March 8, 2024
STIS NEWSLETTERS

About This Article

In this STAN, the STIS Team presents Cycle 32 updates to documentation and news to STIS users submitting Phase I proposals, as well as updates regarding stis_cti and time dependent sensitivity.

Cycle 32 Documentation and Phase I Reminders

Information on STIS in preparation of Cycle 32 proposals has been released in the HST Call for Proposals and Primer, and the STIS Instrument Handbook has been updated to reflect the current status of the instrument. PIs are encouraged to review Chapter 10.1 in the STIS Instrument Handbook (IHB) for a summary and checklist for HST Phase I proposals. 

Special requirements (e.g. ORIENT, scheduling or timing constraints, ToOs, CCD and MAMA observations in one visit, and HST visits that exceed 30 buffer dumps carrying TIME-TAG data) must be justified quantitatively in the Phase I proposal. Proposals to use STIS's FUV- or NUV-MAMA detectors must also demonstrate that the proposed observations do not violate the bright object restrictions. A comprehensive summary, including a table on count rate limits, for drafting a suitable observing plan under these conditions are detailed in Section 7.7.

Special uses of STIS, including recommendations for users interested in coronagraphic observations, can be found in Chapter 12 of the IHB. We encourage PIs to utilize the STIS Coronagraphic Visualization Tool available on the Data Analysis and Software Tools webpage to aid their proposal planning. For coronagraphic observations requiring multiple ORIENTs, PIs are reminded that HST has a limited roll angle at any given time. Observations with large ORIENT differences will be scheduled at different times. Observers can view the "Roll Ranges Report" in the Visit Planner tool in APT to see the available ORIENT ranges when their object is schedulable. 

STIS users preparing a program for Cycle 32 may refer to the Proposal Planning Toolbox, which includes deadlines, further documentation, and tools for submission. For additional inquiries, please contact the HST Help Desk

Updated stis_cti v1.5.2: Bug Fix for Silent Failure during Multiprocessing

Earlier this year, the STIS team was alerted to a new bug that could affect users running stis_cti with multiple processors in parallel, which could cause stis_cti to silently fail to correct data. The bug was due to a combination of Numpy API changes for Numpy versions ≥ 1.24 and the inability of stis_cti to propagate errors raised by subprocesses.  The subprocesses would create the expected CTI-corrected files from the uncorrected input files but fail before CTI corrections were made. Both issues have been fixed in the newly released version 1.5 of stis_cti. Users can upgrade their current version by running: pip install --upgrade stis_cti 

Update on Time Dependent Sensitivity

The STIS team has observed a drop in the time dependent sensitivity (TDS) between 1800 and 2100 Å. The drop occurs in all modes that cover those wavelengths in both the NUV-MAMA and CCD, but the focus of our analysis is on G230L and G230LB. We have correlated the changing TDS with an increase in solar activity expected from the 11-year Solar Cycle. At the extremes, the sensitivity drops to ~5% below the currently adopted TDS corrections (Figure 1). We created a special monitoring program (PID: 17584) to better track the trending decrease, and we will update the TDSTAB reference files for the NUV-MAMA and CCD, and ETC values accordingly. Updated reference files are being created for the upcoming ETC release in early June, 2024, using the latest available data, shown in Figure 2. However, the decreasing TDS trend is not fully characterized, and may decrease in sensitivity more or less rapidly throughout the next year. For the NUV-MAMA in particular, the updated TDS files will adopt a conservative projection of no additional sensitivity drop because the two most recent TDS observations suggest the decrease could plausibly be leveling off, and overestimating the sensitivity decline could result in underestimating count rates for bright object safety screening. Users should therefore be warned that ETC calculations at wavelengths between 1800 and 2100 Å may not be accurately corrected for dates extrapolated into the next year. Calculations will likely be within the promised 10% accuracy for the ETC, but users who have questions or concerns about predicted signal to noise or bright object safety for observations in these wavelength ranges are encouraged to reach out to the HST Help Desk.

An example TDS plot for the G230L mode
Figure 1:  An example TDS plot for the G230L mode with data taken up to Feb. 2024. The title of the TDS plot indicates “{grating}_{cenwave}_{wavelength bin}“, where the wavelength bin is the starting value of that bin, 2000 Å. This wavelength bin is 100 Å wide, and so spans 2000-2100 Å. The red line is the currently adopted pipeline TDS corrections for the NUV-MAMA. Note the decrease in TDS post-2022, and the apparent flattening of the TDS in recent data points.
An example line-segmented fit of the TDS data
Figure 2: An example line-segmented fit of the TDS data shown in Figure 1. In the title, “small” indicates the choice in bin size (still 100 Å for these datasets), and fixed indicates the use of fixed breakpoints to perform the line-segment fits. The blue line is the calculated line-segment fit which is determined by varying the slope of the line, and the location of the breakpoints. The breakpoints are indicated by the vertical dashed lines, and the horizontal dashed line indicates the sensitivity when the instrument was first turned on. Note that the breakpoints and fits shown here are not those used in the pipeline TDS, and merely used in this case for analysis purposes. For the black box section, notice the fit is continually decreasing; however, the two most recent TDS data points may indicate the trend is flattening.  We will adopt the conservative projection of a horizontal fit for our ETC calculations, as demonstrated by the black arrow.

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