The telescope's overall dimensions approximate those of a city bus or railroad tank car—13 m (43 feet) long, 12 m (39 feet) across with solar arrays extended, and 11,000 kg (12.5 tons when weighed on the ground). The heart of HST is a 2.4-m (94.5-inch) diameter mirror designed to capture optical, ultraviolet, and infrared radiation before it reaches Earth's murky atmosphere. The accompanying figure shows the telescope being lifted out of the cargo bay of the space shuttle Discovery in the spring of 1990.

The optical system and scientific instruments aboard HST are compact and pioneering. The telescope reflects light from its large mirror back to a smaller, 0.3-m (12-inch), secondary mirror, which in turn sends the light through a hole in the doughnut-shaped main mirror and into the aft bay of the spacecraft. There, any of five major scientific instruments wait to analyze the incoming radiation. Most of these instruments are about the size of a telephone booth. They include two cameras to image (or electronically photograph) various regions of the sky, two spectrographs to split the radiation into its component wavelengths, and a group of fine guidance sensors to measure the positions of stars in the sky. Most of these instruments have been upgraded or replaced by NASA astronauts since the telescope's launch.

Soon after launch, astronomers discovered that the telescope's primary mirror had been polished to the wrong shape. The mirror is too flat by 2 µm, about 1/50 the width of a human hair. Even though it is the smoothest mirror ever made, this imperfection makes it impossible to focus light as well as expected. This optical flaw (known as spherical aberration) meant that HST was not as sensitive as designed, although it could still see many objects in the universe with unprecedented resolution. In late 1993, astronauts aboard the

Since the 1993 repair mission Hubble's resolution is close to the original design specifications, and the telescope has regained much of its lost sensitivity, enabling it to see very faint objects. In early 1997 a second servicing mission replaced several instruments with more sensitive models, performed maintenance on the satellite's fine-guidance system, and upgraded some of the telescope's data systems. The next service mission is scheduled for 1999.

A good example of Hubble's scientific capabilities today can be seen by comparing the two images of the spiral galaxy M100, shown here. On the left is perhaps the best ground-based photograph of this beautiful galaxy, showing rich detailand color in its spiral arms. On the right, to the same scale and orientation is an HST image showing improvement in both resolution and sensitivity. (The chevron-shaped field of view is caused by the corrective optics inserted into the telescope; an additional trade-off is that Hubble's field of view is smaller than those of ground-based telescopes.) The inset shows Hubble's exquisite resolution of small fields of view.

Given that Hubble was so expensive to build, was meant to be the flagship of a whole new generation of NASA spacecraft, and was greeted with such public fanfare, it is perhaps understandable that the nwes media sensationalized so many aspects of the telescope's problems and of the repair mission. The bottom line, however, is that HST was never really "broken," nor is it now completely "fixed." Despite the fact that it still does not opperate as originally designed, Hubble is arguably the best telescope built by humans to date. Many spectacular examples of its remarkable data appear throughtout this book.