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STIS Cycle 30 Calibration Programs

This Instrument Science Report summarizes the objectives, observations, analysis, and results of the 19 STIS calibration programs executed during HST Cycle 30, providing a snapshot of current instrument performance and referencing ongoing efforts to revise flux calibration.

Original authors: D. Welty, R. Bohlin, J. Carlberg, M. Dallas, S. Hernandez, A. Jones, S. Lockwood, S. Medallon, E. Rickman, D. Stapleton, T. Wheeler

Published 2026-01-27

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Original authors: D. Welty, R. Bohlin, J. Carlberg, M. Dallas, S. Hernandez, A. Jones, S. Lockwood, S. Medallon, E. Rickman, D. Stapleton, T. Wheeler

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Hubble Space Telescope as a giant, high-precision camera floating in space. Inside this camera is a specific tool called STIS (Space Telescope Imaging Spectrograph), which acts like a prism, breaking light from stars into rainbows to tell us what they are made of.

Over time, even the best tools get dusty, their batteries drain, and their lenses shift slightly. This report, written by the team that manages STIS, is like a mechanic's logbook for the year 2022–2023 (known as "Cycle 30"). It details how they checked the health of the instrument, fixed minor issues, and made sure the "ruler" they use to measure starlight is still accurate.

Here is a breakdown of what they did, using simple analogies:

1. The "Daily Check-Up" (Monitoring the Detectors)

STIS has two main types of "film" (detectors): a CCD (like a digital camera sensor) and MAMA (specialized sensors for ultraviolet light). Just like a camera sensor gets hot or develops "dead pixels," these detectors need constant monitoring.

The "Dark" Test: The team takes pictures with the camera lens cap on (no light allowed). This measures the "noise" or static the sensor creates just by being turned on. They found the noise is stable and predictable, like the hum of a refrigerator that never changes volume.
The "Hot Pixel" Annealing: Over years in space, radiation hits the sensor, creating "hot pixels" (permanent bright spots). To fix this, the team occasionally turns off the sensor's cooling system, letting it warm up to about 5°C (41°F), and then cools it back down. Think of this like annealing glass or resetting a computer. This heat "resets" some of the damaged pixels, clearing up the image slightly. They found this works well, though some stubborn pixels remain.
The "Read Noise" Check: Every time the camera reads a picture, there's a tiny bit of static. The team measures this to ensure the "volume" of the static hasn't gotten too loud. It remains consistent.

2. The "Light Meter" Calibration (Sensitivity)

If you take a photo of a candle and a photo of a firework, you need to know exactly how much light the camera is capturing to compare them fairly. STIS needs to know its own "light meter" hasn't drifted.

The Standard Stars: The team points the telescope at three very specific, well-known white dwarf stars (GD153, GD71, and G191B2B). These are the "gold standards" of the universe—like a ruler that never stretches. By comparing the light from these stars to what the telescope should see, they can tell if the telescope is getting dimmer or brighter over time.
The Result: The telescope is still very accurate. The "light meter" has only drifted by a tiny fraction (less than 1%), which is excellent news for astronomers.

3. The "Prism" Alignment (Wavelength Calibration)

STIS uses gratings (like prisms) to split light. If the prism shifts even a tiny bit, the colors (wavelengths) get mislabeled.

The Lamp Test: The telescope has built-in lamps (like tiny lightbulbs) that glow with very specific, known colors. The team shines these lamps through the instrument to check if the "ruler" is still straight.
The Result: The ruler is mostly straight. However, they noticed a very slow, subtle "twist" in the image, like a piece of paper that has been slightly rotated over many years. They are tracking this so they can mathematically correct for it in future photos.

4. The "Slit Wheel" Precision

STIS has a rotating wheel with different-sized holes (slits) to let light in. If the wheel doesn't stop in the exact same spot every time, the image will be blurry.

The Test: They spin the wheel and check if it lands in the exact same place every time.
The Result: It is incredibly precise. The wheel lands within the width of a human hair (or even less) of where it's supposed to be. It's performing better than the original blueprints required.

5. The "Dust Motes" (Flat Fields)

Even in space, tiny specks of dust can get on the lens or sensor, casting shadows.

The Test: They take pictures of a uniform, bright light source to map out where these dust specks are.
The Result: The dust hasn't moved much. The "shadow map" is stable, so the computer can easily subtract the dust shadows from real science photos.

6. The "Lamp Fade" (Appendix A)

The internal light bulbs used for calibration are slowly getting dimmer, just like an old lightbulb in your house. The team is adjusting how long they leave the lights on to compensate for this fading, ensuring the measurements stay accurate.

The Bottom Line

This report is a "health certificate" for the STIS instrument. It confirms that:

The instrument is stable and reliable.
The "rulers" (calibration) are still accurate.
The "noise" (dark current) is under control.
The "lens" (slits and optics) is aligned perfectly.

While the instrument is aging (like a classic car), the team's regular maintenance and monitoring ensure it continues to take high-quality "photos" of the universe. There were no major emergencies, just routine checks and minor adjustments to keep the machine running smoothly for the next cycle of observations.

Technical Summary: STIS Cycle 30 Calibration Programs (Instrument Science Report 2025-01)

Problem and Context
The Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) requires continuous monitoring to maintain accurate data reduction and exposure time predictions. While the instrument's behavior is generally stable, specific characteristics evolve over time, including detector sensitivity (as a function of wavelength), dark count rates, wavelength calibration, detector cosmetics (e.g., hot pixels), and the brightness of internal calibration lamps. Additionally, extended instrument shutdowns ("safing" events) can temporarily alter detector behavior. The STIS team executes a suite of regular calibration programs to generate reference files for the calstis pipeline and the Exposure Time Calculator (ETC). This report details the execution and results of these programs during HST Cycle 30 (November 7, 2022 – November 5, 2023), providing a snapshot of current instrument performance.

Methodology
The study relies on 19 regular calibration programs executed during Cycle 30, utilizing a total of 1,342 internal orbits and 26 external orbits. The methodology is divided into specific monitoring tasks for the CCD and MAMA (Multi-Anode Microchannel Array) detectors:

CCD Monitoring: Programs include monitoring of performance (read noise, charge transfer efficiency [CTE], spurious charge), dark current, bias levels, and hot pixel annealing. Spectroscopic and imaging flat fields are obtained to monitor pixel-to-pixel sensitivity and dust mote stability. Wavelength dispersion solutions are monitored using deep exposures of internal Pt/Cr-Ne lamps. Sensitivity is tracked via observations of standard star fields (e.g., $\omega$ Cen) and specific standard stars (AGK+81°266).
MAMA Monitoring: Similar to the CCD, MAMA monitoring covers dark current (FUV and NUV), flat fields (using Kr and D lamps), and dispersion solutions. Focus and sensitivity are monitored using standard stars (GRW+70°5824, AGK+81°266, BD+28°4211) across various grating settings (L, M, and Echelle modes).
Standard Stars: A dedicated program (PID 16966) observes the three primary white dwarf flux standards (GD153, GD71, G191B2B) to verify the stability of the flux calibration triad.
Mechanical and Optical Checks: Slit wheel repeatability is tested using lamp spectra, and MAMA fold distributions are analyzed to assess microchannel plate (MCP) gain and pulse-height distribution.

Data analysis involves comparing current measurements against historical trends, laboratory values, and pipeline reference files. Techniques include calculating Charge Transfer Inefficiency (CTI) via sparse field tests, measuring dark rates with temperature corrections, and fitting segmented line models to time-dependent sensitivity (TDS) data.

Key Contributions and Results

CCD Performance: The CCD housing temperature remains stable at $\sim$ 21 K. Charge Transfer Inefficiency (CTI) continues a slow, linear increase, with current values of $\sim$ 0.073% (serial) and $\sim$ 0.0024% (parallel). "Spurious charge" (column structure from hot pixels) persists, measuring $\sim$ 1.8 e⁻/pix (gain=1) and $\sim$ 8.3 e⁻/pix (gain=4) at the central row. Read noise remains consistent with recent trends ( $\sim$ 6.3 ADU for gain=1; $\sim$ 2.17 ADU for gain=4).
Hot Pixel Annealing: Annual annealing cycles (warming the CCD to +5°C) successfully reduce the number of hot pixels on average, though persistent hot pixels continue to accumulate over time.
Dark Current: CCD dark rates remained stable at an average of $\sim$ 0.13 cts/s/pix. FUV MAMA dark rates were updated for ETC 31.1, establishing a base rate of $3 \times 10^{-5}$ cts/sec/pix with specific "glow" contributions increasing after orbit 2. NUV MAMA dark rates showed a slow decline below 0.002 cts/s/pix.
Wavelength Calibration: All first-order CCD and MAMA settings remained within desired tolerances for mean wavelength offsets ( $<0.3$ pixels) at the detector center. However, several CCD settings exhibited increasing zero-point offsets ( $>0.5$ pixels) at the E1 pseudo-apertures ( $y \sim 900$ ), suggesting a systematic rotation of the CCD format. Internal calibration lamps (Pt/Cr-Ne) continue to fade, consistent with previous trends.
Sensitivity and Flux Calibration:
- CCD: Most settings showed constant sensitivity or slow declines. However, a sudden dip in sensitivity was detected in the G230LB and G230L modes between 1900–2100 Å.
- MAMA: Most settings were consistent with trends, except for a sudden dip in G230L (1900–2100 Å) and a flattening of the trend for G230L at $\lambda > 2400$ Å.
- Primary Standards: Observations of the three primary white dwarfs confirmed that changes in sensitivity are statistically consistent with those derived from the denser monitoring of secondary standards. The overall change in low-dispersion flux calibration remains much less than 1%.
Mechanical Stability: The slit wheel positioning is highly repeatable, with peak-to-peak deviations of 3.4–6.1 mas (spectral) and 7.1–9.0 mas (spatial), well within performance requirements.
Flat Fields: No significant evolution was found in spectroscopic or imaging flat fields to warrant immediate new reference file generation, though the team is preparing scripts for future updates.

Significance and Claims
The authors state that this report provides a "brief snapshot" of the current instrument performance, serving as a continuation of the annual reports established in previous cycles (7–10 and 17–21). The primary significance lies in the verification that the STIS instrument remains stable and well-understood, with the majority of calibration reference files remaining valid.

The report modestly highlights specific areas requiring attention or future updates:

Flux Recalibration: An ongoing major effort (detailed in Appendix B) is underway to revise flux calibrations for many STIS modes in response to improvements in stellar models.
Specific Anomalies: The detection of sensitivity dips in the 1900–2100 Å range and the systematic rotation of the CCD format are noted as issues that have been identified and will be addressed in future reference file updates (e.g., TDSTAB updates delivered in 2023 and 2024).
Lamp Degradation: The continued fading of internal calibration lamps is tracked, necessitating adjustments in exposure times and settings to maintain adequate signal-to-noise ratios.

The document does not propose new experimental applications but rather reinforces the reliability of existing calibration procedures while documenting the specific, slow-evolving instrumental characteristics that must be accounted for in scientific data reduction.