Hello aspirant,

IISER only takes BS-MS for astrophysics on the basis of JEE advanced, KVPY,SCB only.it takes admission for PhD through GATE/JEST examination.

If you want to pursue Ms astrophysics you can checkout indian institute of astrophysics.

IISER only gives direct admission to those students who have done BS-MS from their institute.

I hope it helps,

Best of luck.