Re: The mirrors, the mirrors
You can switch instruments in an X-ray telescope, like in Chandra. There's a cost associated with doing that, however. The fewer moving parts you have in space, the better, because of reliability. It's also a lot cheaper to have a more simple mission. However, switching instruments can be beneficial. You can move the instrument module (like Chandra), or move the mirrors, like the future Athena mission is planning.
For a mission, it may also be cheaper or easier to build multiple telescopes, rather than one big telescope. The choice of number or size of mirrors also affects its performance at different energies, so there is optimisation for the type of science you want to do.