Hello Piyush,
For the admission in the Indian Institue of Astrophysics, one must have a minimum required eligibility which is M.Sc.(Physics)/Integrated M.Sc. (Physics)/B.Tech/B.E./M.Tech/M.E. with minimum 60% marks from class 10.
To get the admission through NET one must secure JRF in NTA-UGC-CSIR-NET exam.
You can submit your application for the same through their online portal till 21st Oct 2019.

Hope this helps,
Good Luck!

Hii Piyush ,As per the rule to get an admission in Indian Institute of Astrophysics one must possess All India Rank in the range above 280 and the percentage of tenth and twelth should be above 60% rest various colleges have other formalities to admitt the students.Hope my answer had cleared your doubts.

And good luck for your exam