A telescope's mirrors are its key to examining the universe. Hubble's mirrors capture light from the cosmos, transferring it to the science instruments that turn it into data.
People often think the power of a telescope relies on its ability to magnify objects. Telescopes actually work by using mirrors to capture more light at higher resolution that the human eye can see. The larger a telescope's mirror, the more light it can capture.
Hubble has small mirrors by the standards of scientific ground telescopes, but those mirrors combine with Hubble's position 340 miles (547 km) above Earth's atmosphere to create its signature, detail-filled images. Earth's atmosphere is made up of shifting pockets of air that distort the view of telescopes on the ground; this is why stars seem to "twinkle" when you look at the night sky. Other wavelengths of light are blocked entirely by the atmosphere. Hubble solves these problems by simply orbiting above the fray.
Hubble's optics consist of two mirrors, a support system and the openings to the telescope's instruments. Incoming light travels down a tube, is collected by a bowl-like, inwardly curved primary mirror and reflected toward a smaller, dome-shaped, outwardly curved secondary mirror. The secondary mirror bounces the light back to the primary mirror and through a hole in its center. The light is focused on a small area called the focal plane, where it is picked up by the various science instruments.
Hubble's mirrors are very smooth and have precisely shaped reflecting surfaces. They were ground so that their surfaces do not deviate from a perfect curve by more than 1/800,000ths of an inch. If Hubble's primary mirror were scaled up to the diameter of the Earth, the biggest bump would be only six inches tall.
But shortly after Hubble's deployment in 1990, astronomers quickly realized something was amiss. Hubble's primary mirror was just slightly the wrong shape. The tiny flaw — about 1/50th the thickness of a sheet of paper — was enough to distort Hubble's view.
Scientists and engineers came up with a solution: a series of small mirrors that would intercept the light reflecting off the mirror, correct for the flaw and bounce the light to the telescope's science instruments. The Corrective Optics Space Telescope Axial Replacement (COSTAR) was installed by astronauts during the first mission to Hubble in 1993. Since then -- starting with the installation of the Wide Field and Planetary Camera 2 instrument during that same mission -- Hubble's instruments have all had corrective optics built in, eventually making COSTAR unnecessary. It was removed in 2009 and now belongs to the Smithsonian museums.
Hubble's mirrors are made of ultra-low-expansion glass and kept at a nearly constant room temperature to avoid warping. The surfaces are coated with a 3/1,000,000th-inch layer of pure aluminum for reflectivity and protected by a 1/1,000,000th-inch layer of magnesium fluoride.
Hubble was designed to hold six science instruments, each observing the universe in a unique way. Hubble contains two main varieties of instruments: cameras, which capture Hubble's famed images, and spectrographs, which break light into colors for analysis.
Wide Field Camera 3
Wide Field Camera 3 (WFC3) expanded Hubble's reach by giving the telescope greater access to ultraviolet, visible and infrared wavelengths of light. With its high resolution and increased sensitivity in ultraviolet and infrared, WFC3 has become Hubble's predominant camera, responsible for its latest spectacular images. It has imaged everything from nearby star formation to galaxies in the very distant universe.
Cosmic Origins Spectrograph
The Cosmic Origins Spectrograph (COS) is the most sensitive ultraviolet spectrograph ever produced for space. It breaks ultraviolet radiation into components that can be studied in detail. COS is best at studying points of light, like stars or quasars (distant galaxies emitting tremendous amounts of light from their central regions). It has been used to study galaxy evolution, the formation of planets and the rise of the elements needed for life.
Advanced Camera for Surveys
The Advanced Camera for Surveys (ACS) conducts surveys of the universe. It is responsible for many of Hubble's most impressive images of deep space. With its wide field of view, sharp image quality and high sensitivity, the camera doubled Hubble's field of view and expanded its capabilities significantly when it was installed.
Space Telescope Imaging Spectrograph
The Space Telescope Imaging Spectrograph (STIS) is used to study black holes, the composition of galaxies, the atmospheres of planets around other stars and more. In addition to taking detailed pictures of celestial objects, STIS is a spectrograph. It acts like a prism to separate light from the cosmos into its component colors. This provides a wavelength "fingerprint" of the object being observed, which tells us about its temperature, chemical composition, density and motion.
Near Infrared Camera and Multi-Object Spectrometer
The Near Infrared Camera and Multi-Object Spectrometer (NICMOS) is Hubble's heat sensor. The instrument's three "cameras" — each with different fields of view — are specially designed to see objects in the near-infrared wavelengths, which are slightly longer than the wavelengths of visible light (human eyes cannot see infrared light). Infrared light reveals details about distant galaxies, planets and solar systems, and star formation that are not available in visible light.
Fine Guidance Sensors
Hubble's three Fine Guidance Sensors (FGS) — its targeting cameras — provide feedback used to maneuver the telescope and perform celestial measurements. Two of the sensors point the telescope at an astronomical target and then hold that target in a scientific instrument's field of view. The third sensor is available to perform scientific observations. The Fine Guidance Sensors can provide star positions that are about 10 times more precise than those observed from a ground-based telescope.
Hubble is "flown" by commands from controllers on the ground. Several spacecraft systems are in place to keep Hubble functioning smoothly.
Hubble performs in response to detailed instructions from people on the ground. The antennae allow technicians to communicate with the telescope, telling it what to do and when to do it. Four antennae receive and send information to a set of satellites, which in turn communicate with Earth.
Hubble is powered by sunlight. Each wing-like array has a solar cell "blanket" that converts the Sun's energy into electricity. Some of that electricity runs the telescope, some is stored in onboard batteries for the periods when Hubble is in Earth's shadow.
Computers and automation
Several computers and microprocessors reside in Hubble's body and in each science instrument. There are two main computers. One talks to the instruments, sends commands and other information, and transmits data; the other handles pointing control, gyroscopes and other system-wide functions.
Hubble has a "skin," or blanket, of multilayered insulation (MLI), which protects the telescope from temperature extremes. In 2009, astronauts also added panels of insulation called New Outer Blanket Layers (NOBLs) over portions of Hubble. NOBLs replaced sections of blanket that had broken down from exposure to the harsh conditions of space. Beneath Hubble's insulation is a lightweight aluminum shell, which provides an external structure to the spacecraft and houses its optical system and science instruments.
Hubble uses a combination of gyroscopes, reaction wheels and Fine Guidance Sensors to orient itself. Gyroscopes always face the same direction and sense the telescope's angular motion, providing a short-term reference point to help Hubble zero in on a target. The reaction wheels are the steering system. When they spin one way, Hubble turns the other way. They accelerate or decelerate as needed to rotate the telescope. The Fine Guidance Sensors aim the telescope by locking onto guide stars and providing a precise reference point to help the telescope reposition.