August 25, 2023
STIS NEWSLETTERS

About This Article

This STAN is focused on providing information and guidelines for the preparation and submission of HST Cycle 31 GO phase II proposals, for both new and experienced users of STIS. The first section gives an updated listing of policies, tips, and tricks for preparing successful phase II proposals.  The section on STIS instrument news includes new recommendations for wavecal exposure times for some FUV settings.  The section on STIS resources lists some recently developed software tools for the examination and analysis of STIS data and notes several recent STIS presentations and Instrument Science Reports.

Phase II Policies, Tips, and Tricks

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 MAMA detectors, a safety review. Providing incomplete information increases the time and resources needed for STScI to clear observations for scheduling and could ultimately result in losing 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 (UV) 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) as soon as possible if it is missing or incomplete in the Phase I.

Target and field clearance:  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 the 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.

M dwarf clearance:  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 MAMA observations of M dwarfs.

Other variable objects:  A different procedure has been established for other targets that could suddenly enter a bright phase near the time of observation (on time scales of days to weeks), such that the MAMA count rates in that bright phase would exceed the bright object screening limits for the chosen instrumental configuration.  If that is the case, the STIS contact scientist may request a Safety Target Offset Procedure (STOP).  A STOP inserts a spatial offset -- so that the default observation would be of blank sky -- unless the target is determined to be safe shortly before the observation.  To clear the target for observation, the PI will need to provide daily monitoring data of the target during the week before the observation.

Available-but-Unsupported Modes:

STIS has several instrument configurations that are available-but-unsupported (for more details see the STIS IHB Appendix A: Available-But-Unsupported Spectroscopic Capablities and the Phase II Proposal Instructions, Chapter 8 STIS).  Starting in Cycle 30, this list includes the use of the NUV PRISM and the MSMOFF setting.  The justification and explanation for wanting to use one of these modes should be present in the Phase I proposal. The GO should also send an email with the justification to the STIS User Support team for approval.  Once the mode is approved, the GO can use it in the Phase II proposal.  By using an available-but-unsupported mode, the GO takes on extra risks, namely that if the observation fails due to the use of the mode it will not be repeated, and the user support and calibration files for the reduction and analysis of this data may be limited.

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. GOs are reminded that for target acquisitions (ACQ), the exposure times need to be calculated using the STIS Target acquisition ETC (note that imaging ACQ peak-ups are also calculated with with the Target ACQ ETC, but spectroscopic ACQ peak-ups are done with the Spectroscopic ETC).

Acquisitions and Target Coordinate Precision:

The STIS field of view (FOV) for acquisitions is small (only 5''x5'' for point source acquisitions). GOs are responsible for providing precise enough coordinates and proper motions to ensure that 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 in the acquisition FOV is the acquisition target. The November 2018 STAN Article Common Acquisition Errors gives 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).

Additionally, we advise GOs to double check that the coordinates, proper motions, epoch, and units are correct and consistent, by confirming that the target is in the cross hairs on the target confirmation chart available in APT. The space in between the cross hairs is about 5” wide. We refer GOs to the July 2020 STAN Article for additional information on the verification of Gaia coordinates for targets with proper motions in APT.

Targets of Opportunity (ToOs):

To help facilitate efficient safety and technical reviews of ToOs by CSs, GOs should provide early and complete information about their planned instrumental setups and the properties of the anticipated science targets. For disruptive ToOs using one of the MAMA detectors on STIS, it is imperative that GOs provide a representative spectral-energy distribution (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 to be observed, GOs must also communicate how bright those lines are expected to be.  Useful information and perspectives on scheduling ToOs – particularly disruptive ToOs – may be found in Strolger et al. (2020).  The STIS team will supply a checklist of the minimum information that should be provided. 

Disruptive ToOs eligible for the new monthly Flexible Thursday ToO opportunity have additional requirements and restrictions -- e.g., only CCD settings may be used for STIS, and both the trigger and the final, executable phase II must be submitted by 10:00 UT on the Tuesday before the Flex day.  See the phase I proposal instructions for more details. 

Orientation Requirements in APT:

GOs interested in specifying the orientation of the STIS slit on the sky will need to make use of the ORIENT special requirement. We remind users that the ORIENT parameter gives the orientation of the HST focal plane on the sky, so that if the orientation of a long slit is set to be at position angle (PA) X, where X is in degrees east of north, then the ORIENT value is given as X+45 or X+225 degrees. We refer GOs to the STIS Instrument Handbook Section 11.4 for more details on setting the ORIENT parameter.  Users can visualize the specified ORIENTs in APT Aladin, by selecting the “Orient Ranges” control.

Coronagraphy:

STIS coronagraphy users are encouraged to check out the STIS coronagraphy visualization notebook to aid in their Phase II planning. For observations requiring a large range of ORIENTs for azimuthal coverage, users are reminded that HST has a limited roll range at any given point in time, so that large differences in roll are not possible in a single visit. APT’s Visit Planner tool provides Roll Ranges Reports to determine the available roll angles through Cycle 31 for any given target.  ORIENT ranges should be as flexible as the science allows to improve schedulability. However, care must be taken to ensure that desired minimum differences in roll angle between visits are met.  For example, specifying either relative or absolute ORIENT constraints of 0-15 in one visit and 15-30 in an adjacent visit does not guarantee different orientations -- as both could execute at ORIENT=15.

For high-contrast coronagraphic observations, it is best practice to select a PSF reference star that is similar in brightness (or slightly brighter) and similar in exposure time to the science target. This reduces speckle noise being introduced by either the reference star or the science target. In order for the observations not to be dominated by the speckle noise of the reference star, the brighter of the two stars should be the reference star. This also limits the impact of CTE degradation, which is explained in more detail in Section 2.6 of Debes et al. 2019.

STIS Instrument News

The Pt/Cr-Ne hollow cathode lamps used for wavelength calibration and for establishing the wavelength zero points for STIS spectroscopic observations have been fading with time, particularly at the shortest wavelengths  (STIS ISR 2017-04; STIS ISR 2018-04).  Over the last few years, the more rapidly fading LINE lamp has been replaced by the HITM2 lamp as the default for several of the shortest wavelength settings (G140M/1173, G140M/1218, E140H/1234, E140H/1271).  Even with those changes, however, the current default wavecal exposure times are yielding increasingly weaker exposures, and thus potentially less reliable wavelength zero points, particularly when very small apertures are used.  The STIS team has developed recommendations for increasing the default (aperture-dependent) exposure times, by factors ranging from about 1.5 to 4,  for several of the shortest wavelength E140H (1234, 1271, 1307) and G140M (1222, 1272, 1321) settings.  Until those new exposure times are implemented as the new defaults, we encourage observers using those short wavelength settings to consult with their CS on how to specify GO wavecals with longer than default exposure times, in order to ensure more accurate wavelength zero points for their spectra.  Permission for such longer exposures must be specifically requested and approved by the STIS User Support team.

STIS Resources

A number of tools for planning, evaluating, processing, and analyzing STIS observations are available via the STIS Software Tools page, the STIS-Notebooks GitHub repository, and/or the stistools documentation pages:

  • STIS Coronagraphy Visualization Tool -- see also the March 2021 STAN.
  • Defringing STIS G750M and G750L Spectra (defringe) -- see also the July 2021 STAN.
  • DrizzlePac Notebook for combining STIS Images (drizpac) -- see also the April 2022 STAN.

The STIS team has also recently published several Jupyter notebooks demonstrating how to view STIS data products, examine the 1D extraction regions in detail, compare the various 2D CCD calibration steps, perform custom CCD dark calibrations, evaluate STIS target acquisitions, and correct wavelength offsets due to missing wavecals. The first three notebooks were discussed in the January 2023 STAN

Recent STIS Presentations and Instrument Science Reports

STIS Highlights at the 241st American Astronomical Society Meeting:

462.04 – Performance Updates for the Space Telescope Imaging Spectrograph (iPoster) (S. Medallon et al.)

462.05 – Update on the Absolute Flux Recalibration of the HST/STIS Modes (iPoster) (J. K. Carlberg et al.)

STIS Highlights at the 242nd American Astronomical Society Meeting:

230.06 – Updates of the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope (iPoster) (E. Rickman et al.)

230.10 – Investigations of STIS Time Dependent Sensitivity Corrections (iPoster) (D. Stapleton et al.)

 

STIS ISR 2022-07– Update of the STIS CTE Correction Formula for Stellar Spectra (R. C. Bohlin and S. Lockwood)

Correcting for Charge Transfer Efficiency (CTE) losses in the CALSTIS pipeline reduction of CCD data is crucial to the goal of 1% precision in the STIS spectrophotometric fluxes that are the basis for all HST and JWST flux calibrations. Precision in the CTE correction is especially relevant for the faintest flux standards, where the amount of correction can exceed 20%. New STIS spectra and ACS photometry of faint stars have now enabled updates to the parameters of the STIS CTE correction formula. For the very faint star NGC2506-G31, the new parameters yield changes in the spectral energy distribution of 4 to 6% over most of the G430L and G750L spectral wavelength ranges. As this CTE correction depends only on signal level, it should apply to all CCD spectroscopic modes. The STIS, ACS, and WFC3 flux measures are now in accord at the 1% level, not only for the primary standards (and other stars with V<13), but also for fainter stars in the V=16 range.

 

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