Frankly, it is becoming hard-to-impossible to get into astronomy even with a traditional program of education starting with a formal BA in physics or astronomy, and a PhD in astronomy. I would council against entering an astronomy program from anything other than a traditional program in physics and astronomy. There are simply too many undergraduates and graduates in the pipeline already, taking the standard sequence of undergraduate and graduate courses, and many of these will not be able to secure stable jobs after 5 years in graduate school and a PhD. Sorry to be so glum, but times are very hard in astronomy as a career these days.