Prime Focus Corrector

The Prime Focus Corrector, or PFC, is the instrument currently used on the 0.8-meter telescope. The PFC takes images of celestial objects. As a prime focus instrument, it is located at the open end of the telescope tube. Light enters the telescope from an area on the sky and is reflected from the 0.8-meter mirror into a focal plane. The instrument is placed at the front of the telescope, in the focal plane. Once inside the PFC, light passes through a filter and onto a detector called a charged coupled device (CCD).

The PFC has five filters, and each one is designed to allow light of only one color to pass through. The filters are standard UBVRI filters, whose colors range from 360 nanometers, or violet, to 1,000 nm, or infrared. Before starting an observation, the astronomer must choose which of the filters to use, depending on what type of objects or properties he or she intends to study.

The CCD detector on this instrument can capture images in a region of the sky that is three-quarters of a degree across, or a bit larger than the size of the full Moon.

Although it is a relatively small telescope, the 0.8-meter telescope and the PFC make a very useful combination for many types of astronomical projects. Astronomers have used the PFC to make surveys of the sky, search for extrasolar planets, study comets, and search for Near Earth Asteroids.

The PFC also allows astronomers the opportunity to construct color images of celestial objects. All individual images that astronomers take with CCDs are only measures of intensity and are therefore black and white. However, with five filters of different wavelength ranges on the PFC, the intensity can be measured for every visible color. These images can be combined to create one color image of the object. Nearly all color images of astronomical objects are made in this way, including those from the Hubble Space Telescope.


The Prime Focus Corrector mounted on the 0.8-meter telescope.