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Everything You Wanted to Know About the Hubble Telescope,
But Were Afraid to Ask

If mathematical calculations and scientific equations are your idea of fun then read on! This page, provided by our friends at the Space Telescope Science Institute, looks at the inner workings of the Hubble Telescope.

Even if technical jargon isn't your cup of tea, check out a cool diagram of the Hubble Telescope or visit NASA's Lift Off Web site to find out where the Hubble Telescope is right now!

Steve Maran from NASA's Goddard Space Flight Center has also provided his thoughts on the future of the Hubble Telescope. Click here to read his essay.

About the Hubble Telescope

The Hubble Space Telescope is a cooperative program of the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA) to operate a long-lived space-based observatory for the benefit of the international astronomical community. HST is an observatory first dreamt of in the 1940s, designed and built in the 1970s and 80s, and operational only in the 1990s. Since its preliminary inception, HST was designed to be a different type of mission for NASA -- a permanent space-based observatory. To accomplish this goal and protect the spacecraft against instrument and equipment failures, NASA had always planned on regular servicing missions. Hubble has special grapple fixtures, 76 handholds, and stabilized in all three axes. HST is a 2.4-meter reflecting telescope which was deployed in low-Earth orbit (600 kilometers) by the crew of the space shuttle Discovery (STS-31) on 25 April 1990.

Responsibility for conducting and coordinating the science operations of the Hubble Space Telescope rests with the Space Telescope Science Institute (STScI) on the Johns Hopkins University Homewood Campus in Baltimore, Maryland. STScI is operated for NASA by the Association of University for Research in Astronomy, Incorporated (AURA).

HST's current complement of science instruments include two cameras, two spectrographs, and fine guidance sensors (primarily used for astrometric observations). Because of HST's location above the Earth's atmosphere, these science instruments can produce high resolution images of astronomical objects. Ground-based telescopes can seldom provide resolution better than 1.0 arc-seconds, except momentarily under the very best observing conditions. HST's resolution is about 10 times better, or 0.1 arc-seconds.

When originally planned in 1979, the Large Space Telescope program called for return to Earth, refurbishment, and relaunch every 5 years, with on-orbit servicing every 2.5 years. Hardware lifetime and reliability requirements were based on that 2.5-year interval between servicing missions. In 1985, contamination and structural loading concerns associated with return to Earth aboard the shuttle eliminated the concept of ground return from the program. NASA decided that on-orbit servicing might be adequate to maintain HST for its 15- year design life. A three year cycle of on-orbit servicing was adopted. The first HST servicing mission in December 1993 was an enormous success. Future servicing missions are tentatively planned for March 1997, mid-1999, and mid-2002. Contingency flights could still be added to the shuttle manifest to perform specific tasks that cannot wait for the next regularly scheduled servicing mission (and/or required tasks that were not completed on a given servicing mission).

The five years since the launch of HST in 1990 have been momentous, with the discovery of spherical aberration and the search for a practical solution. The STS-61 (Endeavour) mission of December 1993 fully obviated the effects of spherical aberration and fully restored the functionality of HST.

The Science Instruments

Mission Operations and Observations

Although HST operates around the clock, not all of its time is spent observing. Each orbit lasts about 95 minutes, with time allocated for housekeeping functions and for observations. "Housekeeping" functions includes turning the telescope to acquire a new target, or avoid the Sun or Moon, switching communications antennas and data transmission modes, receiving command loads and downlinking data, calibrating and similar activities.

When STScI completes its master observing plan, the schedule is forwarded to Goddard's Space Telescope Operations Control Center (STOCC), where the science and housekeeping plans are merged into a detailed operations schedule. Each event is translated into a series of commands to be sent to the onboard computers. Computer loads are uplinked several times a day to keep the telescope operating efficiently.

When possible two scientific instruments are used simultaneously to observe adjacent target regions of the sky. For example, while a spectrograph is focused on a chosen star or nebula, the WF/PC (pronounced "wiff-pik") can image a sky region offset slightly from the main viewing target. During observations the Fine Guidance Sensors (FGS) track their respective guide stars to keep the telescope pointed steadily at the right target.

If an astronomer desires to be present during the observation, there is a console at STScI and another at the STOCC, where monitors display images or other data as the observations occurs. Some limited real-time commanding for target acquisition or filter changing is performed at these stations, if the observation program has been set up to allow for it, but spontaneous control is not possible.

Engineering and scientific data from HST, as well as uplinked operational commands, are transmitted through the Tracking Data Relay Satellite (TDRS) system and its companion ground station at White Sands, New Mexico. Up to 24 hours of commands can be stored in the onboard computers.

Data can be broadcast from HST to the ground stations immediately or stored on tape and downlinked later.

The observer on the ground can examine the "raw" images and other data within a few minutes for a quick-look analysis. Within 24 hours, GSFC formats the data for delivery to the STScI. STScI is responsible for data processing (calibration, editing, distribution, and maintenance of the data for the scientific community).

Competition is keen for HST observing time. Only one of every ten proposals is accepted. This unique space-based observatory is operated as an international research center; as a resource for astronomers world-wide.

Find out more!

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