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    How space telescopes work, and what Webb, Roman and Rubin each do

    Telescopes collect light; bigger mirrors gather more, and going to space avoids the atmosphere and lets instruments work in the infrared, where light from the earliest galaxies ends up.[1] In 2026 three facilities set the pace: Webb for deep, detailed looks, Roman (launched August 2026) for wide infrared surveys, and the ground-based Rubin Observatory for a ten-year movie of the southern sky.[2][3][4]

    Editor reviewedUpdated Space exploration and astronomySpaceScience

    Collecting light

    A telescope is a light bucket. The bigger its main mirror, the fainter the things it can see and the sharper the detail. Webb’s mirror is about 6.5 meters across, built from 18 gold-coated segments that unfolded in space.[5]

    Earth’s atmosphere blurs starlight and blocks much of the infrared. A space telescope avoids both problems. Webb sits about 1.5 million km from Earth at a point called L2, where it can stay cold and keep the Sun, Earth and Moon behind a sunshield.[2][6]

    Collecting area sets sensitivity and aperture sets diffraction-limited resolution. Webb’s 6.5 m segmented beryllium primary covers 0.6 to 28.5 microns.[5] Infrared observing requires a cold telescope; Webb’s five-layer sunshield, roughly 21 by 14 m, passively blocks infrared radiation from the Sun, Earth and Moon at the Sun-Earth L2 point.[6][2]

    Why infrared: looking back in time

    Light takes time to travel, so looking far away means looking into the past. As the universe expands, light from very distant galaxies gets stretched to longer, redder wavelengths. By the time it reaches us, the light of the first galaxies is infrared, which is why Webb was built to see it.[1] In 2025 astronomers used Webb to confirm a galaxy, MoM-z14, seen just 280 million years after the Big Bang.[7]

    Cosmological redshift moves rest-frame ultraviolet and optical features of high-redshift galaxies into the near- and mid-infrared. Spectroscopic confirmation, rather than photometric estimates, is the standard for distance records: MoM-z14 was confirmed at z = 14.44 with NIRSpec through a sharp Lyman-alpha break, and the implied number of bright galaxies at that epoch exceeds pre-Webb model predictions by more than a factor of 100.[7]

    Deep versus wide versus fast

    The three flagship facilities of the mid-2020s are designed for different jobs:

    • Deep and detailed. Webb studies individual objects in depth, from the early universe to exoplanet atmospheres.[8]
    • Wide. Roman, launched on August 30, 2026, has a field of view at least 100 times larger than Hubble’s and aims at dark energy, exoplanets and infrared astrophysics.[3][9] Its project scientist says a one-month Milky Way survey would take Hubble about a century.[10]
    • Fast and repeated. Rubin Observatory in Chile uses an 8.4-meter telescope and the largest digital camera ever built.[11] It began its ten-year survey in mid-2026 and sends out up to seven million alerts a night about things that changed in the sky.[4][12]

    Splitting light: spectroscopy

    Pictures are only part of the story. Telescopes also split light into a rainbow, called a spectrum. Dark or bright bands in that rainbow act like a barcode that shows which molecules are present.[13] That is how Webb can tell what an exoplanet’s air is made of.[8]

    Spectroscopy underpins distance measurement (redshift), chemistry and kinematics. For exoplanets, transmission spectroscopy during transits measures wavelength-dependent absorption by the planet’s atmosphere.[13] Signals are small, so instrument systematics can dominate: the K2-18 b debate turned partly on how mid-infrared data were binned and reduced.[14] See how exoplanets are found for detection methods.

    Questions readers ask

    Why does Webb look in infrared?

    Light from the earliest galaxies is stretched into the infrared by the expansion of the universe, and infrared light also passes through dust that hides newborn stars.[1]

    What is the difference between Webb and Roman?

    Webb studies objects in great detail with a 6.5-meter mirror. Roman has a field of view at least 100 times larger than Hubble's and is built to survey large areas of sky quickly.[5][9]

    Why is Rubin Observatory on the ground?

    Rubin's job is to image the whole southern sky every few nights for ten years. It does that with an 8.4-meter telescope and the largest digital camera ever built, on a mountain in Chile.[11][4]

    Sources

    Each numbered claim is a statement we checked against the sources listed with it. Status shows how well established it is.

    1. [1]

      Webb observes in infrared because light from the earliest galaxies has been stretched (redshifted) into the infrared by cosmic expansion, and infrared light passes through dust that hides forming stars. confirmedas of 2026-10-10

    2. [2]

      The James Webb Space Telescope, a joint NASA/ESA/CSA mission, launched on December 25, 2021 and orbits the Sun about 1.5 million km from Earth at the second Lagrange point (L2). confirmedas of 2026-10-10

    3. [3]

      NASA's Nancy Grace Roman Space Telescope launched on a SpaceX Falcon Heavy from Kennedy Space Center at 7:26 a.m. EDT on August 30, 2026. confirmedas of 2026-08-30

    4. [4]

      NSF NOIRLab, which operates Rubin with SLAC, announced on June 30, 2026 that the 10-year Legacy Survey of Space and Time had officially started after a period of system optimization and an operational readiness review; Rubin Observatory's own website dates the start of the survey to July 2026. confirmedas of 2026-10-10

    5. [5]

      Webb's primary mirror is about 6.5 m across, made of 18 gold-coated beryllium segments, and the telescope observes wavelengths from 0.6 to 28.5 microns. confirmedas of 2026-10-10

    6. [6]

      Webb uses a five-layer sunshield, about 21 by 14 meters, to block infrared radiation from the Sun, Earth and Moon. confirmedas of 2026-10-10

    7. [7]

      In 2025 a team used Webb spectroscopy to confirm the galaxy MoM-z14 at redshift 14.44, seen about 280 million years after the Big Bang, and found bright galaxies at that epoch to be more than 100 times more common than pre-Webb models predicted. confirmedas of 2025-05-16

    8. [8]

      NASA groups Webb's science into four themes, the early universe, galaxies over time, the life cycle of stars, and other worlds, including measuring what exoplanet atmospheres are made of. confirmedas of 2026-10-10

    9. [9]

      Roman has a field of view at least 100 times larger than Hubble's, carries a Wide Field Instrument and a Coronagraph, and is designed to address dark energy, exoplanets and infrared astrophysics. confirmedas of 2026-10-10

    10. [10]

      Roman's project scientist says its one-month survey of the Milky Way would take about a century with Hubble. reportedas of 2026-08-28

    11. [11]

      The NSF-DOE Vera C. Rubin Observatory on Cerro Pachón in Chile, named after the astronomer Vera Rubin and aimed at questions including dark energy and dark matter, uses the 8.4-meter Simonyi Survey Telescope and the LSST Camera, the largest digital camera ever built, and is operated by NSF NOIRLab and SLAC. confirmedas of 2026-10-10

    12. [12]

      Rubin takes an image about every 40 seconds, collects about 10 terabytes of data a night, produces up to seven million alerts a night that stream to automated alert brokers, will revisit each point in the sky about 800 times over the decade, and will release its billions of catalogued objects openly; its public alerts had already led to hundreds of transient discoveries before the survey began. confirmedas of 2026-06-30

    13. [13]

      Light passing through or coming from an exoplanet's atmosphere can be split into a spectrum whose absorption bands reveal which molecules are present. confirmedas of 2026-10-10

    14. [14]

      A 2025 analysis applying the astrobiology standards-of-evidence framework concluded that instrumental red noise affects the K2-18 b mid-infrared data and that there is not yet statistically significant evidence of biosignatures. confirmedas of 2025-09-03

    Revision history (2)
    1. Page created.
    2. Updated Rubin survey-start citation (NOIRLab June 30, 2026 announcement; Rubin's site says July 2026).

    Created Oct 10, 2026. Last reviewed by an editor on Oct 10, 2026. Next scheduled review: Jan 10, 2027.

    Cite this page

    "How space telescopes work, and what Webb, Roman and Rubin each do." ContentLora, updated Oct 10, 2026. https://contentlora.com/explain/how-space-telescopes-work

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