.515
1.818
2.121
2.424
2.727
3.030
3.333
3.636
3.939
4.242
4.545
4.848
5.151
5.454
5.757
6.060
6.363
6.666
6.969
7.272
7.575
0.303
0.606
0.995
1.993
2.991
3.989
4.987
5.985
6.983
7.981
2
Table 1: MULTIACCUM Exposure Time Sequences
Sequence
Name
STEP2
STEP8
STEP16
STEP32
STEP64
Type
Linear 2 seconds step
Log /8 seconds linear
Log/16 seconds linear
Log/32 seconds linear
Log/64 seconds linear
SAMP-TIME01 to SAMP-TIME25 (seconds)
8.979
9.977
10.975
11.973
12.971
13.969
14.967
15.965
16.963
17.961
18.959
19.957
20.955
21.953
22.951
0.303
0.606
0.995
1.993
3.987
5.981
7.975
9.969
11.963
13.957
15.951
17.945
19.939
21.933
23.927
25.921
27.915
29.909
31.903
33.897
35.891
37.885
39.879
41.873
43.867
0.303
0.606
0.995
1.993
3.987
7.981
15.975
23.969
31.963
39.957
47.951
55.945
63.939
71.933
79.927
87.921
95.915
103.909
111.903
119.897
127.891
135.885
143.879
151.873
159.867
0.303
0.606
0.995
1.993
3.987
7.981
15.975
31.969
47.963
63.957
79.951
95.945
111.939
127.933
143.927
159.921
175.915
191.909
207.903
223.897
239.891
255.885
271.879
287.873
303.867
0.303
0.606
0.995
1.993
3.987
7.981
15.975
31.969
63.969
95.969
127.969
159.969
191.969
223.969
255.969
287.969
319.969
351.969
383.969
415.969
447.969
479.969
511.969
543.969
575.969
0.303
0.606
0.995
1.993
3.987
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Table 1: MULTIACCUM Exposure Time Sequences
Sequence
Name
STEP128
STEP256
MIF512
MIF1024
MIF2048
Type
Log/128 seconds linear
Log/256 seconds linear
MIF 512 seconds
MIF 1024 seconds
MIF 2048 seconds
SAMP-TIME01 to SAMP-TIME25 (seconds)
7.981
15.975
31.969
63.969
127.967
191.965
255.963
319.961
383.959
447.957
511.955
575.953
639.951
703.949
767.947
831.945
895.943
959.941
1023.939
1087.937
0.303
0.606
0.995
1.993
3.987
7.981
15.975
31.969
63.969
127.967
255.961
383.955
511.949
639.943
767.937
895.931
1023.925
1151.919
1279.913
1407.907
1535.901
1663.895
1791.889
1919.883
2047.877
0.303
0.606
0.995
1.993
3.987
7.981
15.975
31.969
63.969
127.967
255.961
511.961
767.961
1023.961
1279.961
1535.961
1791.961
2047.961
2303.961
2559.961
2815.961
3071.961
3327.961
3583.961
3839.961
0.303
0.606
0.909
1.212
1.515
1.818
2.121
2.424
31.994
63.994
127.992
191.990
255.988
319.986
383.984
447.982
511.980
512.283
512.586
512.889
513.192
513.495
513.798
514.101
514.404
0.303
0.606
0.909
1.212
1.515
1.818
2.121
2.424
63.999
127.997
255.991
383.985
511.979
639.973
767.967
895.961
1023.955
1024.258
1024.561
1024.864
1025.167
1025.470
1025.773
1026.076
1026.379
0.303
0.606
0.909
1.212
1.515
4
Table 1: MULTIACCUM Exposure Time Sequences
Sequence
Name
MIF3072
SPARS64
SPARS256
Type
MIF 3072 seconds
Linear 64 seconds step
Linear 256 seconds step
SAMP-TIME01 to SAMP-TIME25 (seconds)
1.818
2.121
2.424
127.995
255.989
511.989
767.989
1023.989
1279.989
1535.989
1791.989
2047.989
2048.292
2048.595
2048.898
2049.201
2049.504
2049.807
2050.110
2050.413
0.303
0.606
0.909
1.212
1.515
1.818
2.121
2.424
127.995
255.989
639.988
1023.987
1407.986
1791.985
2175.984
2559.983
3071.981
3072.284
3072.587
3072.890
3073.193
3073.496
3073.799
3074.102
3074.405
0.303
0.606
63.994
127.992
191.990
255.988
319.986
383.984
447.982
511.980
575.978
639.976
703.974
767.972
831.970
895.968
959.966
1023.964
1087.962
1151.960
1215.958
1279.956
1343.954
1407.952
1471.950
0.303
0.606
255.996
511.996
767.996
1023.996
1279.996
1535.996
1791.996
2047.996
2303.996
2559.996
2815.996
3071.996
3327.996
3583.996
3839.996
4095.996
4351.996
4607.996
4863.996
5119.996
5375.996
5631.996
5887.996
3. Implementation for Phase II Proposal Writing
Since the entry of lengthly sequences of MULTIACCUM “SAMP-TIMES” into phase II
proposal logsheets is both tedious for our observers and prone to errors, RPS2 will have an
optional parameter to simplify the use of these predefined MULTIACCUM sequences.
This way, the sequences indicated in Table 1 are available to GOs using PED or standard
editors for phase II proposal preparation. The new optional parameter is called “SAMPSEQ” and can be given any of the names indicated in Table 1 above.
5
If SAMP-SEQ is used, the number of readouts in the sequence can be truncated by specifying the number of SAMP-TIMES with the new optional parameter NSAMP (= 1-25).
For example, if you want a STEP8 sequence with a maximum time of 120 seconds, you
would choose SAMP-SEQ= STEP8 and NSAMP= 20, and the actual translation of the
sequence into SAMP-TIMEs will be done by Transformation (the feasibility section of
RPS2), and therefore will be transparent. The number of readouts will be NSAMP plus
one for the initial readout, giving a maximum of 26 readouts for a single execution of a
MULTIACCUM exposure. Also note that if SAMP-SEQ is used, the SAMP-TIMEnn
optional parameters are illegal, and vice versa. This makes it easy to specify a predefined
sequence, and there is no chance of changing some of the SAMP-TIMEs in the sequences
listed in Table 1.
6