Can I do M.Sc. in astronomy and astrophysics after completing M.tech. in Aerospace Engineering from IIT?
Yes you can do as initial requirement for MSc is just a graduation degree.
For MTech in Aerospace or Astrophysics you need to score at least 60% for general category and 55% for SC/ST category in Bachelor's degree in Mechanical, Civil, Electrical, Electronics, Instrumentation for admission to specific specializations of Aerospace Engineering