Here is a link on how to make a
solar filter for a large diameter scope using the Baader Solar Observing film. These are the "sub-diameter" filter method.
The main thing to note is that the effective aperture for a solar scope shouldn't exceed 60mm! Otherwise, even with the film, you may still allow too much heat to build up within the scope, and too much light.
Ever noticed how dedicated solar scopes are not all that large in diameter?
Even the 'projection technique' can be done safely with a sub-diameter mask. But with the projection technique, use cheap, really, really cheap eyepieces, such a Huygens, or Ramsdens that come with cheap department store scopes. These a basic eyepieces that have few elements in them, and the elements are not guled to one another nor are they coated. Unfiltered sun light will kill good eyepieces. Projection technique is where you project the image of the sun from the eyepiece onto a screen, such as a sheet of paper.
I know I'd be preaching to the converted, but when you do any solar work, try to kurb the habit of using the finder scope. If solar work isn't big on your list, figure out another way of targeting the sun that doesn't involve using the finder. I came within a whisker of zapping my eyes the first time I set up a scope for solar work due to the finder. Keep this in mind.
