I have proved that UV/IR cut filter-82mm and R2 filter-82mm work as an excellent ERF equivalent for the APM120-BINOCULAR. The effective aperture is 77mm at the set up.
Nextly, I wished to make the ERF-Equivalent for my 20cm-BINO, too. So, I searched for the largest filters available and found 105mm of the UV/IR CUT filter in the market.
But the problem is, neither of R2 nor R64 filter larger than 82mm is in the market. After the long period of search for the R2-Equivalent, I hit on 105mm of #29 dark red filter for the monochromatic photography in the Net market, and will try it with the UV/IR CUT filter. The effective aperture is 100mm.