Target Pillar: Observing the Cosmos: Telescopes, Instruments, and Data
Last Updated: sep 23, 2026 Author: Sarah Mitchell
Introduction
Hubble isn’t a camera in the way most people picture one — it’s a precisely engineered light-collecting instrument that turns faint photons from billions of light-years away into data astronomers can actually analyze.[^1] Launched in April 1990, it remains, more than three decades later, one of the most scientifically productive instruments ever built, and it’s still running today alongside its newer, more famous successor, the James Webb Space Telescope. Understanding how it actually works — the mirror design, the orbit, and the very public mistake NASA had to fix in orbit — explains both why Hubble became so influential and why it’s stayed relevant even after JWST’s launch.
The Basic Optical Design: A Cassegrain Reflector
At Hubble’s core is a 2.4-meter (94.5-inch) primary mirror, arranged in what’s called a Cassegrain design — a layout named after a 17th-century design concept using two mirrors rather than one.[^2] Light entering the telescope travels down its length and strikes the large, bowl-shaped primary mirror, which reflects that light forward to a smaller, dome-shaped secondary mirror suspended above it. The secondary mirror then reflects the light back through a hole in the center of the primary mirror, focusing it into a beam about the size of a dinner plate before it reaches Hubble’s science instruments — cameras and spectrographs — for final focus and detection.[^2][^3]
This two-mirror arrangement lets Hubble achieve a genuinely large light-collecting area while keeping the overall telescope tube far more compact than a single, equally large mirror would require on its own. The mirror’s exceptional polish and considerable size let Hubble detect objects roughly 10 billion times fainter than the human eye can see unaided, with angular resolution precise enough to distinguish detail equivalent to discerning the width of a dime from 86 miles away — about ten times sharper than most equivalent ground-based telescopes typically achieve without additional correction.[^4]
Why Orbit Actually Matters Here
Hubble’s defining advantage isn’t really its mirror size — several ground-based telescopes have larger primary mirrors. It’s Hubble’s position: orbiting roughly 547 kilometers above Earth, entirely above the atmosphere that ground telescopes have to look through.[^5] Earth’s atmosphere is essential for life but genuinely disruptive for astronomy — turbulence bends and distorts incoming starlight, which is why stars visibly twinkle to the naked eye. That same turbulence blurs telescope images, a problem ground-based observatories fight using adaptive optics systems that rapidly correct for atmospheric distortion in real time. Hubble sidesteps the problem entirely simply by operating above the atmosphere altogether, which is also why it can detect ultraviolet light that never reaches ground-based telescopes at all, since Earth’s atmosphere absorbs most UV before it reaches the surface.[^5][^3]
What Hubble Actually Sees: Three Kinds of Light
Hubble’s optical system, formally called the Optical Telescope Assembly, is built to gather infrared, visible, and ultraviolet light — a considerably broader range than the human eye covers on its own.[^3] Its instrument suite includes two cameras and two spectrographs, along with a set of Fine Guidance Sensors that let the telescope point with the extreme precision required to hold a single, faint, distant target steady in view for the extended exposure times deep astronomical imaging requires.[^6]
The telescope’s structural frame, called the Optical Telescope Assembly’s support skeleton, is built from graphite epoxy — the same lightweight, stiff composite material used in golf clubs, bicycles, and tennis rackets — specifically chosen because it resists the expansion and contraction that Hubble’s extreme orbital temperature swings would otherwise cause, changes that could subtly warp the mirrors and throw the entire optical system out of focus.[^3]
The 1990 Mistake That Almost Ruined the Mission
Hubble’s early history included a genuinely serious setback. About a month after its April 1990 launch, astronomers discovered the primary mirror had been ground to a very slightly incorrect shape due to a flawed testing procedure during manufacturing — an error called spherical aberration, producing blurry, out-of-focus images across the telescope’s entire observation record up to that point.[^7] The flaw was tiny in absolute terms — the mirror’s edge was ground about 2.2 microns flatter than the design specified, an amount thinner than a human hair — but Hubble’s optical precision was sensitive enough that even this microscopic error meaningfully degraded image quality.[^8]
NASA’s fix, delivered via a dramatic space shuttle servicing mission in 1993, didn’t replace the flawed mirror itself — doing so wasn’t practical. Instead, astronauts installed a corrective optics package, essentially the equivalent of prescription eyeglasses for Hubble’s other instruments, compensating for the mirror’s precise, known flaw.[^7] The mission worked: image quality improved dramatically, and Hubble went on to produce the vast majority of the discoveries it’s now known for.
Built to Be Fixed: The Servicing Mission Advantage
That 1993 repair was possible because of a deliberate design choice: Hubble was placed in a low orbit specifically so it could be serviced in space by astronauts aboard the Space Shuttle, letting NASA replace instruments as technology improved and repair subsystems as they aged or failed.[^9] NASA carried out four additional servicing missions beyond the initial 1993 repair — in 1997, 1999, 2002, and 2009 — progressively fixing gyroscope and solar array issues while also installing entirely new cameras and spectrographs with each visit, meaning Hubble’s actual observing capability improved substantially over its lifetime rather than simply degrading from its original 1990 specification.[^7]
This serviceable design is a meaningful contrast with JWST, which orbits far too distant from Earth (at the Sun-Earth L2 point) for any comparable in-person repair mission, making Hubble’s continued operation alongside Webb genuinely complementary rather than redundant — Hubble covers ultraviolet, visible, and near-infrared light, while Webb observes further into the infrared spectrum, together giving astronomers a considerably broader combined view than either telescope provides alone.[^9]
How Much Longer Will Hubble Actually Last?
NASA’s engineering assessments have consistently proven more optimistic than early skeptics expected. Formal reliability studies of Hubble’s instruments and core spacecraft subsystems — gyroscopes, reaction wheels, solar arrays, batteries — indicate the observatory should continue operating through the latter part of the 2020s, and potentially into the 2030s.[^9] That’s a genuinely remarkable outcome for a spacecraft that, at points in its history, faced real hardware degradation, including gyroscope failures and, more recently, an instrument reliability report flagging Hubble’s Wide Field Camera 3 for a temporary operational suspension due to a hardware issue.[^10]
Frequently Asked Questions
Is Hubble still operating in 2026, alongside the James Webb Space Telescope? Yes — Hubble remains active and continues producing new observations and discoveries as of 2026, operating concurrently with JWST rather than having been replaced by it. The two telescopes cover different, complementary portions of the light spectrum.
Why couldn’t NASA just replace Hubble’s flawed mirror instead of adding a correction system? Removing and replacing the primary mirror itself wasn’t a practical option for an in-orbit servicing mission. Instead, NASA’s 1993 fix installed a corrective optics package that compensated for the mirror’s precisely measured, known flaw — functioning similarly to how glasses correct human vision without altering the eye itself.
How is Hubble different from ground-based telescopes with even bigger mirrors? Hubble’s key advantage isn’t mirror size — many ground telescopes have larger primary mirrors — it’s location. Operating above Earth’s atmosphere eliminates the atmospheric turbulence that blurs ground-based observations and blocks most ultraviolet light before it ever reaches the surface, letting Hubble see wavelengths and achieve a clarity that ground telescopes need additional correction technology to approach.
Can Hubble be repaired again if something breaks? Not currently in the same way as its five Space Shuttle-era servicing missions — those relied on the now-retired Space Shuttle program specifically. Current spacecraft servicing options look different, and any future Hubble repair would need to use different vehicles and methods than the original astronaut-serviced missions.
What’s the practical difference between what Hubble sees and what Webb sees? Hubble observes ultraviolet, visible, and near-infrared light, while the James Webb Space Telescope is optimized for near- to mid-infrared wavelengths further along the spectrum. This means the two telescopes reveal different physical phenomena and are frequently used together on the same targets to build a fuller picture than either instrument could provide alone.
Sources
- Sentinel Mission — How Does Hubble Telescope Work?
- NASA Science — Hubble Optics
- ESA — Hubble Overview
- NASA Science — HST Overview Fact Sheet
- Sentinel Mission — How Does Hubble Telescope Work? (orbit and atmosphere)
- ESA — Hubble Overview (instrument suite)
- Britannica — Hubble Space Telescope: History, Discoveries, & Facts
- NASA Science — Hubble Design
- NASA Science — Hubble Design (servicing missions and lifespan projection)
- Fox News — Hubble Space Telescope Will Last Through the Mid-2020s, Report Says
Note on methodology: technical details above are drawn from NASA’s official Hubble mission and design pages, ESA, and Britannica, cross-checked for consistency. Hubble’s operational status and instrument health are actively monitored and can change — consult NASA’s Hubble mission pages directly for current status.