Instrument Science Report NICMOS 2007-002
NICMOS Time Dependent
Flat-fields
Tomas Dahlen, Helene McLaughlin, Roelof de Jong
June 29, 2007
ABSTRACT
We use data obtained during the NICMOS flat-field monitoring 2002-2007 to derive a set
of time dependent flat-fields. These ‘epoch’ flat-fields are compared to the current
pipeline flat-fields and differences are quantified. Our results are consistent with a
decrease in DQE over the five-year period on the order of one per cent. We also find that
the spatial structure of the flat-fields change with time. The reasons for these changes are
not fully known, but we note that the changes are consistent with a change in detector
temperature with time. The order of this effect is generally less than one percent. The
‘epoch’ flat-fields are available on the NICMOS web page1.
Introduction
The currently used NICMOS pipeline flat-fields are all based on observations obtained
during the first half of 2002. Since then, the flat-fields have been monitored during each
cycle, mostly in the F110W, F160W and F222M filters, but also occasionally in other
filters. An investigation into the behavior of the flat-fields shows that there has been a
slight change over time to the shape and response of the flat-fields. This change may be
correlated to variations in the detector quantum efficiency (DQE) due to changes in the
detector temperature with time. The effect of this on photometry in most cases is less
than 1%, but may reach a few % in the worst cases. Therefore, the NICMOS team has
created a new set of flat-fields based on the flats monitoring programs since 2002.
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NICMOS Webpage can be found at: http://www.stsci.edu/hst/nicmos/
Copyright  2007 Operated by the Association of Universities for Research in Astronomy, Inc., for the National
Aeronautics and Space Administration
Instrument Science Report NICMOS 2007-002
Observations
The flat-fields included in this investigation were obtained within programs 8974, 8985,
9326, 9327, 9557, 9640, 9996, 10379, 10728, 11016, and 11059. These programs
represent the full span of flat monitor programs post-NCS. Programs 8985, 9327, and
9557 are the post NCS flat-field calibration programs used to create the current pipeline
flat-fields. The main aim of the remaining programs is to monitor any changes in the flatfields and DQE over time. These programs focus on the F110W, F160W and F222M
filters, but observations in a number of additional filters also exist.
Table 1. Program Information Table
Program ID
Program Title
Program PI
Cameras
Date Range
8974
NICMOS Flats and
T. Boeker
1, 2, 3
4/20/2002 – 5/7/2002
temperature dependence of
the DQE
8985
NICMOS Internal Flats
A. Schultz
1, 2, 3
5/13/2002 – 5/19/2002
9326
NICMOS Cycle 10 Early
A. Schultz
1, 2, 3
5/30/2002 – 9/18/2002
9327
Calibration Monitor
NICMOS Flats: narrow filters
for NIC1 + NIC2, NIC3 in
parallel
S. Arribas
1, 2, 3
7/26/2002 – 7/28/2002
9557
NICMOS flats: Camera 3
S. Arribas
3
6/2/2002
narrow filters and grisms
9640
Flats Stability
A. Schultz
1, 2, 3
9/10/2002 – 9/9/2003
9996
Flats Stability
A. Schultz
1, 2, 3
10/15/03 – 9/4/2004
10379
Flats Stability
A. Schultz
1, 2, 3
12/30/2004 – 8/2/2005
10728
Flats Stability
A. Schultz
1, 2, 3
10/28/2005 – 8/6/2006
11016
NICMOS Flats: narrow and
N. Pirzkal
1, 2, 3
10/12/2006 – present
N. Pirzkal
1, 2, 3
11/7/2006 - present
broad filters for NIC1 +
NIC2, NIC3 in parallel
11059
Flats Stability
To examine the change in the shape and response of the flat-fields, we first divided all of
the flats into groups. These groups or “epochs” were picked so that the breaks would fall
in the natural break points between programs and last roughly a year each. The first two
epochs are shorter, because they contain much of the post-SM3B checks, which included
a large number of exposures. The final epoch is longer, because there were a much
smaller number of observations in the monitoring program since August 2005. Below are
plots showing camera and filter combinations that were observed by all programs
between 2002 and 2007. In the figure, we also show the division of the time span from
2002-2007 into five different epochs. The definitions of the epochs are listed in Table 2.
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Instrument Science Report NICMOS 2007-002
Table 2. Date Ranges of each Epoch
Epoch #
Start Date
End Date
1
April 2002
Nov 18 2002
2
Nov 19 2002
Jul 26 2003
3
Jul 27 2003
Jul 05 2004
4
Jul 06 2004
Aug 28 2005
5
Aug 29 2005
Present
Figure 1a: Frequency of observations for all possible camera/filter combinations
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Instrument Science Report NICMOS 2007-002
Figure 1b: Frequency of observations for all possible camera/filter combinations.
Creating the Epoch Flat-fields
The current pipeline flat-field images were created using the methods outlined in
NICMOS ISR 98-003. Calibration and image processing was performed on all flat-field
files using IRAF tasks calnica, mscombine and msarith. Reference NICMOS ISR 98-003
or NICMOS ISR 2002-004 for details concerning the pipeline calibration and explanation
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Instrument Science Report NICMOS 2007-002
of the pipeline “superflats”. These “superflats” are considered to be the current
calibration reference files. These are the files that are available via the HST archive.
Figure 2: Post-NCS Pipeline Flats for F160W filter in camera 1, 2 and 3. Spatial variations are typically
+/- ~40% between brightest and darkest regions, but depends on camera (with largest variations in camera
1). Note that these are inverse flats, so brighter in the images corresponds to lower relative sensitivity. The
bright spot in camera 2 is the coronagraphic hole.
The epoch flats that were created are only available on the NICMOS Temporal Flat-field
web page2 at this time. These files were created using the same methods as the pipeline
flats, with one exception; these flats are split into the time dependent epochs mentioned
above. Individual exposures were first processed using calnica and then combined using
mscombine. Resulting images were then normalized to unity and thereafter inverted.
More on this procedure can be found in NICMOS ISR 1998-003 and ISR 2002-004.
Quality Checks
Detector Quantum Efficiency (DQE)
The flat-fields are created by illuminating the detectors with an internal lamp. We can
assume a uniform illumination over time from the lamps (see discussion in Bergeron &
Bacinski 1999 and Schultz et al. 2003), therefore any change in the measured count rates
in the flat-fields should be attributed to a change in the DQE. To quantify the change in
DQE, we measure the count rates (counts per second) for all individual flat-field obtained
in F110W and F160W filters for the three cameras. An individual flat-field is here
defined as a flat-field created from data taken during a single program, as illustrated by
the points shown in Figure 1. We thereafter normalize the count rates to unity for each
combination of filter and camera. In Figure 3, we show the evolution of the normalized
count rates for the two filters and three cameras. Each point represents an individual flatfield, while the lines are straight-line fits to the data. The figure clearly indicates that the
DQE has decreased over the five-year period since the installation of the NCS. The
2
NICMOS Temporal Flat-field Website
http://www.stsci.edu/hst/nicmos/calibration/reffiles/temporal_flat_files/index.html
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Instrument Science Report NICMOS 2007-002
change is less than a percent in camera 1, but reaches ~two per cent in camera 3. In Table
3, we give the slopes of the best fitting straight lines together with the relative change in
the DQE over the five-year period since the installation of the NCS. A slight decrease in
the DQE is also consistent with results from the ongoing monitoring of NICMOS
standard stars. Note, however, that an overall change in the mean DQE of the detectors is
not compensated for by using updated flat-fields since each flat-field is by construction
normalized to unity.
Figure 3: Change in DQE from 2002-2007. Points show relative count rates for F110W and F160W filters
in camera 1, 2 and 3. Straight-line fits to the data are also shown.
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Instrument Science Report NICMOS 2007-002
Table 3. Slope of the best-fit straight line to the DQE change from 2002 – 2007 and the total change in
DQE over the five-year period derived from the best fit. Results are given for the F110W and F160W filters
in all cameras.
F110W
F160W
NIC1
Slope
ΔDQE(5 yrs)
-2.74E-06
-0.5%
-4.24E-06
-0.8%
Slope
-4.47E-06
-6.72E-06
NIC2
ΔDQE(5 yrs)
-0.8%
-1.2%
NIC3
Slope
ΔDQE(5 yrs)
-1.11E-05
-2.0%
-9.85E-06
-1.8%
Spatial Variance
To investigate the spatial variance in the flat-fields between 2002 and 2007, we derive
ratio images by dividing the new epoch flat-fields by the pipeline flat-fields. In Figure 4,
we show the ratio images for camera 1, 2 and 3 in the F160W filter for available epochs.
There is a clear structure in the ratio images that increases over time. The spatial
variations seen are consistent with temperature induced DQE variations from pixel to
pixel, i.e., each pixel has its own independent DQE – temperature relation (see Section 3f
and Figures 9-10 in Boeker et al. 1999). To quantify the degree of spatial change between
the pipeline and epoch flat-fields, we introduce a “spatial ratio”, which we call R(sp). For
each pixel we calculate the ratio between the two flat-field images (R(x,y)). R(sp) is then
defined as the median of abs(R(x,y)-1) taken over the entire image. Two identical images
will have an R(sp)=0 and a higher R(sp) indicates larger deviations. In Table 4 we list
R(sp) (multiplied by a factor 100 for clarity). In most monitored filters, R(sp) increases
with epoch, consistent with an increased spatial variance with time. The statistical
uncertainty in the measured R(sp) values is <5%, significantly smaller than the trends
seen.
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Instrument Science Report NICMOS 2007-002
Figure 4: Ratio images constructed by dividing the new epoch flat-field by the pipeline flat-fields. Shown
are available epochs for F160W filter in all three cameras.
NIC1
NIC2
EPOCH 1
EPOCH 2
EPOCH 3
EPOCH 4
EPOCH 5
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NIC3
Instrument Science Report NICMOS 2007-002
Table 4. 100*R(sp), per camera/filter combination per epoch
NIC1 F090M
NIC1 F095N
NIC1 F097N
NIC1 F108N
NIC1 F110M
NIC1 F110W
NIC1 F113N
NIC1 F140W
NIC1 F145M
NIC1 F160W
NIC1 F164N
NIC1 F165M
NIC1 F166N
NIC1 F170M
NIC1 F187N
NIC1 F190N
NIC1 POL0S
NIC1 POL120
NIC1 POL240
NIC2 F110W
NIC2 F160W
NIC2 F165M
NIC2 F171M
NIC2 F180M
NIC2 F187N
NIC2 F187W
NIC2 F190N
NIC2 F204M
NIC2 F205W
NIC2 F207W
NIC2 F212N
NIC2 F215N
NIC2 F216N
NIC2 F222M
NIC2 F237M
NIC2 POL0L
NIC2 POL120
NIC2 POL240
NIC3 F108N
NIC3 F110W
NIC3 113N
NIC3 F150W
NIC3 F160W
NIC3 F164N
NIC3 F166N
NIC3 F175W
NIC3 F187N
NIC3 F190N
NIC3 F196N
NIC3 F200N
NIC3 F212N
NIC3 F215N
NIC3 F222M
NIC3 F240M
Epoch 1
0.110
0.170
0.183
0.174
0.108
0.242
0.179
0.124
0.104
0.170
0.165
0.107
0.170
0.154
0.149
0.487
0.112
0.111
0.111
0.233
0.167
0.147
0.141
0.124
0.244
0.171
0.151
0.121
0.105
0.139
0.140
0.207
0.342
0.139
0.109
0.049
0.047
0.050
0.040
0.269
0.040
0.067
0.166
0.080
0.076
0.109
0.125
0.079
0.032
0.032
0.031
0.031
0.127
0.140
Epoch 2
0.572
0.549
0.392
0.347
0.246
0.226
0.369
0.181
0.189
0.171
0.233
0.257
0.190
0.208
0.488
0.338
0.241
0.301
0.201
0.182
0.244
Epoch 3
0.771
0.513
0.443
0.296
0.581
0.235
0.219
0.234
0.208
0.217
0.661
0.420
0.322
0.254
0.293
0.348
Epoch 4
0.773
0.391
0.313
0.561
0.239
0.199
0.203
0.226
0.208
0.669
0.317
0.239
0.325
-
*** Epoch flats-field files do not exist for grisms since these were not monitored.
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Epoch 5
1.150
1.015
0.901
0.953
0.646
0.577
0.423
0.318
0.312
0.972
0.960
0.976
0.222
0.339
0.213
0.197
0.675
0.318
0.212
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Instrument Science Report NICMOS 2007-002
How to use Epoch Flat-fields
The deviations between pipeline and epoch flats increase with time as seen by e.g., the
larger R(sp) values for later epochs. When 100*R(sp) is small (~<0.2), there should be no
need for exchanging the pipeline flat-fields with epoch flat-fields. This is the case for
most of the epoch 1 flat-fields. For epoch 2 and later, you may gain by exchanging to the
epoch flats.
You can either exchange the pipeline flat-fields with the epoch flat-fields during data
reduction, or you can multiply your already pipeline flat-fielded images with the ratio
flat-fields (i.e., “*_cal.fits” images before any drizzling). As a guide to which epoch
flat-field should be used for a particular observation, each flat-field fits image has a
“USEAFTER” header keyword. Note again that this will only correct for the spatial
relative DQE changes, not the suggested changes in absolute photometry.
All epoch flat-field files and ratio images are located on the NICMOS Temporal Flatfield File web page:
http://www.stsci.edu/hst/nicmos/calibration/reffiles/temporal_flat_files/index.html
Future Plans
The aim of the present investigation has been to quantify the time dependent changes in
the NICMOS flat-fields. Next step is to investigate the not fully understood mechanisms
behind these changes. At this point, we do, however, note that the shape and the size of
the temporal changes are consistent with a change in the detector temperature. Therefore,
in the near future the NICMOS team would like to examine in detail the effect of
temperature on the flat-field files. The NICMOS team is working on a “temperaturefrom-bias” algorithm that determines the temperature of the detector from the dark and
bias frames. This algorithm is still in development, however preliminary results have
shown that the detector temperature of all three detectors has dropped about 1.5K since
2002. This is contrast to the current temperature monitoring of the mounting cup sensor
that has showed no discernible change since 2002. Our plan is to create a second set of
flat-fields that vary by temperature instead of time, compare those temperature dependant
flat-field files to the epoch flat-field files presented here, and try to definitively answer
whether or not temperature changes are the cause of the spatial differences over time.
The NICMOS team plans to collect a whole new set of flat-field files for all camera/filter
combinations during Cycle 16, as well as after Servicing Mission 4 (SM4) as part of the
Servicing Mission Orbital Verification (SMOV) program. These sets of flats will be
added to the current epoch flat-field files, and at that time, we will determine whether we
need to update the current pipeline flat-field files with post-SM4 flat-fields or whether we
need to implement the full set of epoch flat-field files into the pipeline. In the mean time,
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there is also a flat-fields stability program in place with monthly monitoring of a subset
of broadband filters in all cameras. Those flat-field files will be added to the epoch flats
as they become available, and ratio images will be updated as needed.
We also note that the NICMOS team is working on updating the pipeline software
calnica. If the new version changes the flat-fields, new images will be announced on the
web page.
Conclusions
Using observations obtained during flat-field monitoring programs 2002 to 2007, we
have created a set of epoch dependent flat-fields. Comparing these flat-fields with the
current pipeline flat-fields (based on observations from 2002), we have found both a
decrease in the overall DQE over the five year period in the order of one per cent, and a
change in the spatial shape of the flat-fields with time, which in most cases is an effect of
less than one per cent. The underlying reason for these changes are not fully known, but
we note that they are consistent with a decrease in detector temperature. The NICMOS
team will further investigate this change.
At this time, the new epoch dependent flats are available on the NICMOS web page.
Future and on-going monitoring of the flat-fields will determine at what point the current
pipeline flat-fields will be updated.
References
Bergeron , L. & Bacinski, J. 1999, NICMOS ISR-99-012
Boeker, T. et al. 1999, NICMOS ISR-99-001
Mazzuca, L. & Schultz, A. 2002, NICMOS ISR-2002-004
Schultz, A. B., Colina, L., Stolovy, S. & Sparks, B. 1998, NICMOS ISR-98-003
Schultz, A. B., Roye, E., & Sosey, M. 2003, NICMOS ISR-2003-003
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