Hey

Firstly, even the scientists at ISRO get a salary of around 1 lakh 10 thousand per month so it is impossible for a graduate from IIT (or any istitute) to get a package of more than 50 lakh at isro.

Secondly, to become an astrophysicist at isro you do not have to graduate from iit. You can do a BSc hons in Physics, then a masters and then finally you will have to do a PhD in Astrophysics. Then you will have to apply and if you are seen capable enough only then they will call you for an interview after which they will decide to whether you deserve a spot in or not(they do not go for campussing).