STIS Update
John Debes, debes@stsci.edu, Sean Lockwood, lockwood@stsci.edu, & Colin Cox,
cox@stsci.edu, for the STIS team
The operations of the Space Telescope Imaging Spectrograph (STIS) have been
remarkably stable, and the COS/STIS team has been focusing on improving calibration
and monitoring the instrument in preparation for Cycle 20 observing.
Time and Temperature Dependence for the NUV-MAMA Dark Rates
In general, the dark rates of the STIS charge-coupled device (CCD) and far-ultraviolet
(FUV) Multi-Anode Microchannel Array (MAMA) continue previous trends. However,
as described in ISR STIS 2011-03 (Zheng, Proffitt, & Sanhow), the near-ultraviolet
(NUV) MAMA dark rates were much higher than expected when the STIS was turned on
after Servicing Mission 4. Now that three years of monitoring have been completed, we
have revisited the analysis of the STIS NUV-MAMA dark rates. The rates have declined
from their previous highs, but not in a consistent fashion. In the first 80 days of the
measurements, the dark rate declined rapidly but displayed large fluctuations (see Figure
1). After this, the fluctuations were reduced and the rate of decline was less steep.
Between October and November 2011, STIS was switched off for a month, and another
discontinuity was observed. Since then, the dark rate has steadily declined.
We analyzed the dark rates separately in the three periods, particularly in checking the
assumption that dark rates were exponentially dependent on detector temperature. As can
be seen in Figure 2, the behavior of the dark rate with respect to temperature is better
described by a quadratic function. The scatter of temperature-corrected dark rates for the
NUV-MAMA is significantly lower after a temperature correction is applied. The time
dependent behavior of the corrected dark rates is well characterized by a two-component
exponential function (See red curve of Figure 1). The COS/STIS team will implement
changes into CALSTIS that reflect this behavior.
STIS CCD CTI Characterization
The harsh radiation environment of space continues to degrade the STIS CCD,
particularly its ability to transfer charge. This degradation, called charge transfer
inefficiency (CTI), is caused by the presence of electron “traps” created by radiation
damage to the CCD detector. These traps impact the signal-to-noise ratio, photometry,
astrometry of fainter sources, and spectroscopy with the STIS CCD. ISR STIS 2011-02
(Dixon) documents how spectroscopy of faint sources is severely degraded when the
source is not placed near the detector amplifier (the E1 aperture position).
The STIS team is in the process of characterizing the charge-trapping behavior of the
STIS CCD. We are investigating the CTI trails of warm pixels (WPs) in various dark
exposures taken during routine calibrations. By measuring the amount of charge in the
trails as functions of position with respect to the CCD amplifier and of number of
electrons in the WP, we will create an empirical model of CTI to apply in a pixel-based
correction scheme.
The planned analysis is similar to that conducted for the Wide Field Camera of the
Advanced Camera for Surveys (ACS) and for the ultraviolet-visible detectors of the Wide
Field Camera 3 (WFC3; see the CTE white paper at
http://www.stsci.edu/hst/wfc3/ins_performance/CTE/CTE_White_Paper.pdf). Depending
on the results and user interest, in the future we may implement a pixel-based correction
for CTI based on algorithms used in CALACS.
Preliminary results are encouraging. The behavior of the WP trails is qualitatively similar
to those seen for both ACS and WFC3, as shown in Figure 3—namely a 30–40 pixel long
trail that contains an increasing fraction of flux as the WP flux decreases. One significant
difference for STIS is a lack of temperature control for the detector, which could add a
layer of complexity to a pixel-based correction. Since the failure of Side-1 electronics in
2001, a constant voltage has been applied to the CCD thermoelectric cooler. A
temperature dependence is observed for STIS CCD dark rates and bias levels, but does
not appear to significantly impact the behavior of WP trails (See Figure 4). This implies
that sufficient characterization of STIS CTI could lead to a robust pixel-based correction.
An example of a pixel-based CTI correction of a STIS CCD dark image using the
PIXCTECORR routine (part of ACSTOOLS in PYRAF) is shown in Figure 5.
Figure 1: Plot of the STIS NUV-MAMA dark rate as a function of time. Blue points are
measured dark rates in the period between September 2009 and July 2012. Significant
scatter is seen due to a dependence of the dark rate on detector temperature, which can
vary with time. When a temperature correction is applied (red points), the scatter is
reduced. The overall behavior of the dark rate is an exponential decrease with time, and a
discontinuity at Oct. 2009 when the detector was briefly turned off.
Figure 2: Plot of NUV-MAMA residuals subtracted from the dark rates of a fiducial
housing temperature for the detector. The residuals are well fitted with a quadratic
dependence on temperature.
Figure 3: Plot of STIS-CCD warm pixel (WP) CTI trails as a function of distance from
the WP for a selection of WP flux levels. Trails are formed from damage to the detector
that traps electrons created by detected photons during readout. As the CCD is read out,
trapped charge becomes released after a delay, causing spurious counts in rows
downstream away from the readout amplifier. This behavior is qualitatively similar to
what is seen in ACS and WFC3.
Figure 4: Plot of counts in the first pixel downstream of a WP (with a flux level of ~6.8
e–/s) as a function of CCD housing temperature, which is a proxy for the true detector
temperature. Our preliminary estimates show changes of ~3.5%/degree, which suggest
that WP trail behavior is only weakly dependent on the temperature fluctuations of STIS.
Figure 5: (left) Uncorrected STIS dark image taken from the bottom of the STIS detector
where CTI is worse. Trails from WPs are clearly seen extending away from the amplifier.
(right) Same area of the detector with a pixel-based CTI correction implemented through
the PIXCTECORR routine in PYRAF. Noise is marginally increased, but the majority of WP
trails are corrected.