Hi Ross: > >A little, off-the-wall, query. The larger the distance from the pinhole to the >white sheet image of the sun, the larger the sun, but also the dimmer too.
This is correct. The sun's image becomes quite dim at the long distance needed to see sunspots. This can be corrected by lowering the amount of ambient light as much as possible. Providing extra shade around the whole setup helps. ie. use a large piece of cardboard for the pinhole, or put the pinhole in the bottom of a box or tube. >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 > >John Carmichael wrote: > >> A bunch of us, including astronomers from Kitt Peak, traveled to southern >> Baja for the July 11, 1991 total eclipse. > >Thanks, > >Ross McCluney > >
