The Space Telescope Imaging Spectrograph (STIS). (NASA)
Faint Object Spectrograph (FOS): The FOS, built by Martin Marietta, was designed to analyze the light gathered by the HST to determine a variety of properties such as chemical composition and quantities, magnetic fields, temperature, etc. The instrument, which used a pair of digicon red and blue detectors, could operate in either high-resolution or low-resolution mode and could make spectroscopic observations across a broad spectrum of light from near ultraviolet to near infrared. It typically operated in the 115 nm to 850 nm wavelength range.

High Resolution Spectrograph (HRS): More commonly known as the Goddard High Resolution Spectrograph (GHRS), this instrument was manufactured by Ball Aerospace & Technologies Corp. Like the FOS, it was designed to analyze incoming light to determine an object’s physical and chemical properties. The primary difference between the two was that the GHRS focused strictly on UV spectroscopy in the 1150 Angstrom (115 nm) to 3200 Angstrom (320 nm) wavelength range.

High Speed Photometer (HSP): The HSP was designed and built at the University of Wisconsin, Madison by a group of scientists, engineers and students from the Space Astronomy Laboratory and the Space Science and Engineering Center. Although it measured 3' × 3' × 6' and weighed 600 lbs., the instrument was unique in that it had no moving parts. The HSP was designed to make very high- speed photometric measurements from near ultraviolet to visible wavelengths. It was equipped with four image dissector tubes and a selection of 23 broad- and narrow-band filters ranging from 1200 Angstroms to 7500 Angstroms. Its apertures offered three different fields of view – 0.4 arcseconds, 1.0 arcseconds, and 10.8 arcseconds – and the instrument was capable of making up to 100,000 measurements per second. Because it had no moving parts, directing radiation from the target through the correct set of filters and apertures was accomplished by aiming the entire telescope, a process that could take 30 seconds or more.

Overruns and Delays

Technicians prepare the replacement SI C&DH for testing. (NASA)
Perkin-Elmer began work on the all-important mirror in 1979 using a special low-expansion glass manufactured by Corning Inc. After numerous de lays, work on the mirror finally concluded in late 1981, forcing NASA to postpone the planned 1983 launch date. Finally, in 1985, the recently renamed Hubble Space Telescope (HST) was ready for launch. NASA established a new launch date of October 1986, but on January 28, 1986, the Space Shuttle Challenger disintegrated on takeoff, forcing NASA to suspend all shuttle operations for almost three years while it investigated the cause of the accident. With no means to deploy it, the HST was put into climate-controlled storage where NASA’s engineers continued to make upgrades including improved solar arrays and more sophisticated computer and communications systems.

The space shuttle program resumed operations on September 29, 1988. On April 24, 1990, the Space Shuttle Discovery lifted off on mission STS-31, carrying the HST into space. When testing of the new telescope began shortly thereafter, it soon became obvious that there was a flaw in the mirror that prevented the optical system from focusing properly. The result was images that, although sharper than those produced by ground-based telescopes, were not as clear as they should be.

The problem was traced to a spherical aberration caused by an incorrectly ground edge on the mirror. This caused light reflecting off the edge of the mirror and light reflecting off its center to focus at two different points. It was later determined that one of the sophisticated instruments used to test the mirror, called a null corrector, was the culprit. Either it had been assembled incorrectly or misused, resulting in the mirror’s outer edge being ground 2.2 microns too flat. Fortunately, there was a way to fix it.

Servicing Missions

SM1 (STS-61) December 2 – 13, 1993

Replacing the Wide Field and Planetary Camera. (NASA)
On December 2, 1993, theSpace Shuttle Endeavour took off to conduct the first-ever servicing mission on the HST. The primary purpose of the mission was to correct the flawed optics caused by the spherical aberration in the main mirror.

To do this, NASA reviewed a number of proposals before settling on one submitted by Dr. Mark Bottema, an optics engineer at Ball Aerospace & Technologies. Bottema proposed installing a device that would place small mirrors no bigger than the size of a quarter in front of the Faint Object Spectrograph, the Faint Object Camera, and the Goddard High Resolution Spectrograph. Called COSTAR (Corrective Optics Space Telescope Axial Replacement), it basically did for the HST’s three instruments what eyeglasses do for the human eye. The hard part was finding room for it aboard the HST. Something had to be removed, and since COSTAR was roughly the same size as the HSP, the least used instrument aboard Hubble, officials decided to make the switch.

Instead of installing corrective vision mirrors on the WFPC, officials decided to replace it with a new unit that incorporated corrective optics, as well as other enhancements including better UV performance and more sophisticated detectors. The astronauts also replaced the HST’s solar arrays and related electronics, replaced gyroscopes and magnetometers, and upgraded the onboard computers.