Hi, and a very warm welcome to the Lounge
Really great to see you jumping into astrophotography from the start, no mean feat, as it's as complicated as taking photographs can get. Regarding field flatteners, these are usually dedicated to telescopes and if you plump for the Evostar 72ED then a field flattener would be a consideration. One piece of advice I give to any budding astrophotographer is to purchase the book "Making Every photon Count." It is written by a member of SGL "Steppenwolf" and is considered "the bible" for budding AP enthusiasts. You can purchase it here from FLO
Steve