SPACE
TELESCOPE
SCIENCE
INSTITUTE
Operated for NASA by AURA
Instrument Science Report STIS 2011-03
Dark Rate of the STIS NUV Detector
Wei Zheng1, Charles Proffitt 2, and David Sahnow1
1
Department of Physics & Astronomy, Johns Hopkins University, Baltimore, MD
2
Space Telescope Science Institute, Baltimore, MD
September 7, 2011
__________________________________________________________________________
ABSTRACT
The dark rate of the NUV detector of the Space Telescope Imaging and Spectrograph
(STIS) has undergone a sharp rise after the instrument’s repair after the fourth Hubble
Servicing Mission (SM4). Variations in the dark rates can be modeled with rapid declines
on the time scale of several weeks plus a long-term one over three years. The dark rate is
known to be dependent on the detector temperature, and the newly derived parameters are
different from those prior to August 2004. Currently, the typical rate is about 3000 counts
-3
per second over the whole detector or 3x10 count/sec/pixel.
Contents:
•
•
•
•
•
•
•
•
•
Introduction (page 2)
Observations (page 3)
Time Evolution of the Dark Rate (page 4)
Temperature Dependence (page 5)
Spatial Variations (page 7)
Long Term Trend (page 8)
Reference File (page 9)
Summary (page 10)
References (page 10)
Operated by the Association of Universities for Research in Astronomy, Inc., for the
National Aeronautics and Space Administration
1. Introduction
The Space Telescope Imaging Spectrograph (STIS) consists of a CCD and two MultiAnode Microchannel Array (MAMA) detectors. The near-ultraviolet (NUV) MAMA
detector is a sealed detector with a semi-transparent cesium telluride photocathode
deposited on the inside of the window. Incident photons between wavelengths ~16003300Å generate photoelectrons at the photocathode, and these electrons enter the front
surface of the microchannel plate (MCP). The charge cloud exiting the back end of the
MCP is centroided by the anode array into 1024×1024 square pixels. The main source of
the background counts is the phosphorescence of impurities in the MgF2 detector
faceplate. Extensive studies of dark currents carried out during the first seven years of
STIS operation suggest that there is a population of impurity sites each having three
levels: (1) a ground state, (2) an excited energy level which can decay immediately to the
ground state, and (3) a meta-stable level that is at an energy level slightly below the one
that can emit radiation. The meta-stable states are initially populated by charged particle
impacts that mostly occur during passages through the South-Atlantic Anomaly (SAA).
Hours or days later, the electrons trapped in these meta-stable states are thermally excited
to an unstable upper level and then emit a photon as they decay to the ground state.
Others causes include charged particles from the environment and scattered light from the
optical system.
Figure 1. History of the STIS NUV dark rate. Between 1998 and 2004, it remained at a
low level of approximately 0.0011 count/sec/pixel. Four surges took place at the
beginning of STIS operation (1997), in early 2000 (SM3a), mid 2009 (SM4) and late
2010 when the detector was turned on after several years in safe mode. The surge shortly
after SM4 is unprecedented in terms of its large scale and slow recovery.
STIS ISR 2011-03 Page 2
The NUV dark rate, as shown in Figure 1, had been generally stable between 1998 and
2004, at a level between 8x10-4 and 1.7x10-3 count/sec/pixel, except for two short periods
after the initial installation and the third servicing mission (SM3a). After a surge, the
dark rate was observed to decline within weeks. However, STIS was in a cold safe mode
for more than four years before SM4. When it was first powered on in mid-2009, the dark
rate was surprisingly high, by nearly tenfold. In late 2009, it became evident that the
behavior of the NUV dark rate was different from previous records: Although it had
declined significantly over several months, it did not return to the nominal level. In this
report, we focus on the trend of NUV dark rate after SM4, between August 2009 and
May 2011.
2. Observations
As part of Servicing Mission Orbital Verification (SMOV) process, frequent
measurements of the STIS dark rates started soon after SM4. As shown in Table 1, we
have collected data from four calibration programs, with 131 useful images and a total
exposure time of 164,124 s. Program 11402 was executed between August 06 and
September 06, 2009 with four 1380s dark images in ACCUM mode per week. Program
11895 was executed on August 24, 2009 (Modified Julian Date [MJD] 55067), with
exposures of 300-800s. Cycle-18 program 11857 took two exposures of 1380s weekly in
ACCUM mode between September 2009 and August 23, 2010, when the STIS
instrument was placed into a safe mode. Cycle-18 program 12415 began on November
16, 2010, when STIS was again powered on, and has taken two exposures bi-weekly in
TIME-TAG mode. These pairs of exposures are arranged in such a way that they are
separated by approximately 4 to 7 orbits to ensure that they are taken in different parts of
the same time block that is free from the SAA.
Table 1: List of NUV Dark Programs since SM4
Proposal
Dates of Observations
Number
of Images
Total
Exposure
Time (sec)
Notes
11395
8/24/2009
15
6700
11402
8/06/2009-9/06/2009
43
55965
4 per week
11857
9/09/2009-8/23/2010
51
70275
2 per week
12415
11/16/2010-5/29/2011
34
43084
2 bi-weekly
STIS ISR 2011-03 Page 3
3. Time Evolution of the Dark Rate
For each raw dark image, we calculate the average count rate over the entire MAMA
detector, and mark hot pixels with count level of 8 and higher. We also retrieve telemetry
data OM2EVQ (NUV MAMA event count) and OM2CAT (detector amplifier
temperature) and select the count rates at temperature of 38°C and near the time when a
dark image is being taken. Since telemetry data are taken under different commands,
there is no guaranty that useful data exist for a given dark time frame. Near an SAA
passage, the dark rate may also be enhanced. As a result, a few data points are excluded
as their significantly high flux level is inconsistent with dark measurements. However,
most of such data points seem to be useful. As shown in Figure 2, we fit the data points
with Marquardt least-square algorithm, with three time components of exponential decay:
“A” as a rapid term shortly after SM4, “B” as a long term, and “C” as a short term after
MJD 55516, November 16, 2010. The best fit yields the following e-folding time scales:
tA=59 days, tB=1427 days and tC=11 days.
Figure 2. Dark rate after SM4, at detector temperature OM2CAT=38°C. The values are
derived from the telemetry data keyword NUV MAMA event count (OM2EVQ), which
gives the total count rate. The data between MJD 55049 and 55453 are fitted with two
exponential terms with e-folding time of 59 and 1427 days, respectively. After MJD
55516, another component C is added in the fitting.
STIS ISR 2011-03 Page 4
4. Temperature Dependence
Most STIS observations are scheduled in blocks of orbits, and dark images are taken at a
variety of temperatures. The temperature of the STIS MAMA detector never reaches
equilibrium, as the MAMA high-voltage power supplies are shut down frequently. The
low-voltage power supply cycles around the SAA passage, and MAMA is only used in
SAA-free blocks, usually 4-6 orbits in one block per day. Usually temperature increases
with time within a given block. As a result, the dark rate fluctuates considerably. The
rate of thermal excitation from a meta-stable state, as discussed in §1, is proportional to
exp(-DE/kT), where DE is the energy difference between the levels. The behavior of the
count rate vs. temperature, as shown in Figure 3, leads to an estimate of 1.1 eV for DE.
The nominal expression of this temperature dependence in the STIS CALSTIS pipeline is
dark rate = norm * scale * exp(-Ck/max(OM2CAT,Tmin))
20
where norm is near unity, scale =1.805x10
Kelvin and Tmin ~308K (35°C).
(1)
, Ck =12211.8, OM2CAT is in units of
Tmin was introduced prior to SM4 because, at low temperature, the dark current often did
not drop as low as the exponential fit predicted. In retrospect, this was a clue that states
with a wide range of parameters contributed to the dark current. Both norm and Tmin were
slowly varying functions of time that are empirically adjusted to give a good match to the
observed dark rate, and which are tabulated in the temperature dependent dark correction
table (tdc) reference file. The temperature for a given observation is taken from the
OM2CAT telemetry value, which is included in the extension header of each MAMA
observation. This approach typically predicts the dark rate with 5 to 10% accuracy,
although the error in individual cases may be substantially larger.
This temperature dependent function, with a limited use in correcting science data and
making a long term prediction, did a good job of predicting the short-term response of the
NUV dark current to temperature changes. The longer-term response is believed to be
more complex. In equilibrium, the number of decays will match the number of excitation
events. A sudden temperature increase would then result in an initial rapid increase in the
dark rate, as the meta-stable states are more easily depopulated at higher temperatures.
However, after several days, the population of meta-stable states would reach a new
equilibrium, resulting in a dark rate that, while higher than the equilibrium rate was at the
cooler temperature, is significantly lower than the short-term response to the same
temperature increase. Such large temporary increases in the dark rate were observed after
the initial installation in 1997 and after the SM3a in 2000.
STIS ISR 2011-03 Page 5
This model of the NUV MAMA window glow as a single population of meta-stable
states with a single band gap and only one time-scale for de-excitation does not work for
the post-SM4 data. We model the dark rate after SM4 with an exponential function of
instrument temperature and multi-component exponential decays over time. Namely,
each term takes the form of
norm * scale * exp(-Ck /max(T,Tmin)) exp(-(t-t0)/t)
(2)
where norm, Ck and t are free parameters. Tmin is fixed at 308.4 K. Time variable t starts
from t0, when a surge took place.
In May 2010, we obtained the following fitting results based on the data in hand at that
time:
dark rate =
24
2.9x10 exp(-18646/max(T,Tmin)) exp(-(t-55050)/29)} +
15
4.9x10 exp(-12515/max(T,Tmin)) exp(-(t-55050)/398),
(3)
where T=273.16+OM2TUBET (temperature of the STIS MAMA tube in units of
Celsius), t in MJD. Fitting with other temperature parameters yields similar results, as
these telemetry data follow a similar pattern. The results suggest that there are now likely
more than one meta-stable state in the population of impurity sites, each of which has a
different depopulation pattern.
While it would be useful to estimate the time scale for the long-term decline, our efforts
are complicated by the lack of data between August and November 2010 and another
surge afterward. Recently we fit all the data points since SM4 with three exponential
terms. For simplicity we adopt a single temperature parameter Ck for all three terms, as
allowing more free parameters does not yield clearer results. The best results suggest that
11
dark rate = exp(-9477/max(T,Tmin)) * (1.24x10 exp(-(t-55049)/52.3) +
10
10
9.54x10 * exp(-(t-55049)/848)+ 7.54x10 * exp(-(t-55516)/14.6)),
(4)
where the last term is only applied for dates MJD >55516. Note that these parameters
display considerable uncertainties because of the limited number of data points. While
the temperature parameter Ck=9477 in the first term is not the nominal value in formula
2
(1), the improvement in c is not significant. For simplicity, we implement the nominal E
value in the current tdc reference file. As shown in Figure 6, the difference in fitting
residuals is acceptable. The time scales tA and tC are in general agreement with the values
derived with a fixed temperature (formula 1), but tB, the long-term scale, is considerably
STIS ISR 2011-03 Page 6
longer. In principal, the values derived from a fixed temperature should be more reliable,
therefore we believe that the long term scale of decline is approximately 1000 days.
Figure 3. Count rate vs. OM2CAT after SM4. Symbol sizes increase with observation
dates, namely the latest results indicate lower dark rates. The solid curve shows the
model fit at the end of May 2011, and the dashed curve for the old fit originally used in
CALSTIS pipeline prior to SM4.
5. Spatial Variations
While the distribution of dark rate in the STIS NUV MAMA detector is more
homogeneous than its FUV counterpart, there is a known gradient: from the top right to
the lower left, the dark rate varies by approximately 30% (Fig. 4a). Most of the difference
is associated with a glow region near the lower and left edges, as the difference between
the detector center and the upper-left region is merely 5%. The comparison between the
dark images taken before and after SM4 suggests that this pattern has not changed
noticeably. The count ratios between these images (Fig. 4b), summed over boxes of 100
x 100 pixel, typically match each other within 3%.
STIS ISR 2011-03 Page 7
Figure 4. Distribution of dark counts on the NUV MAMA detector. The left panel (a):
The summed dark image: there is a gradient from the top right to the lower-left. The right
panel (b): ratio of dark counts between summed images before and after SM4.
6. Long Term Trend
As of May 2009, our model fitting, as shown in Figure 5, suggested a dual-component
decline over time, with the long-term e-folding scale being ~300 days. But afterward the
dark rate seemed to decline more slowly than this model predicts. The monitoring process
was then complicated by another surge observed in November 2010. The latest data seem
to be consistent with an even longer time scale, on the order of 2-3 years. It is extremely
unlikely that the rate will return to its nominal level from 2004. Note that the NUV
MAMA detector of the Cosmic Origins Spectrograph (COS) is a flight spare of this STIS
detector. The COS NUV detector, operating at a lower and controllable temperature,
displays an increasing pattern of the dark rate over time (Zheng et al. 2010), albeit at a
considerably lower level.
The COS window was expected to show a lower level of window phosphorescence than
did the STIS detector. Initially the dark rate was even lower than expected, but it since
has been increasing linearly with time. It is expected that this represents an ongoing
population of similar meta-stable states, and it would take about three years (i.e., 2014)
for the dark rate of the COS NUV detector to reach the pre-SM4 level of its STIS
counterpart.
STIS ISR 2011-03 Page 8
Fig. 5. Model fit to the dark-rate data after SM4. Three terms of exponential decline over
time are assumed, with an exponential factor of temperature dependence exp(-9477/Tk)
assumed, where Tk is OM2CAT in units of Kelvin. In the lower panel residuals are
plotted on the same scale. Except for a few points, the fit yields accuracy with 15%.
7. Reference File
The CALSTIS pipeline utilizes two dark reference files: “drk” and “tdc”. The latest dark
image file was produced by summing 72 dark images taken between August and
-4
December 2009, normalized to a level of 2.84x10 counts/sec/pixel, for raw data at
2048x2048 format. This file has been applied in the calibration pipeline since December
21, 2009. The average signal-to-noise ratio is approximately 14. The latest tdc file,
produced in December 2010 and applied in the pipeline since February 10, 2011,
provides parameters for CALSTIS in formula (1): norm, scale, Ck and Tmin. Since there
is no time-dependence term in CALSTIS, we set parameter norm at close time intervals
to follow the decline trend.
To test the effectiveness of the new reference files, we ran CALSTIS on these dark
images themselves. The results are plotted in Fig. 6, with typical residuals at
approximately 6% level. The current tdc file predicts a dark rate at the December 2010
level (MJD ~ 55550) and assumes such a constant afterward. Should we observe further
significant change in the dark level, a new tdc file will be produced.
STIS ISR 2011-03 Page 9
Figure 6. Dark subtraction with the latest reference TDC file. Solid triangles above are
the average count rates for dark images taken after SM4. Crosses in the lower panel are
the residual values after CALSTIS processing.
8. Summary
The NUV MAMA detector of STIS displays a significantly higher level of dark rate after
SM4. The rate decline can be modeled with exponential terms over time, with short ones
on the order of weeks and a long term over 2-3 years. The temperature dependence can
be modeled with a new set of parameters, which are different from those derived before
the STIS failure in August 2004. We anticipate that the dark rate will remain
-3
approximately at 2.5x10 count/sec/pixel in the next 1-2 years.
9. References
Proffitt, C. et al. 2010 “STIS Instrument Handbook”, Version 9.0 (Baltimore: STScI)
Sahnow, D. et al., 2011, COS NUV Detector Dark Rates During SMOV and Cycle 17,
COS ISR-2010-12.
Zheng, W. et al., 2010, in HST Calibration Workshop, ed. S. Deustua & C. Oliveira, in
press
STIS ISR 2011-03 Page 10