Quote:
Originally Posted by Max Vondel
with a basic 25mm kellner(remember that name!) eyepiece.
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Yep! The key here is that the eyepiece should not have any type of 'glue' to bond any of the various elements to another. The heat of the collected solar light will damage this glue, killing the eyepiece.
Kellner, Huygens & Ramsden are cheap eyepieces, ususally even without coatings, or only basic coatings and few elements forming them, from two to three, and all air spaced. These designs are probably best for solar projection.
I have a couple of nasty Huygens eyepieces just for this purpose.
I'd be careful about how large an apeture you use too. Even the Baader film instructions advocate making a mask to cover the scope with only a 50mm to 60mm opening as the maximum apeture, regardless of the size of the scope. With projection, I wouldn't exceed the apeture by more than 5". Even simple eyepieces can be killed by the amount of heat going through them. Even 4" is plenty.
You can make a simple mask from a piece of stout 'corrugated' cardboard from a box pinched from a fruit and vegie shop. This will give you the protection of three layers of good thick card to block the sun and heat.