greetings,

to study astro physics you need to do phd because in india this is not taught in detail at UG level.

for that you have to appear for the JEST exam which is conducted every year for admission in phd courses.

for that you have to do bsc+msc in physics.

some institutes also accept Btech graduates.

IIA i.e indian institute of astro physics is the best institute in india.

ARIES, BOSE INSTITUTE, IGCAR, IIA, IISER, IISERM, IUCAA, JNCASR are some institutes.

hope this helps.

good luck.