On Mon, 17 May 1999, John Carmichael wrote: > Hi Ross: > > >Getting a small, very long focal length lens (or mirror) to replace the > >pinhole > >can solve the problem, at the risk of introducing some image aberrations, > >including chromatic, visible at the edges of the solar disk. This was of > >course used to great effect on Kitt Peak in the wonderful solar telescope > >there that I visited a few months ago. > > > >The question is one of whether you or your astronomer friends knows of an > >optical system which produces the effect of a long focal length, but with a > >much shorter distance from the optical system to the focal plane. > > Sounds impossible to me, but I'll ask my astronomer friends. > > Take care, > > John
Much easier than it sounds, John! It's called a "telescope", or "telephoto lens"! Seriously, a simple telephoto lens is a basic Galillean telescope. Largish, long focal length positive objective lens, and a small, short focal length *negative* eyepiece lens. The long focal length is mostly consumed between the objective and the eyepiece. The rays coming out of the eye lens can be focussed on a card at any desired distance, giving a large, well focussed image. It will of course be dimmer than the direct sunlight, so you need to shade it like any other method. A small spotting 'scope, half a binocular, the finder scope from a larger instrument, or that spare telephoto lens all will work. Dave
