To view the partial solar eclipse of 10 Jun 2002 (which was an annular
eclipse elsewhere), I built a 26 meter (85 foot) long solar telescope.
It was a reflection pinhole design.
This was adequate not only to see the eclipse but also to see sunspots!
The scope consisted of a masked mirror in one corner of the yard and a screen inside our garage. The mask had a 6 mm hole to reflect a beam of sunlight toward the screen. The beam passed through a doorway to the screen at the far side of the garage. This produced a 24 cm diameter image of the Sun with a resolution equivalent to about 40 pixels across. We could see 3 sunspots including one that was an unresolved sunspot chain. The eclipse was perhaps even anticlimactic after seeing the sunspots.
The basic idea of a pinhole telescope is to let the Sun shine through a small hole, producing an image on a screen. The reflection pinhole telescope is the same, but instead of just a hole you use a small mirror or a mirror covered with a mask that has a small hole. This has the advantage that you can redirect the beam of light toward a conveniently located screen.
The size of the image is larger if the screen is farther away, but the image gets dimmer. The sharpness of the image increases (to a limit) with a smaller hole, but again the image gets dimmer. Also, for a really small hole, the sharpness is limited by diffraction; so there is no point to making the hole smaller than a certain size.
Thus for any distance to the screen, there is an optimum hole size that gives the sharpest image possible while not sacrificing more brightness than necessary. If you put the screen farther away, it turns out that you get the optimum image to be sharper. With a length of 26 meters, I was able to view sunspots.
Also, read a description of an almost identical setup that has quite a bit of theoretical detail.
Last updated: Mon Nov 6 14:42:08 PST 2006