How Hubble Space Telescope Works

By: Craig Freudenrich, Ph.D. & Sarah Goddard

Hubble's Scientific Instruments: ACS and FGS

During a servicing mission in February 2002, astronauts added the Advanced Camera for Surveys (ACS), doubling the Hubble's field of view and replacing the Faint Object Camera, which served as the HST's telephoto lens.

The ACS, which sees visible light, was installed to help map the distribution of dark matter, detect the universe's most distant objects, search for massive planets and examine the evolution of clusters of galaxies. Scientists estimated it would last five years, and right on cue, an electrical shortage disabled two of its three cameras in January 2007.


Diagram of the Hubble Space Telescope. Mouse over the "Telescope Functions" to examine each function. Note: The Faint Object Camera was replaced by the Advanced Camera for Surveys in 2002.

The final instrument on board the HST are its Fine Guidance Sensors (FGSs), which point the telescope and precisely measure the positions and diameters of stars, as well as the separation of binary stars. The Hubble has three of these sensors overall; two to point the telescope and keep it fixed on its target, looking for "guide" stars in the HST field near the target. When each FGS finds a guide star, it locks on to it and feeds information back to the HST steering system to keep that guide star in its field. While two sensors are steering the telescope, one is free to make astrometricmeasurements (star positions). Astrometric measurements are important for detecting planets because orbiting planets cause the parent stars to wobble as they move across the sky.

Multiple repairs to these instruments, along with a few additions, are scheduled for the next servicing mission potentially in early 2009.

Now you know how Hubble takes all those pictures. We'll learn about Hubble's other life as a spacecraft next.