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Hubble Science in the 2030s White Paper: High-Contrast Optical and UV Spectroscopy with HST/STIS

This white paper advocates for the expanded use of the Hubble Space Telescope's STIS instrument in the 2030s to leverage its unique capability for high-contrast visible and ultraviolet coronagraphic spectroscopy, enabling transformative studies of diverse astrophysical targets while serving as a critical pathfinder for the future Habitable Worlds Observatory.

Original authors: K. Ward-Duong (Smith College), J. Debes (Space Telescope Science Institute), J. Aguilar (Space Telescope Science Institute), T. Currie (University of Texas - San Antonio), J. Lomax (United States Nava

Published 2026-06-02

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Original authors: K. Ward-Duong (Smith College), J. Debes (Space Telescope Science Institute), J. Aguilar (Space Telescope Science Institute), T. Currie (University of Texas - San Antonio), J. Lomax (United States Naval Academy), C. Xie (Johns Hopkins University), J. Hashimoto (Academia Sinica Institute of Astronomy & Astrophysics), J. Zhang (University of Hawai'i at Manoa), R. Michelson (Amherst College), E. Vrijmoet (Smith College, Five College Astronomy Department), C. Chen (Space Telescope Science Institute), E. Rickman (Space Telescope Science Institute), K. Hoch (Space Telescope Science Institute), K. Follette (Amherst College)

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

The Big Picture: A Specialized "Star-Blocker" on a Space Telescope

Imagine you are trying to take a photo of a tiny, glowing firefly sitting right next to a blindingly bright spotlight. If you point a camera at the spotlight, the light is so intense that it washes out the firefly completely. You can't see the firefly at all.

This is the problem astronomers face when trying to study planets, dust clouds, or other faint objects orbiting bright stars. The star's light is like that spotlight; it drowns out everything else.

This paper is about Hubble Space Telescope's "STIS" instrument, which has a special trick up its sleeve. It is currently the only telescope in existence (space or ground) that can block a star's light and take a detailed "rainbow" (spectrum) of the faint things hiding right next to it, using both visible light and ultraviolet (UV) light.

The Tool: The "Occulting Bar"

Think of the STIS instrument as having a long, narrow window (a slit) through which it looks at the universe. Inside this window, there are two small, dark sticks called "occulting bars."

How it works: Astronomers aim the telescope so the bright star sits directly behind one of these dark sticks. The stick blocks the star's blinding glare, just like a finger blocking the sun so you can see the clouds behind it.
The Magic: Because the star is blocked, the faint "fireflies" (planets, dust disks, gas jets) sitting right next to the star suddenly become visible.
The Unique Superpower: Most telescopes can just take a picture of these faint objects. STIS can take a picture and break the light from those faint objects down into a detailed rainbow (a spectrum). This tells scientists exactly what those objects are made of (like checking a fingerprint).

Why Hubble is the Best at This Right Now

The paper explains that while giant telescopes on Earth are getting better at this, they have a major problem: The Atmosphere.

The Earth Problem: Earth's atmosphere is like a wavy, shimmering heat haze. It makes the "spotlight" (the star) twinkle and blur. This makes it very hard to block the star perfectly without blurring the faint object next to it.
The Space Advantage: Hubble is floating above the atmosphere. The air is perfectly still. This means Hubble can hold the "dark stick" perfectly steady over the star for hours or days. This stability allows it to subtract the star's leftover "glow" (noise) much better than ground telescopes can, revealing fainter details.

What Scientists Can Do With This (The "Use Cases")

The paper lists several exciting things scientists can do with this "star-blocking rainbow" technique in the 2030s:

Hunting Baby Planets:
- The Analogy: Imagine trying to hear a baby crying in a noisy room.
- The Science: When planets are born, they glow with a specific type of light (Hydrogen-alpha). STIS can isolate this light even if the baby planet is very close to its parent star. It can also look at the "ultraviolet" glow of the material crashing onto the planet, helping scientists understand how fast the planet is growing.
Reading the Recipe of Dust Clouds:
- The Analogy: Think of a dust cloud around a star as a giant, swirling salad.
- The Science: By blocking the star, STIS can analyze the light bouncing off the dust. It can tell if the dust is made of ice, rock, or iron. The paper suggests combining Hubble's UV data with the James Webb Space Telescope's (JWST) infrared data to get a complete "nutrition label" for these cosmic dust clouds.
Watching Stellar Jets:
- The Analogy: Like a garden hose spraying water, young stars often shoot out streams of gas.
- The Science: STIS can zoom in on these streams to see what they are made of and how fast they are moving, even if they are very close to the bright star shooting them out.
Studying Dying Stars and Black Holes:
- The Analogy: Looking at the smoke around a bonfire to see what's burning.
- The Science: It can study the dust clouds around old, dying stars or the bright jets shooting out of black holes (Active Galactic Nuclei), separating the bright center from the surrounding environment.

The "Bridge" to the Future

The paper argues that Hubble is currently the only tool we have for this specific job in the ultraviolet range.

The Analogy: Think of Hubble as a master carpenter teaching an apprentice how to build a specific type of chair.
The Science: A new, massive telescope called the Habitable Worlds Observatory (HWO) is being planned for the future. It will be much bigger and better, but it won't be ready for a long time. By using Hubble now to practice "star-blocking spectroscopy" in the ultraviolet, scientists are learning the tricks and techniques they will need to use when the new telescope arrives. They are essentially "pathfinding" the way forward.

Summary

In short, this paper says: Don't forget Hubble. Even though it's an older telescope, it has a unique ability to block bright stars and analyze the faint light next to them in ways no other telescope can. By using this "dark stick" technique, astronomers can unlock secrets about how planets form, what space dust is made of, and how stars die, all while preparing the scientific community for the next generation of giant space telescopes.

Technical Summary: High-Contrast Optical and UV Spectroscopy with HST/STIS

Problem Statement
High-contrast astronomy has transformed the detection of exoplanets and the characterization of circumstellar environments, yet significant gaps remain in the optical and ultraviolet (UV) regimes. Ground-based adaptive optics (AO) systems on 5–10m telescopes achieve high performance in the infrared (IR) but suffer from reduced image quality and stability in the visible and UV, with typical Strehl ratios of only 10–40% in the V-band compared to 70–90% in the H-band. Furthermore, residual stellar halo light decorrelates rapidly on timescales of minutes to hours, making visible-light high-contrast spectroscopy particularly challenging from the ground. While space-based facilities offer superior stability, there is currently a lack of instruments capable of performing high-contrast spectroscopy across a broad range of optical and UV wavelengths.

Methodology
This white paper details the capabilities of the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST), specifically its under-utilized coronagraphic spectroscopic modes. The methodology relies on the 52" × 0.2" spectroscopic slit, which contains two occulting bars (0.5" and 0.86" wide). By placing a bright astrophysical source behind one of these bars, starlight is suppressed, allowing for the spatially resolved spectroscopy of faint companions and environments.

The paper outlines two primary post-processing techniques for starlight subtraction to achieve high spectral contrast:

Reference Differential Imaging (RDI): Utilizes dedicated observations of reference stars to scale and subtract the Point Spread Function (PSF), requiring close matching of spectral type, brightness, and detector properties.
Spectral Differential Imaging (SDI): Leverages the wavelength and spatial diversity of the PSF without requiring a reference star. This method rescales the image in wavelength space to model and interpolate over the central PSF, leaving residual light from companions or the immediate environment.

These techniques are applied across STIS's extensive wavelength coverage (1150–10,300 Å) using various gratings (G140M/L for MAMA detectors; G750M/L for CCD detectors), achieving resolutions of $R \sim 500 - 10,000$ . The paper emphasizes that advances in PSF subtraction algorithms, originally developed for direct imaging, can now be effectively applied to these dispersed 2D spectra.

Key Contributions and Results

Unique Capabilities: STIS is identified as the sole space-based facility providing visible-light coronagraphic imaging and the only facility capable of both visible and UV coronagraphic spectroscopy. It offers a wide field of view and unparalleled stability (limited only by optical aberrations and thermal breathing) that rivals or exceeds ground-based performance in the optical.
Performance Estimates: Preliminary analysis indicates that starlight subtraction enables visible-light spectral contrasts of $\sim 10^{-4}$ to $10^{-5}$ . Debris disk studies have already demonstrated spectral contrasts (disk-to-star flux ratios) of $10^{-3}$ to $10^{-4}$ at separations interior to 1".
UV Extension: The paper argues that RDI and SDI approaches are readily extensible to UV observations using MAMA detectors, potentially offering improved contrast at closer inner working angles due to smaller PSF sizes at shorter wavelengths. NUV observations can also be conducted using the CCD with G230LB/M gratings.
Science Enablers: The paper details specific use cases enabled by this mode:
- Protoplanets: Resolving H $\alpha$ line emission at $R \sim 6000$ (twice the resolution of current ground-based instruments) and probing NUV continuum emission from accretion shocks.
- Circumstellar Disks: Obtaining the first spatially resolved NUV reflectance spectra to constrain dust grain sizes, composition (e.g., ice absorption edges, Fe-O charge-transfer), and albedo.
- Young Stellar Objects (YSOs): Tracing jet-launching regions and inner accretion zones via forbidden lines ([O I], [S II]) and high-ionization lines (C IV, Si IV).
- Stellar Systems: Characterizing close binaries, white dwarfs, and evolved stars (RSGs, WR stars) to determine mass-radius relationships and dust production rates.
- Active Galactic Nuclei (AGN): Separating central AGN emission from host galaxy populations to trace UV-to-optical continua and emission line ratios.

Significance
The paper posits that revitalizing the use of STIS coronagraphic spectroscopy is critical for the 2030s. It serves as a unique bridge for high-contrast science in the optical and UV, filling a niche that ground-based AO cannot address and that future facilities like the Roman Coronagraphic Instrument (which is limited to bright stars and lower resolution) or JWST (which lacks coronagraphic optical/UV capabilities) cannot fill.

Furthermore, the paper claims that developing these techniques with STIS is essential for "pathfinding" for the Habitable Worlds Observatory (HWO). By characterizing high-contrast UV performance, understanding speckle noise via time-resolved readouts, and refining reference star criteria, STIS can provide the necessary observational and technical groundwork to guide the design and science execution of HWO's UV capabilities. The authors argue that this mode offers transformative science opportunities across planetary science, stellar astrophysics, and extragalactic astronomy while the community awaits the next generation of space telescopes.