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ND v's ERF
Suzy_A,
An ND filter isn't an ERF filter. All the ND will do, if you're lucky, will be to reduce some visible light getting through. The real problems are the UV and IR light that you can't "see" but your etalon ( and your eye) will feel the heat!
Normally you don't need an ND filter with the PST etalon; a Wratten #25 full aperture filter ( Surplus Shed sell them, 110mm diameter). This will filter out all the UV and visible light less than 600Nm (light red) add an IR filter to stop the heat above 700Nm and you have an effective ERF.
(As you probably already know so of the "lighter" ND filters are NOT suitable for visual work.)
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