Astronomy is a fascinating area and there are not only good career options in it but there is ample scope for research as there are so many mysteries about the universe and heavenly bodies yet to be unravelled by the science of Astronomy. Before going into the query further, I would like to suggest here that if you have interest in Astronomy then inculcate it as a hobby also as star gazing and amateur astronomy is something which is very interesting and can be carried out with an ordinary telescope or even with naked eye in a clean environment outside the city limits avoiding the dazzling street lights. Making a career in Astronomy against a backdrop of amateur astronomer is an interesting proposition.
You have asked comparison between doing B.Sc. from IISc and that of B.Tech. from IITs. I suggest that if you have a desire to excel in the field of Astronomy then IISc would be a better option. Though there are many other options also and many colleges and universities are there from where also one can go up in this career line. Only thing is one has to do the commensurate hard work in this direction. One should not depend only on the top colleges as sometimes if admission is not got in them then one should not feel discouraged and can pursue one's dream from other colleges also. I strongly feel that top institution is not the only factors in one's growth and progress though it helps to some extent like campus placement, networking with foreign institutions for research etc.
To make a career in Astronomy the first thing would be to do B.Sc. with Physics and Mathematics as two essential subjects. These two subjects are the main knowledge pillars for learnings in Astronomy. After completing B.Sc. one has to apply for admission in some good college or university or scientific institute where there is a course available in M.Sc. (Astronomy). Some colleges are taking the students for admission to M.Sc. Astronomy course based on the marks obtained in B.Sc. and one should have at least 50% marks in that. Due to the tough competition one has to have very good marks in B.Sc. for this type of direct admission. Many universities/ institutions would have their own entrance test for these admission and one has to prepare for it thoroughly and if you want to see some model questions related to Astronomy then you can visit this page in ISC by clicking here
and can navigate to other similar pages from there to get an idea of questions which can be asked in entrance examination. These questions are only indicative and you have to go exactly by the syllabus of the entrance exam of the institute where you would be finally applying.
Some of the institutions where one can try to get admission for pursuing this course are as follows -
1. Indian Institute of Astrophysics, Bangalore.
2. Indian Space Research Organization, Bangalore.
3. Tata Institute Of Fundamental Research, Mumbai.
4. Raman Research Institute, Bangalore.
5. National Centre For Radio Astrophysics, Pune.
6. Physical Research Laboratory, Ahmedabad.
7. Indian Institute of Science, Bangalore.
8. Inter University Centre For AstronomyAnd Astrophysics
9. Madurai Kamaraj University, Madurai.
10. Andhra University, Visakhapatnam.
11. Jawahar lal Nehru Technological University, Hyderabad.
12. Mahatma Gandhi University, Kerala.
13. Osmania University, Hyderabad.