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I’ve also seen a few images using red filter too. Obviously the advantage is that the further light is towards the RED end of the spectrum the less it is affected by atmospheric turbulence/seeing. Even in good seeing this is an advantage, plus used in conjunction with a mono camera you can get faster frame rates and more signal. BUT- why not just use the IRpass filter like a 642 or 685 (my preference)? This will give even crisper results, and since the moon doesn’t rotate like the planets you are not restricted to a certain capture time - so just go longer to compensate for the slightly less signal you get from a narrower band pass.
But to answer your question specifically, I’d assume they are using a CCD Red (or similar) deep sky imaging filter as opposed to a Red planetary filter as they would be better quality I’d think, but not too sure as I’ve never done it myself.
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