STIS Update

John Debes,, Sean Lockwood,, & Colin Cox,, 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).