A description is provided of the overall performance of the STIS CCD after HST Servicing Mission 4 (SM4) and during Cycle 17 calibrations. Most aspects of CCD performance are found to be consistent with extrapolations of the trends seen prior to the failure of STIS in August 2004. The gain values for 1 and 4 have not measurably changed from pre-SM4 measures. The read noise through Amp D, as determined from unbinned bias images taken during Cycle 17, is slightly higher than pre-SM4 values. As is expected due to the on-orbit radiation environment the dark current continues to increase. The spurious charge has increased as well and has a slope that results in a smaller value at the top of the chip (near the readout amplifier) when compared to the center of the chip. The charge transfer inefficiency (CTI) also continues to increase although the increasing dark current will noticeably minimize actual CTI losses for typical exposure times exceeding several hundred seconds. The post-SM4 CTI measured values agree well with extrapolation of the trend seen through Cycle 17 calibrations. Changes in the sensitivity of STIS CCD modes have been modest. 1. Dark Normalization After the failure of Side-1 electronics and the transition to Side-2, the CCD detector temperature could not be held at a constant value because Side-2 lacks a working thermistor on Space Telescope Science Institute, Baltimore, MD 21218 European Space Agency 449 450 Wolfe et al. the CCD chip. A thermistor located on the CCD housing acts as a proxy to monitor, not control, the CCD thermal variations. Figure 1 depicts the fluctuating housing temperature over the one year since SM4. A scaled measure of the net solar illumination (solid line in Figure 1) appears to account for most of the housing temperature variations and hence can be used as a predictor for the CCD dark count. The measured dark current correlates well with the housing temperature and the fluctuations in the dark current can be scaled using equation 1: normalized dark image = (dark image) × (1 + slope × (TO − T)) (1) where normalized dark image is the result, dark image is the dark image being normalized, slope is a constant scaling factor, TO is the reference temperature, and T is the housing temperature of the exposure. Table 1 shows the old and new slopes and reference temperatures used to normalize the darks. Figure 1: Note that there is less variation in the measured housing temperature well after the SMOV4 testing period. During a portion of the SMOV4 period, a SIC&DH safing led to suspension of STIS and resultant instrument cooling. The solid curve is an appropriately scaled integrated solar flux (average solar radiation times sun illumination time) impinging on HST. After end of SMOV4, variations of the housing temperature appear to track the integrated solar flux variations. Table 1: Reference Temperature and Slope Dates Slope Reference Temperature (TO) Dark Exp Time 07/2001 08/2004 0.070/C 18.0 C 1100 s 06/2009 Present 0.056/C 22.0 C 1100 s Updated Status and Performance of the STIS CCD 451 2. Sensitivities STIS sensitivities for the CCD low resolution grating spectral regions are presented in Table 2 for 2010 and 2004 relative to the initial 1997 measures. Sensitivities for the less fully sampled medium resolution gratings follow the same trends. Table 2: Updated Sensitivities Grating Sensitivity (2004) Sensitivity (2010) G230LB 0.949 0.920 G430L 0.981 0.964 G750L 0.988 0.978 3. Charge Transfer Inefficiency Note that the value quoted for the CTE is in terms of charge transfer inefficiency (CTI). The CTI slope quoted in Table 3 is for the linear equation (Goudfrooij et al. 2009) that evaluates the CTI as time progresses. This time-dependent equation is: CTI(t) = (CTIO) × (1 + α(t − tO)) (2) where CTI(t) is the value of the CTI at a specific time t, CTIO is the CTI extrapolated to zero background, α is the slope of the time dependence, t is the time in years, and tO is 2000.6. Note the value for α can be found in Table 3. The value for α is derived from the Internal Sparse Field Test (Goudfrooij et al. 2006). Another CTI test is the Extended Pixel Edge Response (EPER). This test, while not being an absolute measure of CTI, produces a relative measure of CTI thus allowing a robust measure of time trends (Janesick 2001). Figure 2 plots the CTI as a function of time for both the parallel (top plot) and serial (bottom plot) directions. The parallel CTI increased noticeably more than predicted from a linear extrapolation from pre-SM4 measures while the serial CTI increased marginally. Note that the serial CTI is about an order of magnitude less than the parallel CTI measure. However, the achievable operating temperature for the STIS CCD is ∼ -83 C, well within the temperature range where there is a strong dependence upon temperature. This is demonstrated in Figure 3 which shows the ratio of the derived EPER CTI value, relative to the expected value based upon the linear fit shown in Figure 2, plotted against housing temperature. The parallel CTI appears to show a 20 to 40% increase beginning abruptly at temperatures above 20 C. The serial CTI temperature dependence is noticeable, but dominated by measurement noise and/or additional contributions from other sources.
The ultraviolet energy distribution of the metal-poor supergiant HD 112374 is analyzed based on observations from the International Ultraviolet Explorer (IUE) satellite for the region between 1200 and 2000 A. A discontinuity was found in the UV spectra at 2600 A which confirmed the low-abundance of heavy elements found by Luck et al. (1983). Values for effective temperature and log g in HD112374 were consistent with the star being a very luminous Population II semi-regular variable. The full observational results are presented in a table.
We describe the long term changes in the STIS FUV MAMA detector’s dark current and hot pixels between the initial installation of STIS into HST in 1997 and August 2004, when the failure of the STIS side-2 electronics rendered STIS inoperable. The typical level of the dark current and the number of hot pixels both increased substantially over time. While the low pre-launch measurement of1.6 x 10 −6 counts/hi-res-pixel/s persisted in a small corner of the detector, much of the FUV MAMA was usually covered by a glow that increased in intensity through each daily SAA free period when the detector was in use. The rate at which the bright glow appeared and intensified increased with time. This glow was too unpredictable to make removal of a mean glow image in the OTFR pipeline feasible. The number of hot pixels on the FUV MAMA detector had also been increasing over time. In 1997, only 97 out of the more than 4 million pixels had a dark rate of more than 10 −4 counts/s. By 2004 about 2500 pixels were at this level. Observational and data reduction strategies for mitigating the effects of the increased dark current and the increased number of hot pixels are discussed.
The Near-Infrared Spectrograph (NIRSpec) on board of the James Webb Space Telescope will be the first multi-object spectrograph in space offering ~250,000 configurable micro-shutters, apart from being equipped with an integral field unit and fixed slits. At its heart, the NIRSpec grating wheel assembly is a cryogenic mechanism equipped with six dispersion gratings, a prism, and a mirror. The finite angular positioning repeatability of the wheel causes small but measurable displacements of the light beam on the focal plane, precluding a static solution to predict the light-path. To address that, two magneto-resistive position sensors are used to measure the tip and tilt displacement of the selected GWA element each time the wheel is rotated. The calibration of these sensors is a crucial component of the model-based approach used for NIRSpec for calibration, spectral extraction, and target placement in the micro-shutters. In this paper, we present the results of the evolution of the GWA sensors performance and calibration from ground to space environments.
