NICMOS ∆T-TEST
TIPS May 20 2004
Tommy Wiklind
• NICMOS Cryo-History
• NICMOS temperatures
• ∆-T Test
• Temperature lag
May 20, 2004
TIPS NICMOS Temperature tests
NICMOS CRYO-HISTORY
I. The cold period (1997-1999)
• NICMOS was installed on the HST February 1997
• Solid nitrogen coolant maintained the temperature at 61K
• The dewar temperature slowly increased by 1.5K during 1997-1998
• A thermal short lead to a faster-than-projected sublimation of the nitrogen
• Nitrogen was exhausted on January 4 1999
• NICMOS stopped obtaining scientific data on January 9 1999
II. The warm period (1999-2002)
• NICMOS warmed up to 260K and was useless for scientific applications
III. The cool period (2002- )
• The NICMOS Cooling System (NCS) was installed on March 8 2002
• NCS is a mechanical cooler using neon gas in a closed-loop reverse-Brayton cycle
• The temperature is controlled through the compressor speed
• The dewar temperature is maintained at ~77.1K
• Temperature control is ‘manual’
• The NCS has stopped once since March 2002 (August 2003)
May 20, 2004
TIPS NICMOS Temperature tests
NICMOS Dewar Temperature
The NICMOS detectors show a number of subtle effects sensitive to temperature.
Both the actual temperature and the temperature stability are important
• Detector quantum efficiency (DQE). Higher temperature gives higher sensitivity
• Reset level (the count level immediately following a detector reset) influences the
saturation levels (15% lower for NIC1 and NIC2, 7% lower for NIC3 compared to
pre-NCS era)
• The shading. A noiseless signal gradient (pixel dependent bias) depending on the
amplifier temperature
• The linear dark current
Non-temperature dependent effects
• Read-out noise
• Pedestal
May 20, 2004
TIPS NICMOS Temperature tests
The NICMOS dark signal consists of four components:
• Amplifier glow
Radiation from the read-out amplifiers. In a given read-out, the amount
of signal in each pixel scales directly with the number of read-outs since
the last detector reset.
• Shading
Time-dependent bias that changes across a quadrant as the pixels are
sequentially read out. The amplitude of the signal depends on the time
since previous read-out and read out direction (subtract the median value
of each column/row perpendicular to the fast read-out direction).
• Pedestal
A DC offset, or bias, leftover in an image after it has the dark reference
file subtracted from it. Simple solution: subtract the median value of each
quadrant before flat-fielding.
• Linear dark current
The ‘real’ dark component
May 20, 2004
TIPS NICMOS Temperature tests
NICMOS Dewar Temperature
The aim with the ∆-T test is to quantify the temperature dependence
of the linear dark current
Re-enable temperature dependent dark calibrations in the pipeline
NCS was commanded to change the neon temperature set-point
to three test positions at +0.5K, -0.5K and -1.0 and relative to the
current set-point of 72.4K
Provides data for evaluation of leaving NCS idle during orbit night
May 20, 2004
TIPS NICMOS Temperature tests
NCS
Ne in- and
outlet temp
sensors
(~1.5m)
NIC3 mounting cup
NIC1 mounting cup
NIC2 mounting cup
Dewar = NIC1 mounting cup
May 20, 2004
TIPS NICMOS Temperature tests
May 20, 2004
TIPS NICMOS Temperature tests
May 20, 2004
TIPS NICMOS Temperature tests
May 20, 2004
TIPS NICMOS Temperature tests
May 20, 2004
TIPS NICMOS Temperature tests
May 20, 2004
TIPS NICMOS Temperature tests
May 20, 2004
TIPS NICMOS Temperature tests
May 20, 2004
TIPS NICMOS Temperature tests
May 20, 2004
TIPS NICMOS Temperature tests
May 20, 2004
TIPS NICMOS Temperature tests
∆-T test results:
Comparison of dark
components from largest ∆T pairs are shown for each
camera.
Ampglow is largely
independent of temperature.
The linear dark component
shows significant structure
in NIC3, perhaps due to
physical imperfections in
the detector.
The shading is a thermal
effect that is a complex
function of ∆-time and
pedestal is an essentially
random electronic DC offset.
May 20, 2004
TIPS NICMOS Temperature tests
Linear dark current as
function of
temperature: The
linear dark current is
shown for each
NICMOS camera at two
temperatures.
As expected, the median
dark current in NIC1
and NIC2 increases with
temperature. The
behavior of NIC3 is
anomalous because of a
large pedestal
contribution.
May 20, 2004
TIPS NICMOS Temperature tests
May 20, 2004
TIPS NICMOS Temperature tests
Hot pixel linear dark current as function of temperature
May 20, 2004
TIPS NICMOS Temperature tests
Time lag for NICMOS detector temperature increase: Following the NCS safing event in
August 2003, the thermal response of the NIC1 mounting cup thermal sensor NDWTMP11 is
shown. For almost 2 hours the temperature at the mounting cup is unchanged. During this time
the neon in the cooling loop has been increasing its temperature. The lag in the dewar
temperature response may be related to the fact that the neon average temperature is some 5K
lower than the mounting cup.
May 20, 2004
TIPS NICMOS Temperature tests
Summary
• Linear darks are in the normal range
expected based on dark monitoring in cycles 11 and 12
• Data sufficient for re-enabling of temperature-dependent
darks to calibration pipeline
• No unexpected anomalies where found
• Characterization of the temperature variations continue
exploring options for future NCS operations
May 20, 2004
TIPS NICMOS Temperature tests
Time lag for NICMOS detector temperature increase: Similar plot for
temperature increase during delta-T test.
May 20, 2004
TIPS NICMOS Temperature tests
Time lag for NICMOS detector temperature increase: Similar plot for
temperature increase during delta-T test.
May 20, 2004
TIPS NICMOS Temperature tests
May 20, 2004
TIPS NICMOS Temperature tests
May 20, 2004
TIPS NICMOS Temperature tests