About This Article
This STAN is focused on providing information and guidelines for the preparation and submission of HST Cycle 32 GO Phase II proposals, for both new and experienced users of STIS. The first section gives tips and tricks for the recently announced Reduced Gyro Mode operations, followed by 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.
Tips & Tricks for STIS in Reduced Gyro Mode
Spectroscopy
Due to the recent transition of HST to Reduced Gyro Mode, orbital visibilities are likely to be shorter than originally advertised. One approach to mitigating the loss of spectroscopic exposure time involves disabling automatic wavelength calibrations (wavecals) and replacing them with manually inserted “GO wavecals.” This strategy often allows a subset of wavecals to be placed so they occur during the orbital occultation, thereby preserving time for scientific exposures. See Section 11.2.1 of the STIS Instrument Handbook for more information about GO wavecals.
Unfortunately, this approach also involves several subtle trade-offs that depend on the number of aperture changes and the choreography of buffer dumps, among other things. Optimization will likely require a fair amount of exploration and iteration in APT. You are encouraged to consult with your Contact Scientist for assistance and advice.
Coronagraphy
In Reduced Gyro Mode operations, the schedulability of coronagraphic observations will experience increased difficulty due to a decreased field of regard at a given time. It should be noted, however, that the jitter and focus performance are anticipated to remain the same as HST has already been operating with one gyro while taking data, and therefore the high-contrast performance of coronagraphy is not expected to be impacted. To mitigate the effects of scheduling difficulties, some recommendations for coronagraphy mode observers may involve utilizing aspects of the following. The first recommendation is for users to consider moving to smaller chunks of linked orbits to open up scheduling opportunities. This could mean taking additional point spread function (PSF) reference visits (for example, observe one science target followed by a PSF reference then schedule another opportunity to move the orientation), which will result in more overhead. A second recommendation is to recover azimuthal coverage by using additional aperture locations within a single orbit by taking advantage of the orthogonality of the wedges and bars, (for example BAR5 + WEDGEA1.0 or BAR5+BAR10LL+BAR10LR as described in Debes et al. 2019). To discuss these recommendations, please consult with your Contact Scientist for further guidance.
For more information on the impact of Reduced Gyro Mode operations, please see HST User Documentation Reduced Gyro Mode Tips and Resources and the HST Reduced Gyro Mode Primer.
Phase II Policies, Tips, and Tricks
The Phase II files submitted by General Observers (GOs) via the Astronomer’s Proposal Tool (APT) provide detailed descriptions of the observations to be carried out for each accepted program. Before any planned observation with STIS is executed, it must first undergo a technical feasibility review by a STIS Contact Scientist (CS). Programs that use one of the STIS MAMA detectors must also pass a health and safety review. Incomplete information increases the time and resources needed for STScI to clear observations for scheduling and could result in loss of scheduling windows.
MAMA Health and Safety:
Because the MAMA detectors can be irreparably damaged by overlight conditions, it is extremely important to consider the contributions from all the ultraviolet (UV) light sources that will or could illuminate the detector. Detector safety must be demonstrated for (1) the science target, which must consider 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 concerning points (1) and (2) should already be present in the Phase I proposal. Per the Call for Proposals, “Phase I proposals must include a discussion of the safety of the proposed targets and fields in the Description of the Observations, based on the relevant Instrument Handbook sections and calculations with the appropriate APT and ETC tools.” GOs should provide this information to their CS as soon as possible if it is missing or incomplete in the Phase I.
Target and field clearance:
APT provides a Bright Object Tool (BOT) for identifying potentially unsafe field objects (point (3) above) within a 5″ buffer around the science aperture. All objects labelled as unsafe or unknown 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 using an ORIENT constraint or by changing 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:
Since M dwarfs flare stochastically, special considerations have been defined to verify that these objects can be observed safely with a MAMA detector. STIS ISR 2017-02 describes these procedures. As announced in the November 2018 STAN, Changes to STIS Technical Review Procedure, the CS will provide a spreadsheet that GOs must complete and return as soon as possible to facilitate clearing their MAMA observations of M dwarfs.
Available-but-unsupported modes:
As described in Appendix A of the STIS Instrument Handbook and Chapter 8 of the Phase II Proposal Instructions, STIS has several Available-but-Unsupported configurations. Starting in Cycle 30, this list includes the use of the NUV PRISM and the MSMOFF mode. Although the need for one these configurations should already be justified in the Phase I proposal, GOs must formally request their use (with justification) in an email to the STIS User Support Team. Once the mode is approved, the GO can use it in the Phase II proposal. However, use of an Available-but-Unsupported mode entails several extra risks, which may include lack of calibration reference files and limited support for the reduction and analysis of the data. Additionally, if the observation fails due to the use of the mode, it will not be repeated.
Supplying Exposure Time Calculator (ETC) IDs:
GOs must run ETC calculations for both scientific and acquisition exposures to ensure sufficient S/N and to avoid saturation or overlight conditions. The ETC IDs associated with these calculations must be copied into the appropriate text boxes 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 by using the STIS Target acquisition ETC . Exposure times for Imaging ACQ peak-ups are also calculated with the Target ACQ ETC, but spectroscopic ACQ peak-ups use the Spectroscopic ETC.
GOs are encouraged to rerun older ETC calculations using ETC 32.2 in order to use the lastest instrument performance values, particularly for observations that cover NUV wavelengths between 1800 and 2100 Å to incorporate the latest sensitivity updates. See the March 2024 STAN Update on Time Dependent Sensitivity.
Acquisitions and Target Coordinate Precision:
The STIS field of view (FOV) for acquisitions is small (only 5'' × 5'' for point source acquisitions). GOs are responsible for providing accurate and sufficiently precise 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 corresponds to the desired object. The November 2018 STAN Article Common Acquisition Errors gives some examples of how the algorithm behaves in more complicated scenes.
As discussed in Chapter 3 of the Phase II Proposal Instructions, STIS coordinates are required to be in the ICRS reference system. 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. See the July 2020 STAN Article for additional information on the verification of Gaia coordinates for targets with proper motions in APT.
For multi-orbit, time-critical visits designed to observe unique phenomena, GOs might consider repeating ACQ sequences at the beginning of each orbital visibility period as insurance against loss of data due to issues with the initial acquisition sequence. STIS ISR 2024-01 provides a statistical analysis of the likelihood of data loss, as well as advice on planning multi-orbit visits with additional “safety ACQ” sequences.
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) for 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 CS 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. For example, 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 details.
Orientation Requirements in APT:
The orientation of a STIS aperture on the sky can be specified by using the ORIENT special requirement, which defines the orientation of the HST focal plane. However, it is important to remember that STIS is offset by 45 degrees in the focal plane coordinate system. Consequently, if the orientation of a long slit is desired to be at position angle (PA) X, where X is measured in degrees East of North, then the ORIENT value must be specified as X+45 or X+225 degrees. See the STIS Instrument Handbook Section 11.4 for more details on setting the ORIENT parameter. Users can confirm the specified ORIENTs in APT Aladin visually by selecting the “Orient Ranges” control. Please note that the orientation requirement on APT must be stated during Phase I and that there may be additional difficulty with orientation constraints in Reduced Gyro Mode operations. As always, consult with your Contact Scientist if you have any questions or concerns.
Coronagraphy:
PIs of programs that use the STIS coronagraphs are encouraged to consult the STIS Coronagrapic Visualization Tool to aid in their Phase II planning. Since HST can only accommodate a limited range of off-nominal roll angles at any given time, programs that require a range of roll angles to achieve azimuthal coverage of the source require especially careful planning. The Roll Ranges report provided by APT’s Visit Planner can be used to determine when specific roll angles are available for a given target in Cycle 32. ORIENT ranges should be specified as flexibly as the scientific aims allow to improve schedulability of the program. 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 degrees in one visit and 15–30 degrees 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 yield 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 defaults, we encourage observers using those short wavelength settings to consult with their CS on how to specify GO wavecals with longer 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 the stistools documentation pages. Items of particular interest include:
- 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 244th American Astronomical Society meeting:
Performance Status and General Updates for the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope (Matthew Dallas + STIS team)
Here is a summary of the most recent STIS Instrument Science Reports published:
STIS ISR 2024-01: Safety Acquisitions: Redundancy for non-repeatable multi-orbit STIS visits (Matthew Dallas and Matthew Siebert)
STIS ISR 2024-02: Recalibration of Pre-SM4 STIS Echelle Throughputs (Matthew Siebert, Joleen Carlberg, Svea Hernandez, TalaWanda Monroe)
STIS ISR 2024-03: Rederivation of STIS Secondary Echelle Mode Traces (Matthew Siebert, TalaWanda Monroe, Svea Hernandez)