A focal length of the Earth to the Sun would be an interesting telescope; long focal lengths aren't out of the question; NuSTAR's X-ray telescope has a focal length of about 33'. Has anyone considered a telescope with a focal length of perhaps the Earth to the Moon? https://en.wikipedia.org/wiki/NuSTAR At 12:27 PM 3/5/2015, rcs@xmission.com wrote:

the focal length of the mirror would be about 1/4 of an astronomical unit (distance from Earth to Sun).