Instrument Science Report NICMOS-99-010
NICMOS Temperature-specific Darks
B. Monroe, E. Bergeron
November 18, 1999
ABSTRACT
The various components of NICMOS dark images have been modeled and combined to
make synthetic dark calibration files which are intended for use with observations in a
temperature range from 61 to ~75 K, currently available only for camera 2, with cameras
1 and 3 to follow in a few months. The amplifier glow and the true linear dark current
have been constructed as temperature-independent quantities, while the “shading” component of the darks has been modeled as temperature-dependent. The data used to construct these models was taken with NIC 2, in a temperature range of 61 to 80 K during the
recent warm-up of NICMOS due to cryogen exhaustion. The resulting synthetic darks are
available through a web-based tool on the STScI NICMOS website----------------------http://www.stsci.edu/instruments/nicmos/NICMOS_tools/syndark.html.
1. Introduction
It is well known that NICMOS detectors contain an inherent electronic bias which is
dependent on the temperature of the detectors. This component of the bias has been documented in previous ISRs as “shading”, and will be reviewed momentarily. This shading is
also dependent on the time elapsed since the previous readout (Fig. 1). The changes in
shading with respect to temperature have been severe enough to require recalibration to
eliminate a noticeable gradient across the detector array and other incorrect calibration
computations due to this flawed calibration. The warm-up of NICMOS in late 1998/early
1999 provided an opportunity to gather data on how the NICMOS electronics were
affected by changes in temperature, and to model these changes in order to make dark calibration files which were temperature-specific.
2. Shading Overview
NICMOS darks are comprised of three main components: the true (linear) dark current, the amplifier glow, and the bias. The bias level, or “DC offset”, in a given pixel in a
NICMOS array, is time-dependent. This is the so-called shading, which visually in an
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uncorrected image looks like a ripple and gradual signal gradient across a given quadrant.
The pixels in a given quadrant of a NICMOS detector are read out sequentially. It takes a
little over a µsec to read a single pixel, and so with four readout amplifiers reading in parallel it takes just over 0.2 sec to read the entire 256x256 pixel detector. Considering a
quadrant as an array of i x j pixels, the readout sequence consists of reading sequentially
along a detector row i, clocking j from 1 to 128, then moving to row i+1 and clocking j
from 1 to 128, and so on. Since the amplifier bias changes pseudo-exponentially with time
over the course of the readout, the observed signal, in the absence of external illumination,
varies rather slowly along the rows (referred to as the slow clocking direction), but rather
rapidly along the columns (referred to as the fast clocking direction). This signal is not
accumulated in the pixels during each deltatime period, (the keyword DELTATIM in NICMOS data is used to denote the time since the previous readout, hence the terminology),
but rather is superimposed on the actual signal at the time of each detector readout. The
shading signal is not the same for each readout. Its amplitude and to some extent its shape
are a function of the time since a pixel was last read out (not reset). So readouts with the
same deltatime will have the same bias signal, for a given pixel. The dependence of this
bias on deltatime is nearly logarithmic and quite repeatable.
3. Data Reduction
Data from the warm-up dark monitoring programs 7963 and 8093, which used
STEP64 multiaccum readout sequences exclusively, were used to construct the models of
the true linear dark current and the shading. This data covered a range of temperatures
from ~62.8 K upwards to about 85 K. Additionally, darks from program 7596 were also
used as a complement in the low temperature range (~61.5 to 62.8 K) for the shading,
which were both STEP128 and STEP256-- together, these datasets contain the vast majority of all possible deltatimes. The ampglow component was previously obtained from
darks in program 7795. Over one hundred-eighty observations were examined for the purpose of creating the shading models. In the end, almost one hundred of these observations
were thrown out solely to give the temperature models the right weighting at all points in
the range of temperatures. Fifty-six datasets were used to make the final linear dark current image. Observations severely affected by pedestal or cosmic ray persistence were not
included in the reduction process. Also, any observation obtained within two orbits of the
concurrent flat-monitoring program were excluded to avoid any residual image persistence. All datasets used in the aforementioned reductions were individually inspected for
these and any other defects.
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Figure 1: Temperature and readout sequence dependence of shading in NIC 2.
The image of the true linear dark current was made by directly processing the aforementioned raw data. The steps needed to isolate the linear dark current were the following:
•
Eliminate shading from data.
•
Eliminate amplifier glow from data.
•
Account for pedestal in each quadrant in each dataset.
•
Make an “average” DC offset for each quadrant.
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•
Median pixel values over the data “cube” to create a final image.
First, the individual datasets were truncated, in order to only retain those readouts
from the STEP64 sequence which had an elapsed time of 63.997 seconds since the previous readout. This was done so that the shading profile on all readouts would be identical,
and would not interfere with subsequent processing. The datasets were then run through
Calnica, omitting the ZSIGCORR and FLATCORR steps, while performing the DARKCORR and UNITCORR steps, which were used to remove the shading, and to convert to
counts/second, respectively. Additionally, the DARKFILE was set to a special file containing only the ampglow component of a camera 2 dark image, and the node 2 value in the
NLINFILE was decreased to exclude bad data. In this way, there would be a shading profile in the zeroth read which would be subtracted from all the other reads to produce a
clean dark image. Because of unknown DC offsets (commonly known as ‘pedestal’) in
each of the quadrants in each of the images, a median value in each quadrant for each of
the fifty-six images was determined, and then subtracted from the entire quadrant. These
fifty-six medians (per quadrant) were averaged, i.e. a mean was determined, and the mean
value was added back in to each quadrant, to reproduce an average DC offset for that
quadrant. Finally, each pixel in the array was medianed over the fifty-six images to produce the final image of linear dark current.
To make the shading models, the steps were the following:
•
Eliminate amplifier glow from data.
•
Eliminate linear dark current from data.
•
Fit four polynomials to slow-direction shading profile in each dataset.
•
Make ‘temperature fits’ for slow-direction shading for each deltatime.
•
Fit two polynomials to fast-direction shading profile in each dataset
•
Make ‘temperature fits’ for fast-direction shading for each deltatime.
•
Model bottom fifteen rows in lower left quadrant with an average.
•
Add all three models together to obtain a final model.
To produce the shading profiles for each of the readout times, the following steps were
performed: The raw files were zero-read subtracted (in the case of the 63.997 second readouts where there were multiple readouts with the same elapsed ‘deltatime’ since the last
readout, the chronologically-first one was used), the linear dark current was subtracted by
using dark files constructed in the manner described in the previous paragraph, however
instead of using the composite image of the dark current described above, each image’s
own dark current was used. Of the 180 datasets used for determining the shading, none
were in the same group that was used to make the final linear dark current image in the
previous paragraph. The ampglow was then subtracted. Then a median was taken for each
column of the array. This median was the source of the primary shading profile (See Fig.
1). The individual profiles for each data set were fit to fourth-order polynomials. Four
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fourth-order polynomials are needed to fit the 256 columns, because of the discontinuity
of the first spatial derivative of the bias in the 20th column of each quadrant (described in
ISR-97-026 pp.9-10), and two quadrants per row make up the 256 pixels of the array. This
resulted in 180 sets of four fourth-order polynomials per deltatime. Next, the temperatures
of each observation, obtained from the _spt files of the same rootname as the dark image,
were used to fit a quadratic polynomial of each fourth-order fit vs. temperature. Thus a
way to convert a given temperature and readout sequence to a shading profile was
obtained. In addition to the row profile, the fast amplifier-readout direction results in a row
cross-section per deltatime as well, which has a much smaller amplitude. This y-direction
profile was approximated as a ramp added on top of each column’s x-direction profile.
Only one ramp per quadrant was used, i.e. every column in a quadrant shared the same
ramp. Quadratic fits vs. temperature were also made. Lastly, an attempt was made to
account for the electronically challenged rows at the bottom of the left side of the array. A
profile was constructed for each deltatime, for the bottom fifteen rows on the left-hand
side of the array which is not dependent on the temperature.
4. Results and Caveats
The improvements achieved depend, for obvious reasons, on the temperature of the
science data on which the new darks are used. Users can expect to see much more of a difference at temperatures above 62 K. Not only will the noticeable slow-readout-direction
gradient become much less visible, but often a residual flat-field stamp which was on the
pipeline-calibrated science image will disappear in the recalibrated image (Fig. 2). In
addition, pixel-to-pixel noise is reduced by the new dark current image component. The
standard deviation for sky data values are significantly reduced. Looking at sky data, in
regions at least thirty squares on a side and without an obvious gradient, sky sigma was
reduced by on average roughly 10 to 20% for data at 61.8 K and roughly 30% for data at
62.4 K. However, results are highly variable, depending also on the degree to which pedestal (which these darks are not able to correct) affects the image. Figure 3 shows a
histogram of the old and new dark current.
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Figure 2: Comparison of a science image reduced with both old (pipeline synthetic) and
new (web tool synthetic) darks. Each image was taken at a stretch to show maximal contrast in the features. An empty field was chosen for illustrative purposes. This dataset is a
NIC2 SPARS64 at a temperature of 62.4 K.
old
new
The new temperature-specific darks are available through a web interface which can be
accessed directly from the STScI NICMOS webpage. The inputs to the web tool are
1. The camera.
2. The multiaccum readout sequence.
3. The temperature.
The output is a page with a direct link to a newly created reference file.
It is possible to run the web tool and come up with a dark that does not improve calibrations very much. Most likely this is a result of slight temperature variations, as the
temperature changes over the course of an exposure, typically on the order of a tenth of a
degree. This means that it is worth making a few different darks at temperature intervals
closely spaced, and trying them out to see which dark fits the observations best. It is also
possible that pedestal in the individual observations will obscure improvements in the
recalibration. It is advised to use the existing pedestal correction tools to deal with this
problem. The new darks also do not fix the problem of the “photometrically challenged
column” in the middle of the array (see NICMOS Image Data Anomalies page for description.) Data for deltatimes longer than 64 seconds, at temperatures higher than 63 K does
not exist, so in order to constrain the models (at the high temperature end) for readouts
with these deltatimes, 63.997 second deltatime readouts-- which are the most similar, were
used. Also because of this lack of data, MIF sequences suffer from the same problem with
their short final readouts, and so standard 0.303 second deltatime shading was used in the
models.
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Again, the tool is available at:
http://www.stsci.edu/instruments/nicmos/NICMOS_tools/syndark.html.
Figure 3: Comparison of old and new dark current models. Only a 150 x 150 area at the
center of the array was used, to eliminate the higher dark current due to the warm corners
of the array.
old
new
old
new
5. References
Shading discussion taken directly, with minor modifications, from NICMOS ISR-97026 “Characteristics of NICMOS Detector Dark Observations.”C.J. Skinner, L.E.
Bergeron.
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