These reflecting lenses are small because they fold the light path, but also because they are slow.


Another option is a slow refracting (not reflecting) lens. One can buy an 800mm f/10 telescope (not reflecting) for $400. Not much heavier than a canon 400mm f/5.6. Would require a strong tripod for long exposures, though. (I don't know how it would compare to the reflector, not as portable, better bokeh no doubt, but I dunno about false color with this inexpensive glass... probably not too bad at f/10)