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The Advanced Camera for Surveys is an imaging instrument on the Hubble Space Telescope, designed with separate wide-field, high-resolution and ultraviolet channels. Filters select spectral bands; dispersing elements also provide low-resolution spectroscopy. The different channels have different detectors and optical paths, so the instrument name alone does not specify the relevant wavelength coverage.^[ref]

For an exoplanet transit, a sequence of dispersed images can be divided into spectral channels, each yielding its own light curve. Comparing fitted exoplanet transit depths constructs an exoplanet transmission spectrum. Detector response and pointing variations must be calibrated along with the planetary signal. Historical observations should be interpreted using the channel and observing mode actually employed.

An interacting pair of spiral galaxies undergoing a galaxy merger. Their long tidal tails contain stars displaced by the encounter. They are also a nearby example of a starburst galaxy, with vigorous star formation accompanying the interaction.^[ref]

The Hubble Space Telescope image below combines observations from its Wide Field Camera 3 and Advanced Camera for Surveys. Bright young stars, emitting regions of the interstellar medium and obscuring interstellar dust reveal different aspects of the encounter. The close view emphasizes the interacting central regions; wider views show the long outer tidal tails.^[ref]

The overview compares wide-field and close Hubble Space Telescope views of the Antennae Galaxies and describes their tidal tails and young star clusters.

In the 2016 observational context, appropriate choices are as follows.

For atomic species, use optical transit spectra of Na I near $0.589\mu \mathrm{m}$ and K I near $0.770\mu \mathrm{m}$, with Hubble Space Telescope/STIS or ground-based high-resolution instruments on Very Large Telescope/Keck telescope-class telescopes. Neutral/ionized ultraviolet metal lines offer additional diagnostics with Hubble Space Telescope. Resolving a line's shape and distinguishing stellar contamination matters as much as locating its wavelength.

For molecules, use near/mid-IR spectroscopy: water bands near $1.4$, $1.9$ and $2.7\mu \mathrm{m}$, CO near $2.3$ and $4.6\mu \mathrm{m}$, methane near $3.3\mu \mathrm{m}$, and ${\mathrm{CO}}_2$ near $4.3$ or $15\mu \mathrm{m}$. Hubble Space Telescope/WFC3 samples water around $1.1$--$1.7\mu \mathrm{m}$; ground-based Very Large Telescope/CRIRES or Keck telescope/NIRSPEC spectra target accessible near-IR windows such as CO at $2.3\mu \mathrm{m}$. Warm Spitzer Space Telescope provides $3.6$ and $4.5\mu \mathrm{m}$ photometry, while cryogenic Spitzer Space Telescope spectra/longer-wavelength data are archival by 2016. A broadband eclipse point alone does not uniquely identify a molecule.

For atmospheric escape, use ultraviolet transit spectroscopy of H I Ly$\alpha$ at $121.6\mathrm{nm}$, and suitable C/O/metal lines, with Hubble Space Telescope/STIS or COS. Extended, velocity-shifted absorption beyond the optical planet suggests an escaping envelope, although stellar variability, geocoronal emission and interstellar absorption require treatment. Ground-based H$\alpha$ at $656.3\mathrm{nm}$ probes excited hydrogen and is a complementary rather than interchangeable measure of neutral-hydrogen escape.

Three complementary methods available in the setting of this 2017 paper are:

Transmission probes the limb, eclipses probe the dayside, and direct imaging isolates spatially resolved planetary light. Radial velocities alone measure orbital motion rather than atmospheric composition; high-resolution planetary Doppler spectroscopy can instead detect atmospheric spectral lines if planetary light is isolated statistically.

First refine the ephemeris, planetary mass, stellar radius, and stellar variability using exoplanet transit photometry and the radial-velocity method. Then combine observations that probe different regions rather than relying on one spectrum. A present-day programme could use the following complementary measurements; in the 2019 setting of the paper, James Webb Space Telescope observations would have been a future capability.

Together these address aerosols, molecular composition, elemental enrichment, vertical thermal structure, horizontal heat transport, winds, and escape. Repeat key events and monitor stellar activity, since stellar contamination and instrumental trends can imitate atmospheric signals. Use actual brightness, saturation limits, and predicted feature amplitudes to choose observing modes and exposure times.

The wavelength ranges and time-series capabilities are documented in the NIRISS SOSS guide, NIRSpec overview, MIRI spectroscopy guide, and ESO's CRIRES description.

The Hubble Space Telescope uses ultraviolet through near-infrared transit and eclipse spectroscopy. It established detections of alkali metals, water, aerosols, and escaping hydrogen but has a small aperture and restricted continuous infrared coverage. High-resolution ground facilities such as the Very Large Telescope separate Doppler-shifted planetary lines from telluric and stellar spectra and directly image young giants; the atmosphere and thermal background limit broad-band precision. The James Webb Space Telescope uses NIRISS, NIRSpec, NIRCam, and MIRI for roughly visible-to-mid-infrared transit, eclipse, phase-curve, and direct-imaging spectroscopy. It has measured broad molecular inventories, including the clear carbon-dioxide feature and photochemical sulfur dioxide in WASP-39 b.

JWST improves on earlier facilities through its 6.5-metre collecting area, cold space environment, stable time-series spectroscopy, and broad infrared wavelength coverage spanning several bands of water, carbon dioxide, methane, carbon monoxide, ammonia, and sulfur compounds. The Ariel space telescope is designed for a uniform atmospheric census of about one thousand planets, while the Extremely Large Telescope will add spatial resolution and high-dispersion spectroscopy for nearby giant and terrestrial planets.

Three directions enabled or sharply advanced by JWST are chemically complete retrievals across multiple molecular bands rather than isolated detections; atmospheric tests of smaller and cooler sub-Neptunes and rocky planets; and measurements of photochemistry, clouds, heat redistribution, and vertical thermal structure through repeated transit, eclipse, and phase-resolved spectra.

Sirius is a nearby binary star containing a luminous main-sequence star and a much fainter white dwarf. Their coexistence makes a useful lesson in stellar appearance: a hot object can be faint because its radiating area is small. The resolved Hubble Space Telescope image separates the compact companion from the bright primary's surrounding light.^[ref]

The common orbit provides dynamical information, whereas spectral classification and luminosity constrain the two bodies' physical states. The white dwarf records prior stellar evolution; its current faintness does not mean it was always the less massive or less luminous member. Sirius therefore connects binary star measurements with white-dwarf cooling and the interpretation of the Hertzsprung-Russell diagram.

The Wide Field Camera 3 is an instrument on the Hubble Space Telescope with separate ultraviolet/visible and near-infrared channels. Its grisms disperse light onto the detector without an entrance slit, allowing a time series of stellar spectra during an exoplanet transit. The near-infrared channel uses two grisms covering different intervals; their combined coverage is broader than that of either alone.^[ref]

A spectral channel yields a light curve, and its fitted exoplanet transit depth contributes one interval of the exoplanet transmission spectrum. The resulting molecular bands can constrain water and the atmospheric scale height, subject to exoplanet clouds and reference-pressure degeneracy in transmission spectroscopy. Because the stellar photons dominate, stability and calibration are as important as collecting light.