Harris RTX2000 is a 8Mhz machine with 16-bit data-path, paged program memory, and 1MB addressable memory. This is really an example of "small machine". Philae

Harris RTX2000 is a 8Mhz machine with 16-bit data-path, paged program memory, and 1MB addressable memory. This is really an example of "small machine". Philae had to choose this CPU because there are very few rad-hard low-power CPUs available (and it's not even that low power by modern standarts, 5 mA/MHz), but I am sure they'd choose something bigger if they could. As for radio telescope, I am not sure which ones are you talking about, but those environments are not particularly challenging compared to spaceflight, so those run whatever hardware designers like. I am sure some of them used to run tiny 16-bit CPUs, but I'd be surprised to hear new designs run something that old.