I wonder if anyone has tried to build a reflecting scope similar to the one featured in the March 2004 S&T. The featured one is a 6 inch reflector that folds the light back toward the primary thus allowing the observer to place the eyepiece near the primary rather than at the top of the tube. It seems to me that this configuration would allow for considerably longer focal lengths; but would the law of diminishing returns come into play (light dims with the square of the distance from the source) therefore 2x the distance in the optic path would diminish the light by a factor of four? As you can tell I am searching for a reason to avoid making this fantasy a reality. My dream calls for a 8inch spherical mirror with a focal length of 17 feet. Folding the light would place the secondary at about eight feet. In addition with a focal length that long there would be no need to aspherize the primary. Any comments? Be kind please.
Off Axis fantasy
Started by Philip2, 05/14/2004 06:54PM
Posted 05/14/2004 06:54PM
Opening Post
Posted 05/14/2004 08:08PM
#1
The light loss you thinking of in this case is due to magnification - 2x the focal length means 2x the mag, 4x the area, so 1/4 as bright (for extended objects). For an 8" f25.5, you won't be able to get much more than a 1/2 degree field with a 2" eyepiece, and not much below 100x (or higher, depending on whether you do it with a 55mm plossl or a 41 pan or a 31 nagler....). This won't be a wide field instrument.....
Jarad
Jarad
Funding Member
