500mm reflex lens for astrophotography?

I'm a current EPL-1 user contemplating getting an EM-5. One of my interests is astro-photography, primarily moon shots. I don't have a telescope and I'm not yet ready for that kind of investment. What do you think about the EM-5 and a 500mm reflex and what lens would you recommend (Olympus, Rokinon, ...?)

Thanks!