New NICMOS temperature
dependent reference files
Tomas Dahlen
TIPS 11/20/2008
With contributions from
Elizabeth Barker, Deepashri Thatte, Denise Smith,
Eddie Bergeron, Nor Pirzkal, Dave Grumm, Robert Jedrzejewski,
Megan Sosey, Roelof de Jong, Alex Viana and others
TIPS, Nov 20, 2008 - Tomas Dahlen
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New NICMOS temperature dependent reference files

Aim: Improve NICMOS calibrations with CALNICA
 CALNICA is
the IRAF task that performs instrumental
calibration of NICMOS data:
dark current subtraction
 correction for detector non-linearity
 flat-fielding
 conversion to count rates
 population of photometric keywords
 cosmic ray identification and rejection

is used by pipeline/OTFR to produce final calibrated
NICMOS images
 CALNICA is available in STSDAS so that users can customize
calibration
 CALNICA
TIPS, Nov 20, 2008 - Tomas Dahlen
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New NICMOS temperature dependent reference files
Previously on TIPS (4/2008):
Updates to improve CALNICA
• Optimum weighting when calculating count rates from up-theramp sampling.
• New way of handling cosmic rays.
• Detecting and flagging curvature in detector response
• Improved error maps
• See NICMOS ISR 2008-2
TIPS, Nov 20, 2008 - Tomas Dahlen
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New NICMOS temperature dependent reference files
Next steps to improve calibrations:
Create reference files that depends on temperature
 Implement the temperature dependence into CALNICA
 Use accurate temperatures from detector bias levels (Bergeron)
 Significant improvement over using the mounting cup temperatures

Why do this?
The NICMOS detectors are very sensitive to changes in temperature,
we expect the detector “properties” to change with temp (DQE, dark
current, bias)
 NICMOS detector temperature as measured from the bias (“tempfrom-bias”) shows a continuous decrease with time

Plot shows change in NIC1 temperature with
time 2002-2008
NDWTMP11: Mounting cup temperature
TFBTEMP: Temp-from-bias
TIPS, Nov 20, 2008 - Tomas Dahlen
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New NICMOS temperature dependent reference files
We have looked at the temperature dependence of :
v Darks
v Flat-fields
v Photometry
TIPS, Nov 20, 2008 - Tomas Dahlen
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New NICMOS temperature dependent reference files
1.Darks
The NICMOS dark reference files consists of three
components
The Ampglow - the amplifiers in the corners of
the detectors warms up during read-out and emits
infrared radiation that is detected by the chip
NIC3 Ampglow
The “normal” linear dark current
The shading - noiseless signal caused by
changes of the pixel bias levels as a function of
time since last read-out, “delta-time”.
NIC3 Lindark
All components should be temperature dependent
NIC3 Shading
Δt=256s
TIPS, Nov 20, 2008 - Tomas Dahlen
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New NICMOS temperature dependent reference files
1.Darks - Ampglow
Ampglow images are created from the shortest available dark
images (0.2s SCAMRR)
Red symbols show the mean counts in the corners (30x30
pixels) of individual ampglow images (each is a mean of 25
reads)
Black dots show the mean of the counts in temperature bins
Blue star is the mean of all measurements
 Higher counts at higher temp-from-bias.
 Correlation at a 5σ significance.
Decrease in response by 1.5-2% during the ~6 year period is
consistent with what is found from flat-field and photometry
monitoring .
Does this matter?
The signal in the corners is ~20 counts.
With a maximum NSAMP=25, the change in ampglow due
to the change in temperature is:
Δcounts = 20 x 25 x ~2% = ~10 counts
TIPS, Nov 20, 2008 - Tomas Dahlen
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New NICMOS temperature dependent reference files
1.Darks - Ampglow
Comparing pre NCS and post NCS Ampglow
Typical pre NCS temp: ~61K
Typical post NCS temp: ~76K
Full line shows a straight line fit to post NCS data only with thin
lines indicating +/- 1sigma
The pre NCS response at ~61K is fully consistent with the
temperature dependence derived using post NCS data only.
Assuming that the ampglow signal is constant, this supports
the relation between DQE and temp-from-bias
Implementation
A master ampglow image is created for each camera
Scaling factor is calculated from temperature
Scaled amplow is subtracted from the data
TIPS, Nov 20, 2008 - Tomas Dahlen
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New NICMOS temperature dependent reference files
2. Linear dark
Linear darks created from long exposure DARK images after
subtracting the ampglow
Two methods are used to investigate the temperature
dependence of the linear dark
1)
Median dark current vs temperature for individual
images is fitted (top plot)
2)
Dark current in individual pixels are fitted to temperature
(bottom plot)
Neither of these cases shows a temperature dependence that
is statistically significant
E.g., mean change in NIC3 is ~2% +/-10%
Would a ~2% change matter?
Example: 2% x 1000s x 0.2 e-/s / 6.5 e-/DN = ~1 DN
Implementation
A master Linark image is created for each camera
No temperature scaling at the moment
TIPS, Nov 20, 2008 - Tomas Dahlen
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New NICMOS temperature dependent reference files
3. Shading
Shading images are created from DARK exposures after
subtracting ampglow and linear dark components.
For each of the 12 existing delta-times, a shading profile is
created for each camera by collapsing the dark exposure
along the fast readout direction.
Delta-times
0.20s
0.30s
0.39s
0.99s
1.99s
3.99s
7.99s
15.99s
31.99s
63.99s
127.99s
255.99s
TIPS, Nov 20, 2008 - Tomas Dahlen
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New NICMOS temperature dependent reference files
3. Shading
Shading images are created from DARK exposures after
subtracting ampglow and linear dark components.
For each of the 12 existing delta-times, a shading profile is
created for each camera by collapsing the dark exposure
along the fast readout direction.
For each shading profile we calculate the temperature
dependence
The change in shading over the ΔT=2K can be ~10 DN and
therefore important to correct for
Example of change in shading vs
temperature for NIC3 (four different
delta-times).
Implementation
Temperature dependent shading correction is applied
on pre NCS data
Post NCS images to be completed
TIPS, Nov 20, 2008 - Tomas Dahlen
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New NICMOS temperature dependent reference files
Flat-fields

The flat-field calibration images show a decrease in
response with decreasing temperature (time)

Flat-fields are normalized to unity by construction
and a constant “per pixel” DQE change would not
affects flats

The change in flat-field structure seen is therefore a
second order change in the DQE caused by
temperature (hotter pixels are relatively more
affected than cooler).
Implementation
Five flats are created for each combination of camera&filter.
Each flat is used within a specified temperature range
TIPS, Nov 20, 2008 - Tomas Dahlen
2002
2003
2004
2005
2006
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New NICMOS temperature dependent reference files
Photometry
Example: Count rates for NIC3, NIC2,
and NIC1 (top to bottom) 2002-2007.
Despite the relatively large scatter, there
is a trend of a decreased count rate
with time, consistent with the decrease
in temperature. The trend is clearest for
NIC2 with a decrease of ~2-3% in the
count rate over the ~5 year period.

The standard star calibration images show a
decrease in response with decreasing temperature
(time), though with some scatter

NIC2 shows clearest signal with a decrease of 2-3%
over the post NCS period

Ongoing work and results are still preliminary
Implementation
A wave-length dependent scaling factor is calculated
Science data is scaled by this factor to correct the
counts for the temperature dependence of the DQE
(keeping the ZP unchanged)
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New NICMOS temperature dependent reference files
Summary / Status update
CALNICA is updated to include temperature dependent reference
files (a few checks remain)
Temperature dependent flat-fields, darks/ampglow and darks/lindark
have been created
Temperature dependent darks/shading (post NCS) and photometry
references file are being worked on
Updates will go into opus 2008.5a
Will go into future stsdas
Temperature dependent reference files part of the upcoming
reprocessing of NICMOS data
TIPS, Nov 20, 2008 - Tomas Dahlen
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