March 25, 2021
STIS NEWSLETTERS

About This Article

This STAN provides information regarding the updates made to the STIS Exposure Time Calculator (ETC) for Cycle 29 Phase I, an update to the new STIS CCD spatial scanning mode, STIS news from the recent AAS, and the preliminary release of a Coronagraphic Visualization Tool for preparing STIS coronagraphic observations.

 

STIS Exposure Time Calculator Updates for Cycle 29 Phase I

In November 2020, the 29.1 version of the ETC was released. Two updates specific to the STIS ETC were introduced in this release.

  • STIS throughputs for the CCD modes were updated based on changes to the underlying Time-Dependent Sensitivity (TDS) and updated to the cycle 29 MJD; MAMA modes were updated just to the new cycle 29 MJD .
  • STIS CCD dark rates increased slightly from [0.022, 0.027, 0.032] to [0.023, 0.028, 0.033] cts/s/pix for [low, medium, high] settings, respectively.

In addition, 29.1 provided general updates to the ETC that may affect your calculations with the STIS ETC:

  • The ETC evaluation date has been updated to 2022-Apr-01 (MJD 59670)
  • Updates to many CALSPEC model versions, listed spectral types, and V mag.
  • The zodiacal light scaling table was updated to fix some transcription errors. The locations of corrected values have been marked with an asterisk; neighboring regions will have been affected by interpolation. Users concerned about background-dominated calculations should recalculate their results.
  • Fixed typos in metallicity for Non-Stellar Objects SSP 25 and SSP 5.

In January 2021, the 29.1.1 version of the ETC was released. This update fixes an issue retrieving archived calculations with the COS ETC. The full release notes for 29.1 and 29.1.1 can be found on the HST ETC website.

Spatial Scans with the STIS CCD - New Data Coming for Testing Exoplanet Characterization

Spatial scanning with the STIS CCD is an available-but-unsupported mode for obtaining high S/N ratio spectra of relatively bright targets by trailing the target in the spatial direction within one of the long STIS apertures.  Possible scientific applications include the reliable detection of weak stellar and interstellar absorption features (particularly in the red and near-IR where ground-based observations can be severely compromised by strong telluric absorption) and the accurate monitoring of stellar fluxes (both broad-band and in narrower spectral intervals) for characterizing transiting exoplanets.  As discussed in the 2017 March STAN and in section 12.12 of the STIS Instrument Handbook, spatial scanning offers the potential advantages of improved flat fielding, improved fringe removal (beyond about 7000 Å ), and better correction for cosmic rays, compared to the previously used method of deliberately saturating the CCD detector at a fixed pointing.

Several observing programs utilizing STIS spatial scans have yielded encouraging results.  Cordiner et al. (2017, ApJL, 843, L2; 2019, ApJL, 875, L28) have presented detections of absorption from weak lines of interstellar C60+ near 9500 Å in high-S/N scanned G750M spectra of several reddened early-type stars.  The 2020 September STAN discussed the analysis of a set of 20 scanned G750L spectra of the well-known exoplanet host 55 Cnc that were obtained under STIS special calibration program 15383 (C. Proffitt, PI), which was designed to test the reproducibility of fluxes that can be achieved with this mode.  After custom corrections for cosmic rays and fringing, application of linearized de-trending fits (commonly used in analyses of such time-series data) yielded residuals in the total fluxes of order 30 ppm – which is comparable to the best precisions achieved for exoplanet host fluxes via other HST observing modes.  A more detailed discussion of those results may be found in Welty et al. (2021, BAAS 237, 350.06).

While these results are encouraging, they are based on somewhat limited data.  A follow-on STIS special calibration program (16442; D. Welty, PI) has therefore been approved for cycle 28, in order to better understand the instrumental systematics of this observing mode.  Of order 45 short spatial scans of 55 Cnc (very similar to those obtained in program 15383) will be obtained over five consecutive orbits – providing both a longer baseline within a single visit and a comparison with the data from that prior program.  A transit of the super-Earth 55 Cnc e will also be included, in order to see how well the transit parameters can be reproduced via spatially scanned spectra. As for the data from program 15383, these new spectra will be made publicly available as soon as they are obtained.

As noted in the 2020 September STAN, spatial scanning with the STIS CCD may be advantageous for some applications – e.g., for obtaining higher resolution spectra of brighter targets (V < 7.5) – particularly at wavelengths greater than about 6000 Å, where WFC3/UVIS spectra will exhibit saturation and contamination from overlapping orders.  More detailed information regarding observing strategy – e.g., determining the scan parameters for observing a particular target (exposure time, scan rate, initial target positioning) and obtaining custom flat-field exposures – may be found in Sec. 12.12 of the STIS Instrument Handbook; specific questions may be addressed to the STScI HelpDesk.  The spectra of 55 Cnc obtained for calibration program 15383 are publicly available via the MAST archive portal; the data for program 16442 will be available as soon as they are obtained.  We would encourage those who may be interested in using STIS spatial scans to download and examine those data, in order to gauge the potential utility of this technique for their own research projects.

Up-to-date scheduling information on the program and links to the data can be obtained from the Visit Status page for 16442

New STIS Instrument Updates and Scientific Results from the 237th American Astronomical Society Meeting

The 237th AAS Meeting was held as a virtual conference from January 10-15, 2021. Here we describe two poster updates from the STIS team that capture recent instrument-related news in order to aid the user community in understanding the capabilities of the instrument. These posters offer 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.

HST/STIS Recent Instrument Highlights and Calibration Updates: CCD Rotation, Coronagraphy, Spectral Defringing and Flux Calibration

This poster summarizes a variety of recent updates: (1) an investigation into the long-term rotational evolution of the CCD position angle, with comparisons to recent studies of rotation in the spectral traces; (2) a summary of existing coronagraphic modes and available tools for planning high-contrast observations with the STIS coronagraphs; and (3) the official releases of new spectra defringing tools for the G750M and G750L modes and new reference files for the re-measured flux sensitivity for echelle spectra taken in the E140M mode.
https://aas237-aas.ipostersessions.com/?s=77-15-F5-80-8C-5E-2B-88-AD-B5-53-DB-81-F8-B4-EE

Performance of HST/STIS Spatially Scanned Spectra for Time Series Photometry of Exoplanet Host Stars

The newly-available spatial scanning mode with STIS has applicability for a variety of science cases, including measuring stellar and interstellar absorption in the red and near-IR and obtaining very accurate time series measurements of stellar fluxes (e.g., for transiting exoplanet atmospheric characterization). This poster describes results from a calibration program to assess the performance of this mode and its orbit-to-orbit reproducibility based upon multiple spectra of a known exoplanet host star.
https://aas237-aas.ipostersessions.com/?s=C9-CC-BC-15-EC-55-01-B6-41-11-D4-59-87-2F-0C-69

(For more details on Spatial Scanning, see a detailed summary in the September 2020 STAN)

For previous AAS presentations from the STIS team, recent archived posters are also available from 2020 and 2019.

For interested readers, we also provide links to some recent scientific results using STIS, which were presented as iPosters and talks at the virtual AAS meeting:

 

Galactic Science iPosters:

 

Galactic Science Talks (recording requires AAS log-in at https://my.aas.org/services/AAS237)

 

Extragalactic Science iPosters:

 

Extragalactic Science Talk (recording requires AAS log-in at https://my.aas.org/services/AAS237)

 

STIS Coronagraphy Visualization Tool (Preliminary Release)

Some of the most important steps in planning and preparing coronagraphic observations with STIS involve (1) selecting the appropriate occulter position (from the various supported fiducial apertures, visualized below in Figure 1 on an on-orbit lamp flatfield) and (2) determining the appropriate orientation of the observatory to conduct science observations, often at multiple telescope roll angles to provide angular diversity of imaging for the purposes of post-processing. Considerations for these decisions include avoiding the locations of the central star’s diffraction spikes relative to any scientific feature(s) of interest, and verifying the true north angle of the CCD detector to understand where a given companion or disk feature might be located.

Figure 1

Figure 1. Supported fiducial positions for STIS coronagraphy, showing the various positions along WEDGEA (vertical) and WEDGEB (horizontal), in addition to the two occulting BAR positions. (Image from section 12.11 of the STIS Instrument Handbook).

To this end, in advance of the Cycle 29 deadline, we offer a preliminary version of a coronagraphic aperture visualization tool for STIS that users may find beneficial in visualizing their scientific scenes in various STIS coronagraphic configurations. The tool is provided as a Jupyter Notebook with some examples that users can adapt and modify for their scientific planning and specific use cases (see Figures 2-4). This tool is still in development, but users may find the early version of the notebook valuable as an approximate illustration of the various configurations and options for STIS coronagraphy. The tool is currently available through the STIS website software tools page.

Figure 2

Figure 2. Example 10”x10” field of view from the visualization tool for a point source companion (magenta circle), with the central star placed behind the BAR10 occulter (blue). The field of view is shown in the detector reference frame, with the true North angle shown as the lime green arrow at 0º on the polar plot, and the telescope ORIENT position of 195º provided in the upper right. The four diffraction spikes from the central star are shown as orange arrows, with a 20º zone of avoidance for each spike (red shaded regions). The companion’s position is at 1.75” with a position angle of 45º; at this ORIENT angle, the companion would be visible and unaffected by scattered light from the diffraction spikes.

Figure 3

Figure 3. The same companion configuration and observing set-up as shown in Figure 2, but with a different ORIENT position of 33º. At this ORIENT angle, the companion would not only lie within the diffraction spike avoidance region (red shaded area), but would also be partially obscured by the BAR10 occulter itself.

Figure 4

Figure 4. An example of a simple disk feature, with the central star placed behind the WEDGEA1.0 occulter, in a 15”x15” field of view. The ORIENT angle of 45º means that the true North angle (green arrow) is aligned with the detector Y-axis (AXIS2). The disk has a semimajor axis of 6”and position angle of 300º. Zooming into the detector at this selected field of view shows both of the vertical WEDGEA and horizontal WEDGEB occulters (shown in blue), to aid in observation planning.

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