Dark Rate of the STIS NUV Detector

SPACE TELESCOPE SCIENCE INSTITUTE Operated for NASA by AURA Instrument Science Report STIS 2011-03 Dark Rate of the STIS NUV Detector Wei Zheng 1, Charles Proffitt 2, and David Sahnow 1 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 per second over the whole detector or 3x10-3 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 Multi- Anode 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 ~1600-3300Å 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 MgF 2 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) 11395 8/24/2009 15 6700 Notes 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: t A =59 days, t B =1427 days and t C =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(-c k /max(om2cat,t min )) (1) where norm is near unity, scale =1.805x10 20, C k =12211.8, OM2CAT is in units of Kelvin and T min ~308K (35 C). T min 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 T min 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(-c k /max(t,t min )) exp(-(t-t 0 )/t) (2) where norm, C k and t are free parameters. T min is fixed at 308.4 K. Time variable t starts from t 0, 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 = 2.9x10 24 exp(-18646/max(t,t min )) exp(-(t-55050)/29)} + 4.9x10 15 exp(-12515/max(t,t min )) 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 C k for all three terms, as allowing more free parameters does not yield clearer results. The best results suggest that dark rate = exp(-9477/max(t,t min )) * (1.24x10 11 exp(-(t-55049)/52.3) + 9.54x10 10 * exp(-(t-55049)/848)+ 7.54x10 10 * 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 C k =9477 in the first term is not the nominal value in formula (1), the improvement in c 2 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 t A and t C are in general agreement with the values derived with a fixed temperature (formula 1), but t B, 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/t k ) assumed, where T k 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 December 2009, normalized to a level of 2.84x10-4 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, C k and T min. 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 approximately at 2.5x10-3 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