[masraum]

Originally Posted by Pazuzu

Correct, the "filter" is in the diagonal. You MUST use a refractor with this, so there are no actual surfaces near the focal point. The Herschel wedge inside turns 4% of the sunlight up towards the eyepiece, and 96% gets dumped onto a ceramic puck that absorbs the energy. Then there is a built in ND3 (.001) neutral filter, then a UV blocking filter, then the eyepiece. Very comfortable brightness level.

The trick is that it's easy to make a diagonal super flat, so it won't distort the image at all. Making a 4, 6, 8 inch solar filter which is optically flat is incredibly expensive, so no one does. The glass and mylar filters induce distortion. They also change the color of the sun (green, blue, orange, depends on the brand), and can sometimes allow potentially dangerous amounts of UV light through.

Interesting. Good stuff. Thanks