Yeah the RASA is a little ridiculous in how much photons it collects
This is an oversimplification and I'm no PhD but you can get a rough calculation of this comparison concept with:
f-ratioSlower^2 / f-ratioFaster^2
5.9*5.9 / 2.2*2.2 = 34.8 / 4.8 = 7.25
So in theory it would take 14.5 hours to equal the 2 on RASA. However it doesn't really work that simply and there other factors. The biggest one would probably be the aperture and image scale. Larger image scale=more light per pixel. Your 73mm has 430mm FL and the RASA has 620mm, so you'd be sampling at 1.8"/px and I am at 1.25"/px. Then there's stuff like scope obstruction, reflectivity, transparency, skyglow, etc. This would shorten it a bit so maybe I'd guess like 10 hours?
Thanks! Yeah it is very nice for adding some H-alpha to RGB images. I wouldn't use it for pure narrowband false color images, but you probably could get a decent enough result, just way less efficient.
Thank you for doing that calculation Jarrett.