Astronomy
My 10 inch Mirror with a 1/48 P-V Wavefront Error @ 550 nm
Now, in the year 2001, 8" mirrors are the norm for home made telescope mirrors. The vast majority of these, both home made, and commercial, are ground to a Radius of curvature of 160" giving them a Focal Length of 80". The resulting Focal Ratio (FR) of 80 / 8 or F:10 is a standard for 8" mirrors. The Clear Aperture of an 8" mirror can never be 8". Using a small Secondary Mirror of 1.333" leaves a Clear Aperture of 6.667" making it neccessary to reduce the focal length to 66.67" to keep the F:10 Focal Ratio. If the focal length is not shortened for the Clear Aperture, then the resulting telescope will have a Focal Ratio of 80 / 6.667 = 11.999 or F:12.
Being a photographer I wanted a brighter FR than F:10. I knew I would have to shorten the Focal Length of the mirror, because the mirror's Clear Aperture was now fixed at 8". I also knew that by reducing the focal length to 50", I could get an FR = F:6.25, which like my camera's f:6.3, is 1.5 f stops brighter than F:10. In Photography, a difference in 1.5 f stops represents 3X difference in the intensity of light. Thus a 50" focal length would yeild almost 3X the intensity of light of the 80" standard design. Before I settled on a 50" Focal Length, however, I decided to investigate the optical properties of the eyepiece and its interaction with my mirror.