JULY 3, 2019
STIS NEWSLETTERS

July 2019 STAN

This STAN provides information about best practices when submitting Phase II proposals, notification of a recently-published paper in JATIS regarding coronagraphy with STIS, details about the STIS data handbook update, and information about a new job opportunity at STScI.

STIS Best Practices for Submitting Phase IIs

The Phase II files submitted by Guest Observers (GOs) are the detailed descriptions of the observations to be carried out for each accepted program. Before any planned observation is executed, it must first undergo a technical feasibility review and, for programs using one of the STIS MAMAs, a safety review. Providing incomplete information increases the time and effort needed for STScI to clear observations for scheduling and could ultimately result in lost scheduling windows. 

Health and Safety

Because the MAMA detectors can be irreparably damaged by overlight conditions, it is vital to consider the contribution of all ultraviolet light sources that will or could illuminate the detector.  Detector safety must be proven for (1) the science target, which must address the maximum UV brightness of time-varying sources, (2) physically associated objects (e.g., hot companions to cool stars or unresolved UV bright regions in galaxies), and (3) incidental field sources within 5’’ of the science aperture.

Information for points (1) and (2) should already be present in the Phase I material.  Per the Call for Proposals, "proposers must address the safety of their targets and fields with respect to the appropriate count rate limits of the photon-counting detectors."  In Cycle 26, only half of relevant STIS proposals had sufficient discussion of safety. GOs should provide this information to their Contact Scientist (CS) ASAP if it is missing or incomplete in the Phase I. 

The Astronomer's Proposal Tool (APT) provides a Bright Object Tool (BOT) for identifying potentially unsafe field objects (point (3) above) within 5’’ of the science aperture. All unknown or unsafe objects identified by the BOT as well as unidentified bright sources visible in the DSS or GALEX images (e.g., bright extended sources) must either be cleared for safety or avoided by use of an ORIENT constraint or changes to the instrument setup. See Sections 7.7.6 and 12.4 of the STIS Instrument Handbook for more details. While GOs are ultimately responsible for ensuring the safety of their observations, they should consult with their CSs if they need additional guidance.

Furthermore, special considerations have been defined to verify the safety of M dwarfs, which flare stochastically in the UV.  STIS ISR 2017-02  details these procedures. As announced in the November 2018 STAN Changes to STIS Technical Review Procedure, the STIS team will provide a spreadsheet that GOs should fill out and return to facilitate clearing their M dwarf observations.

 

Supplying Exposure Time Calculator (ETC) IDs

GOs should run ETC calculations for both scientific and acquisition exposures to ensure sufficient S/N and to avoid saturation or overlight conditions. These ETC IDs should be copied into the ETC ID field for the corresponding exposures in the APT file.  As announced in the November 2018 STAN  Changes to STIS Technical Review Procedure, CSs will request this information from GOs if it is missing.

Acquisition/Target Coordinate Precision

The STIS FOV for acquisitions is small (only 5x5'' for point source acquisitions). GOs are responsible for providing precise enough coordinates and proper motions to ensure the acquisition target (which may or may not be the science target) falls within the FOV during the blind pointing stage. Because of the small FOV, neglecting to specify even modest proper motions can place the intended target well off-center even with perfect pointing.  Furthermore, the acquisition centering algorithm will center the brightest object in the scene. It is the GO’s responsibility to verify that the brightest object is the acquisition target. Check out the November 2018 STAN Article Common Acquisition Errors for some examples of how the algorithm behaves in more complicated scenes.

STIS coordinates are required to be in the ICRS reference system (see Table 3.3 in the Phase II Proposal Instructions). When using the Simbad target generation tool in APT, the “Reference Frame” is auto-filled with “Simbad,” and GOs must manually update this to “ICRS” after verifying the reference frame.

 

JATIS Article Describes Best Practices for STIS Coronagraphy

The Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS) contains the only currently operating coronagraph in space that is not trained on the Sun. In an era of extreme--adaptive-optics--fed coronagraphs, and with the possibility of future space-based coronagraphs, we re-evaluate the contrast performance of the STIS CCD camera.  The 50CORON aperture consists of a series of occulting wedges and bars, including the recently commissioned BAR5 occulter.  In a newly published JATIS article, we discuss the latest procedures in obtaining high contrast imaging of circumstellar disks and faint point sources with STIS. For the first time, we develop a noise model for the coronagraph, including systematic noise due to speckles, which can be used to predict the performance of future coronagraphic observations. Further, we present results from a recent STIS calibration program that demonstrates better than 10-6 point-source contrast at 0.6", ranging to 3X10-5 point-source contrast at 0.25” (First described here).  These results are obtained by a combination of sub-pixel grid dithers, multiple spacecraft orientations, and post-processing techniques.  Some of these same techniques will be employed by future space-based coronagraphic missions. We also discuss the unique aspects of STIS coronagraphy relative to ground-based adaptive-optics--fed coronagraphs as well as specific science use cases.

The article can be found either at the STIS ISR documentation page (STIS ISR 2019-03) or directly in JATIS.

Examples of angular coverage gain with the STIS 50CORON mask
Examples of angular coverage gain with the STIS 50CORON mask. Panel a) demonstrates a hypothetical observing program that achieves full angular coverage of a target with a combination of the WEDGEA0.6 and WEDGEA1.0 masks using six separate spacecraft orientations as described in the text. The red circle denotes an inner working angle of $0\farcs45$. Panel b) shows an example mask from one spacecraft orientation that demonstrates regions where there is no data due to the diffraction spikes of the star and the occulter. Panel c) demonstrates a hypothetical observing program that combines BAR5 and WEDGEA1.0, obtaining full angular coverage beyond $0\farcs5$, and an inner working angle of $0\farcs25$ with a total of 3 {\it HST} orbits as described in the text. Panel d) shows an example orientation as in panel b).
Test of the STIS coronagraphic noise model.
Test of the STIS coronagraphic noise model. The measured azimuthally averaged RMS as a function of time per pixel for 24 20s exposures of HR 8799 taken behind the WEDGEA1.0 aperture location compared to predicted noise sources including detector noise (green dashed-dotted line), empirically derived speckle noise (dashed orange line), photon noise from the PSF wings (red dashed-triple dotted line), as well as the total noise (black solid line). 

STIS Data Handbook Updates

A new version of the STIS Data Handbook (version 7) was published in April 2019. The handbook contains useful information about the STIS data formats, analysis methods, and the calibration pipeline. This is the first update in 8 years, and the new version of the Data Handbook has significant changes including the replacement of IRAF/PyRAF examples with Python codes, information about the pixel-based CTI correction, and the introduction of stistarg, the STIS target acquisition simulator. A PDF copy of the new version is available here.

 

Job Opportunity: Senior Research & Instrument Analyst

The Space Telescope Science Institute (STScI) has openings for Senior Research and Instrument Analysts (RIAs) in our Instruments Division. Opportunities on HST instrument and James Webb Space Telescope (JWST instrument teams exist. Senior RIAs provide support in all phases of instrument calibration, including the collection, reduction, organization, analysis, and interpretation of data to optimize the scientific return of our flagship missions. 

This is a great opportunity to join a dynamic and collaborative team operating a cutting-edge science facility. A PhD in relevant fields is preferred but all academic levels will be considered. Experience with spectroscopy, either from ground- or space-based telescopes, is a plus. Salary will be commensurate with you education and experience. STScI provides a comprehensive benefits package, flexible work schedules, and generous retiement. 

The application deadline is July 12, 2019. Full details and a link to the online application are available here

Please Contact the HST Help Desk with any Questions

https://hsthelp.stsci.edu.