May 2026 STAN

In this STAN, the STIS team presents an update to the calstis wavelength calibration procedure for STIS CCD observations using the E1/E2 pseudo-apertures. The STIS team also announces the publication of a new Instrument Science Report that validates the STIS time dependent sensitivity trends with the primary CALSPEC standard stars and a recently developed introductory Jupyter Notebook that helps users with TIME-TAG data.

Update to calstis Wavelength Calibration Procedure

Long-term CCD Rotation Impact on Wavelength Calibration Accuracy

Multi-cycle monitoring of the STIS CCD has revealed a gradual detector rotation over time (Dressel et al. 2007; Nguyen et al. 21; Ward-Duong et al. 22). This slow rotation has produced wavelength-calibration offsets that increase with time toward the CCD edges, reaching up to ∼1 pixel relative to the detector center, that will be described in detail in an upcoming Instrument Science Report (ISR; Mingozzi et al. in prep.). While the standard wavelength calibration remains accurate near the nominal detector center (<0.2 px), systematic offsets affect spectra obtained at the E1/E2 pseudo-aperture positions, where targets are placed closer to the CCD amplifier (i.e., around column ∼900) to mitigate charge-transfer inefficiency effects. This leads to systematic wavelength offsets in the calibrated science data that can be significant for velocity-sensitive applications, such as redshift/blueshift measurements and spatially resolved kinematic studies.

A calstis Update to Improve Wavelength Calibration Accuracy at E1/E2

The STIS team has now updated the STIS CCD wavelength-calibration step in the calstis pipeline, implementing an improved procedure for CCD observations obtained with the E1/E2 pseudo-apertures. This update is included in calstis version 3.5.0, distributed as part of hstcal version 3.2.0.

The wavelength calibration in calstis is handled through a series of dedicated modules that process contemporaneous wavecal exposures and propagate the resulting spectral shifts to the science data (see Hodge et al. 1998 for details). In particular, the wavelength-calibration module calstis4 processes the 2-D rectified wavecal exposures (with the rectification performed by calstis7) to determine shifts in the dispersion (SHIFTA1) and cross-dispersion (SHIFTA2) directions introduced by Mode Selection Mechanism (MSM) non-repeatability and thermal effects. In the standard procedure, the module produces one spectrum summing the wavecal over the full illuminated extent of the slit and cross-correlates it with a reference lamp spectrum to derive wavelength and spatial zero-point offsets in pixel units. With the new update, when the E1/E2 pseudo-aperture is identified from the science-header keyword PROPAPER, calstis4 performs the cross-correlation using only the detector region around the E1/E2 location, rather than the full detector. This allows the pipeline to derive a SHIFTA1 more appropriate for spectra extracted at E1/E2.

Important Notes:

• This calstis update affects only the calstis4 wavelength-calibration step; all other calibration modules and procedures remain unchanged.
• The updated pipeline has been applied to all the datasets in MAST; see the following section, "User Guidance and Caveats," for more information.
• The STIS team has verified that the agreement between spectra extracted at the center and at the E1/E2 positions has been improved (see Fig. 1), and is reliable for a large majority of datasets obtained at the E1/E2 position.

User Guidance and Caveats:

1. Users downloading data from MAST or reprocessing observations with calstis through the standard stistools workflows (with calstis version ≥ 3.5.0) do not need to take any additional action, as the updated pipeline is already applied automatically. Users who run the individual modules directly instead of using stistools must specify the -e flag when running cs4.e to ensure that the cross-correlation is performed at the E1/E2 position (e.g., cs4.e -e rectified-file). The -e flag is not required when running cs0.e.
2. Archival G230MB datasets may frequently show unreliable wavelength-calibration solutions because the wavecal cross-correlation at the E1/E2 position can fail, primarily due to low-S/N wavecal spectra and spurious features near the detector edges. When this occurs, the pipeline usually produces anomalous SHIFTA1 values (stored in the flt or crj header), with shifts reaching tens of pixels instead of the expected few-pixel offsets. Users working with these modes are encouraged to inspect the wavelength calibration quality of their observations. This issue can also affect a small percentage of G230LB and G430M configurations. The STIS team will be modifying any Phase II recommendations for these gratings and pseudo-apertures in the future.
3. The updated pipeline correction is intentionally limited to standard E1/E2 observations identified through the PROPAPER keyword in the science data header. This is because the wavelength-calibration offsets are determined before the location of the science-target spectrum along the slit is fully established by the pipeline. As a result, observations obtained with non-standard target placements, such as POS-TARG offsets, or observations of extended sources sampled at multiple detector positions still retain residual detector-position-dependent wavelength offsets.

A forthcoming Instrument Science Report will describe the implementation, validation tests, affected configurations, and scientific impact of the updated wavelength calibration procedure in detail. A companion Jupyter Notebook will also be released to demonstrate how to refine the wavelength calibration for observations obtained at arbitrary detector positions, including extended-source extractions and cases where the E1/E2 wavecal cross-correlation does not provide a reliable solution.

New STIS Instrument Science Report

We are pleased to announce the publication of a new STIS Instrument Science Report. The full list of STIS ISRs can be accessed here: https://www.stsci.edu/hst/instrumentation/stis/documentation/instrument-science-reports

ISR 2026-01: Verifying the STIS Time Dependent Sensitivity Trends with the Primary CALSPEC Standards

Daniel Stapleton, Svea Hernandez

The STIS team monitors the time dependent sensitivity (TDS) of each grating with one from a set of three secondary CALSPEC standard stars: GRW+70D5824, AGK+81D266, and BD+28D4211. Here, we use the three primary CALSPEC White Dwarf standard stars, dubbed the standard star “triad” (GD71, GD153, G191B2B), as an independent set of standards to verify the accuracy of STIS TDS corrections derived from the TDS monitoring stars, increasing the sample for each STIS L-mode from one up to three or four standard stars. We focus on triad star observations using the STIS L-mode gratings (e.g., G140L, G230L, etc.) with the same configuration as our standard TDS monitoring programs, and compare the triad observations to the TDS pipeline trends. Our analysis indicates the relative net count rates inferred from the triad standards agree with the TDS trends derived from the TDS monitoring stars with average residuals < 2% across the full wavelength range of STIS, suggesting our current TDS L-mode trends are reliable and robust. We note that the dispersion in the residuals does vary with wavelength, with the NUV showing the lowest spread (± 0.32% at 2400-2500 Å) and the NIR the largest (± 1.32% at 9500-9900 Å); however, this scatter is also seen in our measurements of the TDS monitoring stars and is more indicative of other instrumental effects. Our findings rule out long term deviations, such as variability in our TDS monitoring stars, within measurement uncertainties.

New Introductory Notebook on STIS TIME-TAG Data

The STIS team contributes to the HST Notebooks Git repository, which contains a set of Jupyter Notebooks to aid the STIS user community with observation planning and data analysis. We have a series of introductory notebooks, meant to provide walk-through examples of concepts in our Data Handbook. The STIS team has developed a new notebook, Introduction to STIS TIME-TAG Data, which teaches users how to navigate TIME-TAG file structures, display the raw data with several methods, calibrate the full exposure, and split data into sub-exposures. The notebook also covers a few special cases, such as creating light curves with TIME-TAG data and how to address extraction position drift in sub-exposures. To view the notebook, we recommend using the rendered HTML version or downloading the notebook. If you have any questions, issues, or suggestions, please contact the HST Help Desk.