Unlike many other scientific investigators, astronomers cannot move the subject they study, stars for example, into the laboratory. Inside the solar system, we can use active optics such as lasers in addition to passive optical remote sensing for probing matter. Outside our solar system, with the unique exception of the Voyager spacecraft, we rely exclusively on passive detection and measurement instruments and methods.
The classic astronomical telescope system consists of two primary parts: (1) the telescope, sometimes called the fore-optics, which collects radiation and focuses it onto a mask at the image plane that clearly defines the area to be analyzed by an instrument; and (2) the image plane instrument, which processes the radiation using optics, filters, spectrometers, interferometers, or polarimeters, and reimages the field onto a focal plane where the processed radiation is converted into electronic signals for scientific analysis and archiving. Large modern ground- and space-based telescopes require the addition of a third section between the fore-optics and the image plane instrument. This section performs wavefront sensing and control (WFSC). The WFSC subsystem measures the telescope wavefront and corrects it using actuators that move portions of surfaces. In addition, many modern telescopes employ an optical metrology subsystem that measures distances between optics and holds them to a predefined distance.
Telescopes are optimized for broad classes of applications in astrophysics such as x-ray, solar, and far-IR astronomy, and astrometry (the precise measurement of positions on the sky). The instrument, however, is often configured to respond to a specific set of astronomy questions. The modern telescope is the most expensive part of the system and is usually built to serve many instruments. Astronomical optics for space applications are different from those used for ground applications. Astronomical telescopes for space applications are optimized to use the unique environment provided by space, and with only one exception, the Hubble Space Telescope (HST), are not able to be serviced for updating the instruments and their focal planes.