Hubble Space Telescope is not only the most famous optical telescope on (or off) Earth, it also may be the busiest. It operates 24 hours a day, seven days a week, requiring a large support team of scientists, engineers, and technicians.
Hubble gathers the light from distant astronomical objects in the same way as its ground-based cousins. Light enters the top of the telescope tube, reflects off the 94.5-inch (2.4-meter) primary mirror, then off a smaller secondary mirror, and back through a hole in the center of the primary mirror. There, it is directed to one of six instruments that record the light.
The instruments include cameras and spectrographs covering optical, ultraviolet, and infrared wavelengths.
The cameras record their images through sets of filters that isolate individual wavelengths of light. Scientists combine these monochrome images to produce the spectacular color shots for which Hubble is famous. Some of the final images are “true” color, showing us how the objects would like to our eyes. Others are false-color images that emphasize the energy emitted by specific chemical elements, such as hydrogen, oxygen, or carbon. Such images help astronomers measure the composition and structure of their target objects.
The spectrographs split the light into a full “rainbow” of colors, allowing astronomers to measure the intensity of each wavelength. The spectra reveal details about an object’s composition, temperature, and motion through space.
Hubble records its observations in computer memory and transmits them to Earth several times a day through a constellation of five orbiting communications satellites. The observations are relayed to a central ground station at White Sands, New Mexico, then to Hubble’s control center at NASA’s Goddard Space Flight Center in Maryland, and finally to the Space Telescope Science Institute on the campus of Johns Hopkins University in Baltimore.
The control center operates around the clock, with engineers and technicians keeping an eye on Hubble’s systems. They also transmit instructions for each observation: target object, length of the observation, which instrument to use, which filters, and so on.
Those observations are planned well in advance at the Space Telescope Science Institute. Astronomers from around the world submit proposals for Hubble projects. A review committee selects the best proposals (only about 1 in 10 is approved). Each accepted project is allotted a certain number of “orbits” for observations. Each orbit is about 97 minutes long. Scheduling experts then work out the most efficient way to accommodate all of the projects and compile the detailed instructions that will be transmitted to the telescope.
Because Hubble is in orbit, of course, its operators face many challenges that don’t confront the operators of ground-based telescopes, or that require different solutions.
One challenge in the “different-solution” category is keeping the mirrors at a constant temperature. On the ground, a telescope is enclosed in a dome that is sealed and air conditioned during the day to maintain a cool temperature for the telescope’s nighttime viewing.
Because Hubble is in space, however, it is always staring into darkness, so it operates around the clock. It spends most of its time in direct sunlight. But for a part of each orbit, the telescope is in Earth’s shadow, so the temperature of its skin drops by more than 100 degrees. Such an abrupt change inside the telescope could cause its mirrors and lenses to expand and contract. That would change their curvature, blurring their view. So the mirrors, lenses, and instruments are wrapped in thick blankets of insulation, while the spacecraft’s skin is coated with a material that reflects most of the sunlight that strikes it, which keeps Hubble from overheating.
Hubble must provide its own power as well, which is supplied by two wing-like solar arrays. They convert sunlight to about 5,600 watts of electricity.
One critical challenge is accurately pointing the telescope. Since Hubble is drifting through space, there is no solid platform from which to guide it. Yet even a tiny misalignment, or a small drift during observations, can ruin the view.
Hubble maintains its alignment by using instruments known as Fine Guidance Sensors, which are the primary science instrument for Fritz Benedict’s extrasolar-planet work, to lock onto “guide stars” near its target. The guide stars, whose positions are precisely catalogued, serve as astronomical signposts to help Hubble maintain its aim. Gyroscopes and reaction wheels move the telescope to keep these guide stars locked in place. With these systems, Hubble can aim at a single tiny patch of sky for hours or even days on end without a wobble.
This system has kept Hubble working for more than two decades, allowing it to conduct thousands of observations of the universe.