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July 6, 2022

July 2022 STAN

This STAN discusses policies, tips and tricks for preparing and submitting Phase II Proposals ahead of the Cycle 30 Phase II deadline. It also includes STIS instrument news on exposure times for wavecals.  The third article summarizes the latest software tools for data analysis, namely Coronagraphy Visualization tool, Defringing package, and a DrizzlePac notebook. The final article provides summaries of recent STIS presentations, Instrument Science Reports, and papers.

Phase II Policies, Tips, & 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.

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.

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.


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 NUV PRISM and MSMOFF setting.  The justification and explanation for wanting to use one of these modes should be present in the Phase I. The GO should also send an email with justification to be approved by the STIS User Support team.  Once the mode is approved, the GO can specify this set-up in their Phase II.  By using an available-but-unsupported mode, the GO is taking on extra risks, namely that if the observation fails due to the usage of this mode they 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 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. 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.

Additionally, we advise GOs to double check that the coordinates, proper motions 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 observation setups and 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 of observation, GOs must also communicate how bright these lines are expected to be.  Useful information and perspectives on scheduling ToOs – particularly disruptive ToOs – may be found in Strolger et al. (2020).


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 place on the sky, and if the orientation of the long slit is set to be the 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.

STIS Instrument News

Exposure Times for Wavecals

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 narrow apertures are used.  The STIS team is currently developing recommendations for increasing the default exposure times for the shortest wavelength FUV settings.  Until those new default exposure times are implemented, we encourage observers using the shortest 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.

Software Tools for Data Analysis

STIS Coronagraphy Visualization Tool:

The STIS team is offering a preliminary version of the coronagraphic aperture visualization tool to aid users in visualizing their scientific scenes using the various STIS coronagraphic configurations. The tool is available on the STIS website software tools page and the STIS-Notebooks GitHub respository. More details on the use of this software (with examples) can be found in the March 2021 STAN.


Defringing STIS G750M and G750L Spectra:

While the interference fringes visible at longer wavelengths in G750M and G750L spectra are not removed in the automated calstis pipeline reductions, software for defringing had been available as an IRAF/pyraf package (Goudfrooij & Christensen 2018b).  Those routines have been converted to a new Python package (defringe) by several members of the STIS team (2020 July STAN).  See the defringe documentation pages for detailed instructions and examples of its use; some additional examples are given in the 2021 July STAN.


DrizzlePac Notebook for STIS Imaging:  

A Jupyter Notebook that can be used to align and combine STIS images using tools from DrizzlePac is available on the STIS Software Tools webpage and in the STIS-Notebooks GitHub respository. The Notebook includes examples for all three STIS detectors: the CCD, NUV MAMA and FUV MAMA. An example of applying the pixel-based charge transfer inefficiency (CTI) correction to STIS CCD data with the stis_cti code is also included.

Recent STIS Presentations, Instrument Science Reports, and Papers

STIS Highlights at the 240th American Astronomical Society

To keep the astronomical community informed regarding ongoing calibrations and new software tools, the STIS Team presented a new poster at the 240th meeting of the American Astronomical Society. The poster “Highlights of the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope” (iPoster) includes helpful information both for users preparing to observe with STIS and for those in the later stages of calibration and analysis: (1) the new defringing tool and successful applications, (2) ongoing flux recalibration efforts, (3) news regarding spatial scans with the STIS CCD, (4) the coronagraphy visualization tool, and (5) updates on the STIS NUV dark rate.


ISR 2022-01: Long-Term Rotational Evolution of the STIS CCD Flatfields

March 18, 2022 — K. Ward-Duong, S. Lockwood, J. Debes, R. J. De Rosa

We confirm a long-term rotational drift of 0.0031 degrees/year of the STIS CCD format, based on an analysis of a set of stable, high-S/N dust motes visible in 50CCD flat field images spanning over 20 years of calibration data. The derived rotation rate is similar to the values found in analyses of two related effects: (1) the progressive rotation of the spectral traces for the grating L modes and (2) an increasing offset in detector true north position angle relative to the FITS header orientation keyword in science images.  This rotation may also be the cause of an apparent increasing zero-point offset in the wavelengths for L-mode spectra obtained at the E1 pseudo-aperture (currently under investigation).  (STIS ISR 2022-01)


ISR 2022-02: STIS CCD & MAMA Full-field Sensitivity & its Time Dependence

April 28, 2022 — L. Prichard

The three detectors on STIS (one CCD and two MAMAs) are subject to time-dependent sensitivity (TDS) changes on both short- and long-timescales. These variations are corrected for in the STIS calibration pipeline (CALSTIS), using TDS models derived from spectroscopic data.  From aperture photometry of sources in two standard stellar fields, we derive magnitude trends and residual TDS effects for each detector, covering the full STIS lifetime.  While the magnitude trends are within the ~1% STIS flux calibration accuracy for data up to 2012, there appears to be an increasing over-correction (i.e., sources appearing brighter with time) in more recent data.  (STIS ISR 2022-02)


ISR 2022-03: Comparison of STIS CCD CTI Corrections on Photometry

May 12, 2022 — L. Prichard

The STIS CCD detector suffers from charge transfer inefficiency (CTI) which can be corrected for using either a pixel-based or an empirical flux correction.  We compare the corrected magnitudes and their spatial, time and magnitude dependences for those two methods, for data obtained between 2010 and 2022. The offsets are smallest for the brightest stars and deviate further from zero with increasing magnitude. Stars brighter than 19 mag are marginally over-corrected with both CTI methods. Stars fainter than 19 mag are slightly under-corrected by the pixel-based CTI method and slightly over-corrected with the empirical flux CTI method. In general, however, the results from the two methods agree within ~1%.  (STIS ISR 2022-03)


ISR 2022-04: Recalibration of the STIS E140M Sensitivity Curve

May 19, 2022 — J. Carlberg, T. Monroe, A. Riley, S. Hernandez

In 2012, the blaze function shapes (normalized sensitivity as a function of wavelength) of E140M’s spectral orders began exhibiting changes that could not be accounted for with simple blaze shift coefficients in the PHOTTAB reference files. To best characterize the evolving shape changes across all post Servicing Mission 4 data, 3 new PHOTTAB files and 2 new RIPTAB files, based on new spectra of G191B2B, were delivered in 2020.  All five reference files were then redelivered in 2022 based on the new CALSPEC v11 models. This ISR primarily describes the work and methods implemented for the 2020 sensitivity curve derivations, but also describes both the blaze shift updates prior to 2018 that motivated that work and the more recent updates to the new flux standards. (STIS ISR 2022-04)


ISR 2022-0n: Scattered Light in STIS Grating G230LB

Coming soon — G. Worthey, T. Pal, I. Khan, X. Shi, R. C. Bohlin

The G230LB grating used with the STIS CCD detector scatters red light.  The effect is particularly evident as a spurious upturn in the apparent flux at the shortest wavelengths, for spectra of objects cooler/redder than about G0.  This ISR describes a procedure for correcting for the scattered light, based on comparisons of G230LB (CCD) and G230L (NUV-MAMA) spectra of several late-type stars.  (see the AAS240 iPoster by Worthey et al. for a summary)


On the Same Wavelength as the Space Telescope Imaging Spectrograph

2022 Feb — T. Ayres (AJ, 163, 78)

The low-order polynomial dispersion model used in the pipeline wavelength calibration of STIS echelle spectra cannot completely remove the effects of subtle camera distortions, leaving small systematic undulations in the resulting wavelengths.  This paper, based on deep wavecals taken for GO calibration program 15948, displays some of those systematic deviations and discusses methods for refining the wavelengths in extracted STIS echelle spectra. (full paper)

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