June 2020 STAN
Ahead of the upcoming Cycle 28 Phase II deadline, this STAN provides information regarding STIS best practices for submitting Phase II's. In addition, information regarding updates to the STIS Exposure Time Calculator for Cycle 28 Phase II, new posters from the recent American Astronomical Society Meeting, and information regarding the recent redelivery of several super-dark and super-bias reference files is available in this STAN.
STIS Best Practices for Submitting Phase II's
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.
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." 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) with a 5’’ buffer around 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 corresponding places 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.
Acquisitions and 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 GOs 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.
Targets of Opportunity (ToOs):
To help facilitate efficient safety and technical reviews of ToOs by contact scientists, GOs should provide early and complete information about their planned observation setups and properties of the anticipated science targets. For disruptive ToOs using one of STIS’s MAMA detectors, it is imperative that GOs provide a representative SED of the type of object to be observed before the ToO is triggered as well as a clear explanation of how the absolute scaling of the UV flux will be determined once a ToO object is identified. If the class of objects has emission lines in the UV passband of observation, GOs must also communicate how bright these lines might reasonably be expected to be.
New STIS Posters from the 236th American Astronomical Society Meeting
At the 236th (Summer 2020) 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.
Here we summarize the posters presented at the most recent AAS meeting in June. The STIS team presented posters detailing a new method for FUV-MAMA dark creation and subtraction, the impact of various factors on echelle observations, and general STIS updates.
Updates and Highlights of STIS
STIS has been successfully operating on HST for over 20 years with a wide range of capabilities, including imaging, low to high resolution spectroscopy, coronagraphy, and long-slit spatial scanning on the CCD. This poster presents the latest highlights and updates of STIS, which includes discussions of the redelivery of CCD super-darks and super-biases, current work on Python-based defringing tools for STIS CCD observations, and updates to the STIS Time Dependent Sensitivity.
Principal Component Analysis of STIS MAMA Darks
The HST STIS FUV-MAMA detector has a glow region dark background that varies over timescales of both years and hours as the detector warms up. The HST/STIS pipeline does not currently correct for this background structure beyond a 1-D background subtraction during spectral extraction, which can lead to under-subtraction. This poster details initial work to fit the morphological changes to this glow region using a Principal Component Analysis (PCA) model of the dark structure.
Getting the Most Out of Your STIS Echelle Observations
HST/STIS echelle modes are used frequently due to their high resolution and ability to maximize spectral coverage in a single exposure. The STIS team works to understand what observatory factors impact echelle observations in order to optimize observations. This poster discusses the effects of observatory focus and jitter on echelle throughput and spectral quality. This poster also provides an update on the general focus behavior of HST, calibration of E140M, and the health of the STIS MAMA detectors as it related to echelle data.
STIS ETC Updates for Cycle 28 Phase II
For Cycle 28 Phase II, a new version of the STIS Exposure Time Calculator (ETC 28.2) has been released. STIS ETC 28.2 maintains most of the functionality and underlying STIS detector characteristics as in STIS ETC 28.1. The main notable STIS change affects the CCD Gain=1 Readnoise, which was changed from 6.1 e- to 6.2 e- based on trends from the STIS CCD Readnoise Monitor (pictured below in Figure 1). In addition, there were some minor updates to header data for the Phoenix M dwarf models.
Full release notes of ETC 28.2 can be found on the main ETC site here.
Redelivery of Recent CCD Super-Dark and Super-Bias Reference Files
In March 2020, the STIS team redelivered weekly CCD super-dark and super-bias reference files from 2019-03-16 onward. During four of these weeks in 2019 (Mar 16-25, Apr 10-29, May 8-17), we found that previously delivered files were created incorrectly. This was due to an error in the reference file creation software, which we quickly found and fixed. Our fix also includes various automatic tests to ensure that the reference files produced are valid and correctly made. To check if a bad file was used, users may check the ext=0 BIASFILE keyword for any of these files:
If you have data taken between 15 March 2019 and now*, we recommend re-retrieving your data from the MAST portal, or downloading the relevant reference files from https://hst-crds.stsci.edu and re-calibrating your data.
* Updated super-dark/super-bias files for data taken in the last month are being worked on and will be coming to CRDS soon.