ZWO Duo Band hack?

[Ags]

I just noticed this very cheap filter: https://www.firstlightoptics.com/zwo-filters/zwo-125-2-duo-band-dual-narrowband-filter.html

Unfortunately I have a mono camera so it is less useful. But I do have dichroic RGB filters so why not stack this filter with R to get pure Ha, and stack with G or B for Oiii?

Also the docs make a great deal about the incident angle of light being 8 degrees or less. So what focal ratios does the filter work with?

[JamesF]

It's a much wider bandpass than a normal Ha (or OIII) filter.

As regards the focal ratio, it's a bit late in the day for this, but I am vaguely thinking along these lines:

If focal ratio = focal length / aperture diameter, then 2 x focal ratio = focal length / aperture radius = 1 / tan ( incident angle )

So focal ratio = 1 / ( 2 x tan ( incident angle )) = 1 / 2 x tan(8) = 3.56.

I may be completely wrong though.

James