Hoya 82mm red (25A) filter

[Peter Drew]

Would this filter be any good/safe as an ERF for Ha viewing with a modded PST assembly?

[Merlin66]

Peter,

I've tried a few Wratten photographic filters and added 2" UV-IR filters behind them to solve the $$$ problem of an ERF. In each case I found the surface accuracy of the photographic filters wasn't good enough to give good clean definition.( the larger sizes weren't cheap either!)

The basic answer is yes. a Good quality Wratten red combined with a UV-IR would function as a safe ERF.

Ken

[Peter Drew]

Thanks for your reply Ken, I will give it a try. I already have 3 ERF's with no obvious provinence and they give excellent results. I use them whenever the Sun is available with no harmful effects yet noticeable. My query stemmed from not being sure whether the filters needed to have other radiation modifying attributes other than the attenuation provided by the red glass.

[Merlin66]

Peter,

I'd strongly recommend adding a UV-IR filter to the optical train.

The 25A filter passes all the light and heat above 610-620nm including NIR and IR.

[Peter Drew]

Thanks again Ken, will do.