Instrument Science Report NICMOS 2000-004
NICMOS Warm Darks
A.B. Schultz, L. Bergeron, J.J. Bacinski, W. Baggett
September 26, 2000
ABSTRACT
NICMOS was commanded to Observe on June 19, 2000 and on June 30, 2000. A series of
MULTIACCUM and ACCUM darks were executed as part of the flight software (FSW
4.0C) installation. As expected, the darks were saturated at the current temperature of
~230o K. Real-time monitoring of the instrument during the dark observations indicates
that NICMOS is healthy and functioning as expected.
Introduction
The NICMOS cryogen was exhausted on January 4, 1999. The filter wheel commanding
was inhibited on January 6, 1999 at 14:27 UT. At that time, all temperature sensors
reached their limits and the filter wheels were stored in their “blank” positions. Data taking was suspended on January 11, 1999, at which time the instrument temperature had
reached ~115o K. NICMOS was commanded to SAAOPR and the Pupil Alignment Mechanism (PAM) to HOLD.
Subsequent to this suspension, the NICMOS detectors were not operated nor the
Pupil Alignment Mechanism (PAM) exercised until June 2000 when a series of MULTIACCUM and ACCUM darks (program ID: 8869) were obtained in support of the
installation of the NICMOS flight software (FSW) 4.0C. Data were obtained prior to
installation of the new NICMOS flight software to ensure that the NICMOS was functioning properly. This data was used as a comparison reference set for a second set of data
obtained following the installation to verify that changing the software did not affect the
instrument.
Instrument Science Report NICMOS 2000-004
FSW 4.0C - a solution to TPG resets
A Timing Pattern Generator (TPG) reset occurs when a spurious signal is generated
caused by a high energy particle event. The TPG reset sets all the digital-to-analog converter (DAC) voltages to zero which effectively powers off the detector. This was traced to
a component (Opto-Isolator) on the TPG board. An opto-isolator is an electronic component used to reduce electronic noise in a signal path by electrically isolating one part of a
circuit from another. This isolation is accomplished by converting electrical pulses to optical pulses and back. An energetic charged particle event can trigger an optical pulse which
results in a TPG reset. In the past, a TPG reset would suspend the instrument.
The NICMOS flight software (FSW) was modified to handle TPG resets. The new
FSW detects a TPG reset, sets a flag, and allows the observations in the other two NICMOS cameras to continue. Stored command instructions will conditionally recover the
reset TPG during normal instrument mode transitions. If a TPG reset occurs before the
start of an observation, the TPG timing pattern is loaded nominally by the FSW and data
are read from the analog-to-digital converter (A/D). However, the detector will be powered off so the data collected is noise. NICMOS data sets that are affected by a TPG reset
will be flagged in OPUS: the NICMOS keyword EXPFLAG is set to read DETOFF any
time a TPG reset occurs during an exposure.
The NICMOS 4.0C FSW release was installed on June 27, 2000 and activation was
completed at 19:28 GMT. After activation, NICMOS was commanded to the nominal state
of SAAOPR and the PAM was transitioned to HOLD in preparation for the June 30, 2000
on-orbit test.
NICMOS Operation
During normal operation at the start of an exposure, the NICMOS detectors are commanded to drop out of autoflush mode and to run the pixel reset pattern three times. It
takes ~0.6 seconds to complete this task. No data is saved and this task is transparent to the
user. The array is then read out to determine the amount of remaining charge on the array.
This read is a bias frame and is saved. It is called the “zeroth-read”. The zeroth-read is the
first read in a MULTIACCUM exposure. For MULTIACCUM mode, the camera is readout non-destructively and each read of the camera is saved. During OPUS pipeline
processing, the zeroth-read is subtracted from each readout and the readouts are individually calibrated. For ACCUM mode, the NREAD parameter determines the number (N) of
reads that are read and averaged. ACCUM mode observations are processed on the spacecraft. The detector is read N times, and these reads are averaged to form the initial read.
The process is repeated to form the final read. The initial read is subtracted from the final
read, and the resulting image is sent to the ground. The intermediate reads are not sent to
the ground.
2
Instrument Science Report NICMOS 2000-004
Data
NICMOS was commanded to OBSERVE mode on June 19 and June 30, 2000. For each
test, the PAM was powered on one hour prior to transition to OPERATE to allow the temperature sensors to warm up. The PAM was moved to the prime camera position, Camera
1. The detectors transitioned from off (SAAOPR) to on (OPERATE), and to OBSERVE
mode. The temperature sensors were disabled on transition to OBSERVE mode. A series
of MULTIACCUM (NSAMP=25, SAMP-SEQ=STEP8) observations were obtained.
After these observations, the cameras were transitioned to SAAOPR (off), and then again
to OBSERVE mode. A second set of observations were obtained, a series of MULTIACCUM (NSAMP=25, SAMP-SEQ=STEP8) and ACCUM (NREAD=1) observations.
These sets of observations were designed to test that the NICMOS instrument was functioning properly. Table 1 presents the 8869 observing parameters.
Table 1. 8869 warm dark observation parameters.
Camera
OBSMODE
Filter
Sequence
NSAMP
NREAD
NIC1,NIC2,NC3
MULTIACCUM
blank
Step8
25
-
NIC1,NIC2,NC3
ACCUM
blank
-
-
1
Real-time monitoring of these activities indicated that the NICMOS instrument was
healthy and was functioning as expected. No status buffer messages were issued throughout the activities. The PAM mechanism was transitioned from HOLD to PAMI, and then
positioned to the correctly commanded location (PAM1). The detectors and TPGs functioned as expected. Data were collected and dumped successfully. After the second set of
dark observations, NICMOS was transitioned back to SAAOPR with the PAM in HOLD.
It will remain in this state until HST Servicing Mission 3B (SM3B) unless any special
tests are required prior to the servicing mission.
Data Characteristics
The NICMOS darks were obtained in parallel with other science observations, and the
Cameras were operated in parallel with one another. No problems were reported in execution or in OPUS pipeline processing of the data. The data were successfully ingested into
DADS. The NICMOS darks were processed with OPUS 11.1. Some of the darks were also
reprocessed with OPUS 12.0 to test the new SAA dark keywords The keywords were correctly populated. For example:
3
Instrument Science Report NICMOS 2000-004
/ POST-SAA DARK KEYWORDS
SAA_EXIT=’2000.171:12:34:49’/time of last exit from SAA contour level 23
SAA_TIME=
1529 / seconds since last exit from SAA contour level
SAA_DARK= ’N/A
’ / association name for post-SAA dark exposures
SAACRMAP= ’N/A
’ / SAA cosmic ray map file
A check of the Standard Header Packet file, spt (SHH), for 8869 visit 01 and visit 02
observations indicated the flight software version number keyword was updated to reflect
FSW 4.0C.
The 8869 darks were taken at a temp of ~230o K. The darks were saturated and exhibited some structure. Figure 1 presents a raw zeroth-read from each camera on the left, and
the same data with the low temperature (~63o K) “super zeroth-read” subtracted on the
right. The super zeroth-read subtraction was performed to remove the fixed bias pattern.
This worked to some degree, although the zeroth-read shading is different at the two temperatures and does not subtract completely.
Part of the observed structure in the images is the fixed bias pattern which normally is
subtracted off by the zeroth-read (as is the zeroth-read shading). Some of the spatial structure looks like the large-scale high-temperature dark current observed during the end-oflife (EOL) monitoring. The EOL darks were obtained during the warm-up of the instrument following exhaustion of the cryogen (Böker et al. 1999, Böker and Jones 1999,
Bergeron et al. 1999). The difference in appearance between the 8869 darks and the EOL
darks is that the intensity level is flatter, and there is no “salt-and-pepper” appearance from
warm pixels. For reference as a comparison of the EOL darks to the observed structures,
Figure 2 presents a comparison of the EOL dark images to the “bright-earth” persistence
image.
Shading
The shading is quite well behaved and more-or-less as expected. All three cameras
show the usual shading v. delta-time dependence, but interestingly they all have shading of
approximately the same amplitude. At these high temperatures (~230o K), the NIC3 has a
*slightly* higher DC level, while at low temperatures (~63o K), the NIC2 has a much
larger amplitude shading than the other two cameras. Figure 3 presents the 8 sec. deltatime shading images for all three cameras. The images have been rotated so that the readout orientation is the same for all three cameras.
4
Instrument Science Report NICMOS 2000-004
NIC3
NIC2
NIC1
Figure 1: Zeroth-read from each camera. Raw image (left) compared with the same image
after subtraction of the low temperature (~63o K) super zeroth-read (right).
5
Instrument Science Report NICMOS 2000-004
Figure 2: Side-by-side comparison of various signals in all three Cameras. Dark current
images (left) during dewar warm up (temperature 82o K). Persistence images (middle) following bright-earth saturation. Dark current images (right) during dewar warm up (temperature 97o K). The dark spot in the upper left of the NIC2 “earth persistence” image
(middle) is where the coronagraphic hole was at the time of the observation. (From Bergeron et al. 1999.)
6
Instrument Science Report NICMOS 2000-004
Figure 3: Warm dark shading images (delta-time 8 second) for each camera, NIC1 (left),
NIC2 (middle), and NIC3 (right). Images rotated so that the readout orientation is the
same for each camera. The 20 pixel vertical bands are due to the over-read timing buffer.
These pixels are read non-destructively and not written to the image scratchpad.
Analysis
The signal seen in the 8869 darks is invariant with exposure time (i.e. saturated), and
nearly zero when compared to the expected zeroth-read bias level of 24,500 DN. Each of
the 26 reads for the MULLTIACCUM observations seem to have the same amount of signal in them. After the 160 sec. exposure time, there is no measurable signal accumulation.
This means one of two things (or some combination of both):
1). The dark current is saturating every pixel in the array in the zeroth-read (0.203s), giving a dark current per pix of > 800,000 e-/s (which appears to be very high assuming a
well depth at 63o K of approximately 180,000 e-).
2). The well-depth, which is known from the EOL data to decrease with increasing temperature, has dropped to nearly zero. At a temperature of 230o K, the detectors are no
longer sensitive to radiation of any kind and are useless for science observations, as
expected. The observed pattern in the warm dark images is just the well-depth structure of
the array at this temperature (the saturation levels).
7
Instrument Science Report NICMOS 2000-004
Conclusions
The NICMOS instrument is healthy and functioned as expected. The detectors and TPGs
functioned as expected. The NICMOS instrument functioned properly before and after
installation of the FSW version 4.0C. Dark observations were successfully obtained with
each camera. The data successfully completed OPUS pipeline processing and ingestion
into DADS. NICMOS performance was nominal.
The 8869 darks are saturated and the zeroth-read bias levels are consistent with the
high temperature. The shading is well behaved and as expected. There is no signal accumulation for the MULTIACCUM observations - all of the 26 reads for the
MULTIACCUM observations have the same amount of signal. This is likely due to a combination of low saturation levels and high dark current.
References
Bergeron, L., Böker, T., Bacinski, J. and Mazzuca, L. 1999, “Observed Characteristics
of the HST/NICMOS Dark Current During the end-of-life Warm-up”, internal document.
Böker, T., Bacinski, J., Bergeron, E., Gilmore, D., Holfeltz, S., Monore, B., and Sosey,
M. 1999, Analysis, results and assessment of the NICMOS warm-up monitoring program,
NICMOS ISR-99-001.
Böker, T. and Jones, M.R. 1999, NICMOS Dark Current Anomaly: Models and Test
Plans, NICMOS ISR-99-009.
8