STIS Update

John Debes,, on behalf of the STIS Team


The Space Telescope Imaging Spectrograph (STIS) is still a significantly used instrument on the Hubble Space Telescope, accounting for 15% of prime General Observer orbits in Cycle 19. We continue to monitor and improve upon the performance of the instrument since its revival in 2009, with two Hubble cycles of monitoring complete since Servicing Mission 4 (SM4). Here we report on corrections for geometric distortion, time-dependent sensitivity of spectral modes, and dark currents in STIS’s charge-coupled device (CCD) and near-ultraviolet (NUV) multi-anode micro-channel array (MAMA).


Geometric distortion corrections for STIS

Geometric distortion is most important for precision astrometry, but it can also affect photometry if incorrectly characterized. After SM4, a re-analysis of each detector’s geometric distortion found that the CCD and NUV MAMA detectors were well represented by the older distortion corrections. For the far-ultraviolet (FUV) MAMA, however, the distortion correction was discrepant by two–four pixels on average (see Figure 1). We have developed new corrections, which we will integrate into the calstis calibration software. The average error of the new FUV-MAMA distortion corrections is less than two pixels.












Time-dependent relative sensitivities of STIS CCD spectroscopic modes

The relative sensitivities of most STIS spectroscopic modes continue to decline with time. In the optical and near infrared, the relative sensitivities of the STIS CCD grating modes show rates of decline similar to those encountered before and after SM4 (–0.21%/yr and –0.17%/yr for G430L and G750L, respectively). In the NUV, however, we find two notable exceptions: G230L shows a moderate increase in relative sensitivity (0.09%/yr; see Figure 2), and G230LB shows a flat trend with time. Since the Cosmic Origins Spectrograph is showing a similar increase in NUV relative sensitivity, this trend may be due to a change in the Hubble optical telescope assembly, rather than to instrument-specific causes. In the FUV, the relative sensitivity of the G140L mode has been declining after SM4 at a rate of –0.23%/yr.