Key Components of the Survey Telescope

Channeling the light

The telescope's optical system is dominated by two reflecting mirrors. The focusing system includes two corrective lenses that minimize distortion. The diagram at right shows how that incoming starlight strikes the 2.5-meter primary mirror, bounces back and strikes the smaller secondary mirror, then is reflected back through a hole in the primary mirror. The light passes through the first correcting lens and then through the second lens on top of the camera. Images from the system appear in good focus from an area of sky equal to about 30 full moons.

Making the two-dimensional image

The inner sanctum of the SDSS telescope contains what may be the most complex camera ever built. It includes 30 silicon electronic light sensors called charge-coupled devices, or CCDs (seen at left), that are each two inches square. Scientists encase each column of five devices in a vacuum-sealed chamber. In order to enhance sensitivity, liquid nitrogen cools each chamber to -80 degrees Celsius. Each CCD is made up of more than four million picture elements, which release electrons as light is absorbed. The electrons in turn are amplified into electronic signals that can be digitized, recorded on tape and ultimately fed into a computer. A night's observing will produce up to 200 gigabytes of data on a dozen tapes. Each of the five rows of CCDs receives the light through a different colored filter, so each row records the brightness of objects in a different color.

Into the third dimension

A spectrograph, a device that disperses light into many colors so the spectrum can be recorded, analyzes the distance, composition and age of each celestial object. Astronomers drill 640 holes in an aluminum plate, with each hole corresponding to the position of a selected galaxy, quasar or star in the sky. Scientists plug the holes with optical fiber cables (right). The fibers simultaneously capture light from the 640 objects and record the results in CCDs. The plug plates are interchangeable with the CCD camera at the focal plane of the telescope. On a good night, observers will use six to nine plates.