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I can assure you that the red filter plus UV-IR combo makes an excellent ERF.
The real problem is getting suitable optical filters. I find the photographic filters are not always up to the job. The Baader ERF for instance is almost 6mm thick and has a 1/10 wave surface accuracy....
An easy, quick test of an ERF is to focus the image of the sun on the back of your hand - if you feel any heat or discomfort then it's no good.
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