March 2019 STAN
Here we summarize the posters presented at the most recent AAS meeting in January. The STIS Team presented posters detailing available modes with STIS, visible light observations using coronography, spatial scanning with STIS, and an update on the STIS software tools.
At the 2019 Winter American Astronomical Society Meeting, the STIS team presented several new posters to aid the user community in understanding the capabilities of the instrument. These posters are useful reference materials for anyone interested in submitting a proposal to observe with STIS, and therefore this article serves to provide them to the scientific community.
STIS Available Modes Poster
STIS is an incredibly flexible instrument due to the large variety of available (supported and unsupported) observing modes. With this flexibility comes the challenge in that it can be difficult for users to pinpoint which modes best serve their needs. In an effort to summarize all STIS modes, the STIS team presented a poster that contains all of the available and supported observing modes, provides their specifications, and highlights example scientific use cases.
Click herefor the STIS Available Modes Poster.
Unique Visible Light Coronagraphy with STIS
STIS is home to the only currently operating coronagraph in space not trained on the Sun. With the potential for future space-based coronagraphs, and the progressing state of ground-based coronagraphy spurred on by extreme adaptive optics, the STIS team has created a poster that evaluates the high contrast capabilities of the STIS coronagraphic aperture 50CORON.
Click here for the STIS Coronagraphy Poster.
Spatial Scans with the STIS CCD
Spatial scanning with the STIS CCD is a recently enabled, available-but-unsupported observing mode. As advertised in several recent STANs, spatial scanning allows the observer to obtain high-S/N ratio spectra of relatively bright targets by trailing the target in the cross-dispersion direction within one of the long STIS apertures. The STIS team has created a poster that details its methodology and applications.
For more information, click here.
The State of Software Tools for STIS
STIS was installed in 1997, and through continued development, the ecosystem of software support for STIS calibration and data analysis looks much different today compared to when STIS was first installed. Recently, this has been made even more apparent through the transition away from the venerable IRAF/PyRAF to a pure Python software suite. In light of this, the STIS team created an iPoster for the past AAS that summarizes the current suite of software tools available to STIS users for data analysis and calibration. This poster also details the status of the IRAF/PyRAF to Python transition and lays out the next steps the STIS team will take to improve the STIS software ecosystem.
Click here to find the STIS software tools iPoster.
On October 5, 2018, the Hubble Space Telescope (HST) experienced a failure of Gyro-2, and normal HST operations were suspended. Gyro-2 had been experiencing elevated jitter rates and increased failed acquisitions for the past year. After the failure, the team at NASA Goddard powered on Gyro-3 (the last of the backup gyros). Upon start-up, however, it was found that the Gyro-3 rate bias was significantly higher than normal and higher, even, than operational levels. Because it was not performing at the required level for operations, Gyro-3 was reset and exercised in an attempt to restore it to its operational limits. HST remained in a suspended state until October 27, 2018 when the Gyro-3 bias drift rate was finally brought within operational levels, though still larger than expected (see Figure 1). The telescope jitter improved from before Gyro-2 failed, and over the past 4 months, the Gyro-3 bias drift rate has slowly decreased. All three of the gyros now in use are enhanced models, with a longer expected life than the three that have failed since SM4.
Figure 1: The RMS jitter in the STIS MAMA and CCD since SM4. The y-axes show the RMS jitter in the V2 (top panel) and V3 (bottom panel) frames. The purple points show that, although the jitter after switching to Gyro-3 is high, it’s still an improvement from the previous configuration.
After the almost month-long suspension, the STIS team monitored the first science and calibration data taken for all detectors very closely. While the CCD bias and the dark levels for both the CCD and the FUV-MAMA were nominal and performing as expected, the NUV-MAMA detector dark rate was initially ~2 times higher than normal, and took about one month to settle back into the normal regime. Since then, the NUV-MAMA dark rate has remained at this nominal level (see Figure 2). When STIS was last not in operation for an extended period of time in 2010, the elevated dark current took a few months to return back to normal levels, so this behavior was not unexpected.
Figure 2: The NUV-MAMA count rate from July 1, 2018 through the present. The greyed-out section is when HST was in safe mode.
While all of the STIS detectors have returned back to nominal values, we are still seeing some effects from the new Gyro-3. Although the Gyro-3 bias is continuously decreasing, it is still high, and can occasionally cause the loss of science observations. Users should always check their data promptly to make sure their data have not been adversely affected by any unforeseen instrumental effects.
ETC 27.1 was updated to use an evaluation date for calculations from mid-cycle 26 (MJD 58604; 2019-May-01) to mid-cycle 27 (MJD 58940; 2020-Apr-01). Though the underlying time-dependent sensitivity (TDS) trends remain unchanged, we modeled a small linear decrease in system throughput over this period of time. The greatest throughput changes occurred at the blue end of the FUV gratings, on the order of -1%, with typical changes much less.
Likewise, we extrapolated the STIS/CCD dark rate from [0.021, 0.025, 0.030] to [0.021, 0.026, 0.031] counts/s/pixel at the [top, middle, bottom] of the detector to reflect the new evaluation date. These trends were derived from temperature-corrected darks, and the "low" value at the top of the detector was adjusted upward by 1σ of the temperature correction distribution to give a conservative value for the detector's lowest dark rate region (near the read-out amplifier and E1 position).
The NUV and FUV dark rates and CCD read noise parameters remain unchanged.