Instrument Science Report NICMOS-2002-004
SM3B Science Flats
L. Mazzuca, A. Schultz
October 22, 2002
ABSTRACT
This report describes the results from the NICMOS pointed flat-field calibration program
for SM3B. During programs 8998, 9557, and 9327, multiple sky background and lamp-on
exposures were obtained for all filters with all cameras. Analysis of the DQE reveals that
the relative countrates are at expected levels with respect to the operating temperature of
77K and are all within 8% of the ratio of modeled QE at 77K-to-62K performed during the
1997/1998 warm-up program. The resulting lamp-on minus background images were normalized to create the final calibration reference files.
Introduction
As part of the NICMOS SM3B calibration program, on-orbit sequences of NICMOS camera flat-fields were obtained to determine the relative changes of the detector quantum
efficiency (DQE) for all detectors. Since the DQE changes as a function of temperature,
the sensitivity through all filters in all cameras is affected by the higher operating temperature of 77K with the NICMOS cooling system (NCS). Relative changes of the DQE were
measured from flat-field exposures generated from a pair of “lamp-on” and “lamp-off”
exposures for each filter. Both are exposures of the random sky through a particular filter,
but the lamp-on has the additional signal from a flat-field calibration lamp. At each dithered position, repeats were obtained for lamp-off, followed by the same pattern plus
repeats with the lamp-on. The DQE increase is calculated by differencing the two types of
exposures.
Instrument Science Report NICMOS-2002-004
Observations
During SM3B, NICMOS calibration program 8985 obtained flat-field images of several
filters for each of the three cameras. Program 9557 continued flat-field observations for
Camera 3 narrow band filter and grism flat-field observations. Later in the cycle, program
9327 obtained narrow band filter flat-fields for Camera 1. All three calibration programs
executed as Continuous Viewing Zone (CVZ) programs on May 18th, June 10th,, and
August 29th 2002, respectively. Multiple exposures were taken of background and lampon observations, equally. Exposure times for all observations using a given filter were
identical. Background observations were obtained first followed by lamp-on observations.
The data were obtained using MULTIACCUM mode. Table 1 presents the specifics for the
observations.
Table 1. Filters used for Calibration Programs 8985, 9557, & 9327
SAMP-SEQ
# Exposures
(lamp off + on)
nic1 f090m
STEP32
10
95.96
8985
nic1 f095n
STEP128
10
895.92
9327
nic1 f097n
STEP128
10
767.93
9327
nic1 f108n
STEP128
10
895.92
9327
nic1 f110m
STEP32
10
95.96
8985
nic1 f110w
STEP8
20
3.196
8985
nic1 f113n
STEP128
10
767.93
9327
nic1 140w
STEP8
20
23.96
8985
nic1 f145m
STEP32
10
95.9
8985
nic1 f160w
STEP16
10
63.95
8985
nic1 f164n
STEP128
10
767.93
9327
nic1 f165m
STEP64
10
127.96
9327
nic1 f166n
STEP128
10
767.93
9327
nic1 f170m
STEP64
10
127.96
8985
nic1 f187n
STEP128
10
895.92
9327
nic1 f190n
STEP128
10
767.93
9327
nic1 pol0S
STEP32
10
95.96
8985
nic1 pol120S
STEP32
10
95.96
8985
nic1 pol240S
STEP32
10
95.96
8985
Camera/Filter
2
Exposure
Time (s)
Program
Instrument Science Report NICMOS-2002-004
SAMP-SEQ
# Exposures
(lamp off + on)
nic2 f110w
STEP1
20
7.97
8985
nic2 f165m
STEP8
20
3.196
8985
nic2 f171m
STEP32
10
95.96
8985
nic2 f180m
STEP32
10
95.96
8985
nic2 f187n
STEP128
10
383.95
8985
nic2 f187w
STEP8
20
39.95
8985
nic2 f190n
STEP128
10
383.95
8985
nic2 f204m
STEP64
10
127.96
8985
nic2 f205w
STEP8
20
23.96
8985
nic2 f207m
STEP32
10
95.96
8985
nic2 f222m
STEP32
10
95.96
8985
nic2 f237m
STEP32
10
95.96
8985
nic2 pol0L
STEP32
10
95.96
8985
nic2 pol120L
STEP32
10
95.96
8985
nic2 pol240L
STEP32
10
95.96
8985
nic3 f108n
STEP16
18
31.96
9557
nic3 f110w
STEP1
10
3.989
9327
nic3 f113n
STEP16
16
31.96
9557
nic3 f150w
STEP1
10
3.989
8985
nic3 f160w
STEP1
10
22.951
8985
nic3 f164n
STEP16
16
31.96
9557
nic3 f166n
STEP16
16
31.96
9557
nic3 f175w
STEP1
10
3.989
8985
nic3 f187n
STEP16
18
31.96
9557
nic3 f190n
STEP16
16
31.96
9557
nic3 f196n
STEP16
16
31.96
9557
nic3 f200n
STEP16
16
31.96
9557
nic3 f212n
STEP16
16
31.96
9557
nic3 f215n
STEP16
16
31.96
9557
nic3 f222m
STEP2
10
13.957
9327
Camera/Filter
3
Exposure
Time (s)
Program
Instrument Science Report NICMOS-2002-004
SAMP-SEQ
# Exposures
(lamp off + on)
STEP2
10
11.963
8985
nic3 g096
MCAMRR
14
2.72
9557
nic3 g141
MCAMRR
14
1.81
9557
nic3 g206
MCAMRR
14
1.81
9557
Camera/Filter
nic3 f240m
Exposure
Time (s)
Program
Detector Quantum Efficiency (DQE)
The 8985/ 9557/9327 flat-field observations were used to determine the DQE for the current operating temperature of 77K. One representative background and lamp-on exposure
per filter was used for analysis. The second column of Table 2 presents the median countrates computed from differencing the two exposures. The countrates were calculated
using the integration times and median fluxes in the area [1:256,56:256] for all cameras.
The DQE scaling factor data points from the warm-up phase (at 77K) and post-NCS era
are seen in the 3rd and 4th columns, respectively, of Table 2.
There is overall good agreement between the DQE warm-up and the SM3B NCS values.
The DQE was projected to be approximately 40% higher at 77K (using data from the
1997/1998 warm-up program). On average the actual DQE increased by 53% for NIC1,
32% for NIC2, and 32% for NIC3, with an increased sensitivity toward the blue end of the
wavelength spectrum, as expected. The last column of Table 2 indicates the difference
from projected values to be within ± 8 %.
Table 2. Median Countrates and Relative DQE Values.
Camera/Filter
Median
Countrate
(cts/s)
Modeled
Ratio DQE
Actual
Ratio DQE
%
Difference
nic1 f090m
248.88
1.7
1.79
5
nic1 f095n
12.80
1.7
n/a*
n/a*
nic1 f097n
14.70
1.7
1.61
(5)
nic1 f108n
15.62
1.7
1.66
4
nic1 f110m
354.50
1.7
1.81
6
nic1 f110w
980.50
1.6
1.66
4
nic1 f113n
16.85
1.6
1.59
(1)
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Instrument Science Report NICMOS-2002-004
Camera/Filter
Median
Countrate
(cts/s)
Modeled
Ratio DQE
Actual
Ratio DQE
%
Difference
nic1 140w
1059.49
1.5
1.54
3
nic1 f145m
244.87
1.5
1.53
2
nic1 f160w
442.16
1.5
1.50
0
nic1 f165m
210.20
1.5
1.47
(2)
nic1 f170m
203.32
1.5
1.44
(4)
nic1 f187n
14.56
1.4
1.34
(4)
nic1 f190n
13.52
1.4
1.3
(7)
nic1 pol0S
377.64
1.7
1.84
8
nic1 pol120S
380.30
1.7
1.86
9
nic1 pol240S
376.53
1.7
1.85
9
nic2 f110w
3508.11
1.5
1.49
(1)
nic2 f165m
715.61
1.4
1.35
(3)
nic2 f171m
266.79
1.4
1.39
(1)
nic2 f180m
231.35
1.4
1.36
(3)
nic2 f187n
53.67
1.3
1.38
6
nic2 f187w
598.19
1.3
1.32
2
nic2 f190n
50.57
1.3
1.33
3
nic2 f204m
240.40
1.3
1.32
2
nic2 f205w
1356.47
1.3
1.29
(1)
nic2 f207m
305.66
1.3
1.30
0
nic2 f222m
250.24
1.3
1.27
(2)
nic2 f237m
231.05
1.2
1.15
(4)
nic2 pol0L
250.45
1.3
1.33
3
nic2 pol120L
249.04
1.3
1.35
4
nic2 pol240L
249.34
1.3
1.20
(8)
nic3 f108n
302.31
1.6
1.60
0
nic3 f110w
19254.90
1.6
1.63
2
nic3 f113n
342.93
1.6
1.59
(1)
nic3 f150w
18336.6
1.4
1.42
1
nic3 f160w
8065.60
1.4
1.40
0
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Instrument Science Report NICMOS-2002-004
Camera/Filter
Median
Countrate
(cts/s)
Modeled
Ratio DQE
Actual
Ratio DQE
%
Difference
nic3 f164n
331.38
1.4
1.30
(7)
nic3 f166n
311.91
1.4
1.31
(7)
nic3 f175w
19279.0
1.3
1.40
7
nic3 f187n
251.51
1.3
1.21
(7)
nic3 f190n
255.05
1.3
1.22
(6)
nic3 f196n
252.05
1.3
1.20
(8)
nic3 f200n
249.14
1.3
1.20
(8)
nic3 f212n
214.57
1.2
1.17
(2)
nic3 f215n
194.26
1.2
1.18
(2)
nic3 f222m
1292.74
1.2
1.19
(1)
nic3 f240m
1489.34
1.1
1.09
(1)
nic3 g096
1.772.70
n/a
1.76
n/a
nic3 g141
13630.50
n/a
1.47
n/a
11464.20
n/a
1.30
n/a
nic3 g206
*no
pre-SM3B archive exists to compare DQE ratios
During the NICMOS flat monitoring program 7691 in Cycle 7, flats were also taken multiple times a day in a number of filters in all cameras in order to follow the quantum
efficiency (QE) variations as a function of temperature and wavelength. By determining
the exact DQE as the detectors heated up, the DQE for all filters could be modeled for the
NICMOS Cooling System (NCS) operating temperature (see NICMOS ISR 99-001).
Expectations were that low sensitivity pixels would experience a significant increase in
DQE, especially at shorter wavelengths.
A comparison between Figures 1 and 2 further indicates a nominal difference between
projected and actual values. Figure 1 shows the ratio of the modeled DQE at 77 K to the
DQE at 62 K, as a function of wavelength. The data points are DQE scaling factors, which
were calculated during the warm-up monitoring for a subset of wavelengths (filters). The
continuous lines are linear fits extrapolated from the average measured in the three cameras. For all regions, the DQE increased roughly linearly between 62K and 77K.
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Instrument Science Report NICMOS-2002-004
Figure 1: Ratio of modeled DQE at 77K to DQE at 62K.
During SM3B, the scaling factors were computed directly from the countrates of the lampon minus the background images compared to the countrates computed from archive files
before warm-up. Figure 2 gives the new data points and linear slopes based on the scaling
factors computed.
Figure 2: Scaling factors of DQE based on a comparison of SM3B flat-field calibration
proposals to the 1997/1998 pre-warmup values (at 62K)
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Instrument Science Report NICMOS-2002-004
The small differences in scaling factors between the modeled warm-up data and the postNCS data are graphically represented in Figure 3 (see also the last column of Table 2).
Figure 3: Difference between the SM3B observed DQE scaling factors and prediction of
the factors based on warm-up data analysis.
The resulting wavelength dependence of the expected DQE for NICMOS operations at
77K is shown in Figure 4. Here, we have scaled the pre-launch DQE curve, which was
derived from ground testing of the detectors, to reflect the changes measured at the wavelengths used in the SM3B and calibration programs. There is a clear increase in sensitivity
for all cameras.
Figure 4: Actual NICMOS DQE as a function of wavelength for operations at 77K, compared to pre-launch measurements at 63K
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Instrument Science Report NICMOS-2002-004
Creating the Flat-fields
Calibration reference flat-fields were created similar to the method used during Cycle 7
and 7N (see ISR 98-003). The STSDAS tasks clanica, mscombine, msstat, and msarith
were used to perform calibration and image processing. See the ISR for details of the calibration switch settings used. The sets of background and lamp-on images were combined
separately and then subtracted. The resulting lamp-on minus background image was normalized and inverted to create the final “superflat”. These flats are now considered to be
the current calibration reference files. They are available from the HST Archive to all
NICMOS observers. Figure 5 gives a representative view of a superflat for each camera.
Figure 5: An example of a generated “superflat” flat-field for each camera: F110M
(NIC1), F187W (NIC2), F113N (NIC3) from left to right. Note: these flats were generated
with a blank bad pixel mask. Once a mask is created, it will be applied to the images. The
images are inverted to better display the grot; therefore, the dark regions have higher relative QE while those that are bright have lower relative QE.
Grot
Upon visual inspection of the flat-fields after the servicing mission, it became obvious that
more grot accrued on the detectors. Figure 6 gives an example of an individual flat-field
exposure before and after SM3B for each of the three cameras. The most notable addition
is a group of 27 pixels located at the bottom right quadrant on the NIC1 detector. A more
detailed analysis of grot will be performed at a future date.
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Instrument Science Report NICMOS-2002-004
Figure 6: Representative individual flats taken for filters F110M (NIC1), F187W (NIC2)
and F113N (NIC3) before and after installation of the NCS. The histograms show the
“flattening” of the arrays at the higher temperature. Again, the dark regions have higher
relative QE while those that are bright have lower relative QE.
Conclusions and Recommendations
Overall, there are no obvious changes to the morphology of the flat-field images when
compared to the pre-NCS images. As expected, detector sensitivity has increased for all
cameras. One flat-field per filter from proposals 8985, 9557, and 9327 has been created
and delivered to OPUS. NICMOS observations being processed through the OPUS pipeline will be calibrated using these flat-fields.
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