Illumination Size

Illumination Size: The size of the area at the focal plane (the virtual image) illuminated by the primary optics.

General:

The focal plane is generally fully illuminated in the center, and gradually tapers off in brightness toward the edge. A common way of measuring the illuminated area is by defining the zone of full illumination (the 100% zone), and the area where the brightness has tapered off to 75%.

The 100% zone is the area at the focal plane which is fully illuminated by the primary mirror. This area will have 100% of the brightness available from the primary mirror. This is the area produced by the light cone from the primary, reflected from the diagonal, as long as there is no vignetting. Changing the diagonal minor axis is the easiest way to change the size of this zone.

The 75% zone is the area at the focal plane which is 3/4 illuminated by the primary mirror. This area will be dimmer than the 100% area, tapering off in brightness from the edge of the 100% zone until only 75% of the brightness from the primary mirror is available at the edge of the 75% zone.

Visual Use:

An eyepiece will usually have approximately the same field lens diameter as its focal length. So to fully illuminate the field of a 12 mm eyepiece, a 12 mm (1/2 inch) area of 100% illumination is required. Full illumination is not absolutely required, and in fact usually drops off to around 75% near the edges of the eyepiece field.

The larger the eyepiece field lens, the larger areas of 100% and 75% illumination required. This is also impacted by the diagonal mirror minor axis and any possible vignetting by other elements of the telescope, such as the focuser inside diameter.

Some practical limit must be reached, however, because increasing the diagonal size will also decrease contrast and light gathering ability. One possible rule of thumb is to limit the size of the 100% zone to one half of the field lens size of the largest eyepiece you expect to use.

Contrast is very important in a telescope. To see fine details in planetary images and faint nebulae alike, you need the maximum contrast possible. In a newtonian telescope, one of the biggest contrast killers is an oversized diagonal mirror. If possible, the diagonal minor axis should be kept under 20% of the diameter of the primary mirror. This is easy with high focal ratio telescopes but can be very difficult with shorter focal ratios. See the Improper Design section.

Photographic Use:

Generally, to attain the brightest image (and utilize the full potential of the telescope's light gathering ability), the film in the camera should be as fully illuminated as possible. This requires a substantially larger diagonal mirror than does visual work.

In a 35mm camera, the short dimension of the film is 24mm (about 1 inch). The camera body requires the focal plane to be moved farther out from the focuser as well. Adding 2 inches of focal plane height for the camera body, and requiring a 1 inch area of 100% illumination will call for a fairly large diagonal mirror.

The other components of the telescope must be redesigned to accommodate photographic work. The focuser inside diameter must be larger to prevent vignetting of the light cone, and the diagonal mirror spider mount must be strong enough to prevent the heavier mirror from vibrating or sagging.

A telescope which is optimized for photographic use does not usually perform well for visual work.