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Hi Michelle
The specialised, astronomy related, solar filters designed for telescopes are generally called “full aperture solar filters”.
Basically they fit at the front end of the telescope and cut down the eye-damaging visible, infra red and ultraviolet spectrum before it enters the telescope objective, be it a (refractor) lens or a (reflector) mirror.
They are designed for refractors (i.e. lenses) as well as reflectors, such as Newtonians and SCTs, etc.
Some of the full aperture solar filters are made from specially coated optical quality glass, like those made by 1000 Oaks; others use the specialised astro solar film made by Baader.
Some full aperture solar filters do not use the entire surface area – they only have a smaller “hole” that is covered with the specialised solar filter, and these are still fitted to the front of the optical tube to prevent the harmful rays entering the system, be in lens or mirror.
Shops like Bintel, Sirius Optics, My Astro Shop and others sell these approved filters and they should be able to advise you on the size for your particular telescope.
You are very wise to ask these questions, as a poorly designed or ill fitting filter can be very dangerous. Never use a small solar filter placed only at the eyepiece end – the solar radiation will have already been magnified and dangerously concentrated the moment it enters the unfiltered front of the telescope and the viewer will risk permanent eye damage.
Cheers
Dennis
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