Focal Position for Reflecting Telescopes
Reflecting telescopes can be designed for many kinds of astronomical work through choice of the focal arrangement to suit the type of observation. For photography, photometry, and spectroscopy of faint objects the prime focus is best because its small focal length lessens the exposu re time required. The Newtonian focus, most useful for small telescopes, is now little used by professional astronomers. In both these arrangements the observer works at a considerable distance above the observatory floor since both focal positions are near the entrance of the telescope.
I n the Cassegrain focal arrangement a convex secondary mirror positioned at the top in front of the focus slows the rate at which light rays converge, effectively increasing the telescope's focal length. The secondary mirror reflects the converging rays to the bottom of the telescope and through a hole in the objective mirror to focus behind the objective. This is a much more convenient observing position since it is near the floor and behind the telescope. Of all the observations made with the 5.1-meter Hale telescope on Palomar Mountain 75 percent are from the Cassegrain focus.
We might think that putting the secondary mirror and its supports or the observer's cage for the prime focus into the path of the light rays would obscure part of the image; but the only effect is to cut down the amount of light reaching the objective; the loss is small, and the quality of the image is not affected.
Equipment that is too heavy and bulky to be attached to the back of the primary mirror or is sensitive to changing gravitation as the telescope moves can be placed in a room below the observatory floor. An auxiliary flat mirror diverts the long converging beam down the hollow polar axis around which the telescope rotates, and with this coude focal arrangement the focus can remain stationary no matter which way the telescope points.
Reflecting telescopes can be designed for many kinds of astronomical work through choice of the focal arrangement to suit the type of observation. For photography, photometry, and spectroscopy of faint objects the prime focus is best because its small focal length lessens the exposu re time required. The Newtonian focus, most useful for small telescopes, is now little used by professional astronomers. In both these arrangements the observer works at a considerable distance above the observatory floor since both focal positions are near the entrance of the telescope.
I n the Cassegrain focal arrangement a convex secondary mirror positioned at the top in front of the focus slows the rate at which light rays converge, effectively increasing the telescope's focal length. The secondary mirror reflects the converging rays to the bottom of the telescope and through a hole in the objective mirror to focus behind the objective. This is a much more convenient observing position since it is near the floor and behind the telescope. Of all the observations made with the 5.1-meter Hale telescope on Palomar Mountain 75 percent are from the Cassegrain focus.
We might think that putting the secondary mirror and its supports or the observer's cage for the prime focus into the path of the light rays would obscure part of the image; but the only effect is to cut down the amount of light reaching the objective; the loss is small, and the quality of the image is not affected.
Equipment that is too heavy and bulky to be attached to the back of the primary mirror or is sensitive to changing gravitation as the telescope moves can be placed in a room below the observatory floor. An auxiliary flat mirror diverts the long converging beam down the hollow polar axis around which the telescope rotates, and with this coude focal arrangement the focus can remain stationary no matter which way the telescope points.
