Astronomy

You could try Indian Institute of Astrophysics, Bengaluru. It is one of the oldest institutions in India (1786 AD) and right now, the premier institute for Astrophysics (in my opinion). They offer three courses:
M.Tech, Ph.D. Dual degree GATE written test)
M.Sc, Ph.D. Dual degree JEST written test)
PhD. Currently they operate out of their campus in Koramangala, Bengaluru, though the CREST campus - Hosakote, Kodaikanal Solar Observatory, Vainu Bappu Observatory, the Gauribidanur radio telescope and the Handle Observatory are under them. For more information visit Indian Institute of Astrophysics
2. You could also try IUCAA (Inter University Center for Astronomy and Astrophysics), Pune. Visit IUCAA (qualifying exam: JEST)
3. If you are interested in theoretical Sciences, You could try TIFR (Tata Institute of Fundamental Research) or JNCASR (Jawaharlal Nehru Centre for Advanced Scientific Research) or RRI (Raman Research Institute) (qualifying exam: JEST or GATE)
4. IISc (Indian Institute of Science, Bangalore) provides courses related to astronomy. The advantage is the beautiful campus of the best Institute of India. GATE or JEST or written test)
5. Not really Astronomy, but Indian Institute of Space Science and Technology (Home : IIST) runs courses in Engineering related to Aerospace and Avionics. In-so-far as job opportunities are concerned, Research and teaching would be primary. For example, the Handle telescope in Leh is controlled through CREST, Hosakote. So that's a more technical job. IIST, however is quite different from the rest. It is a company (ISRO) backed institute and the placement might be different. The institute is possibly the most beautiful one you will see.

Minimum Eligibility Criteria for UG Courses:
Only Indian citizens are eligible to take admission in UG courses offered at IIST. Candidates meet IIST admission eligibility criteria, if they have secured minimum 75% aggregate marks in their Class 12th Board exams or equivalent examination. Age Limit: General and OBC category candidates who are born on or after October 1, 1992 are eligible for admission whereas SC/ST/PwD category candidates meet IIST eligibility criteria if they are born on or after October 1, 1987. Candidates are offered admission in B.Tech programmes offered at IIST on the basis of their performance in JEE Main and JEE Advanced examinations.

Hello Girish,
It is very nice to see your desire and aim of going into space. However, every country allows only it's citizens to go up in space. Last Indian to go in space was from Indian Air Force. However, if we see the background of different people who gave gone to space, there is wide variety. Scientists, naval aviators, air force aviators normally are the ones. Hence practically speaking you need to be either a scientist or aviator from IAF to be able to go to space. However this all may change if you want you go as tourist. Shortly two private companies will take paying passengers aboard their spaceships. So, if you have money then perhaps you can go in space in one of these also. Best wishes.