An observatory class national large optical-IR telescope (NLOT), is proposed to be built and located in the country. The telescope consists of a 10–12 m segmented primary. In order to cater to a diversity of observational programs, the telescope is designed with high throughput in both the optical and IRregions (0.3–5 $\mu$m). It should perform reasonably well up to 30 $\mu$m. The telescope and instruments should have remote operations capability, allowing for the queue as well as classical scheduling and high reliability and robustness. This article provides a brief description of the science cases that drive the telescope requirements, activities related to optics design and some thoughts on the instruments.

The future of astronomy in the coming decades will be shaped by the upcoming three extremely large optical telescopes, the Thirty Meter Telescope (TMT), the Giant Magellan Telescope (GMT) and the European Large Telescope (ELT). The USA astronomy and astrophysics 2020 decadal survey and the Canadian long-range plan for astronomy have recently recommended these large observatories as a top priority for ground-based astronomy for the upcoming decade. India is a 10% partner in one of these large observatories, the TMT, which is jointly funded by the Department of Science and Technology (DST) and Department of Atomic Energy (DAE). Here, we highlight India’s contributions to the development of the telescope and science instruments. The size of back-end science instruments scale with telescope aperture, hence, science instruments for TMT will be the biggest ever built for any telescope. Designing and building them requires broad collaboration within India, across TMT partnership and industries. India contributes >30% of the work share towards the development of wide field optical spectrometer (WFOS). India is part of the development of other first-light instruments, the infrared imaging spectrograph (IRIS) and multi-object diffraction-limited high-resolution infrared spectrograph (MODHIS). Infrared guide star catalog is an important contribution from India to these adaptive optics (AO)-assisted instruments. India leads the development of high-resolution optical spectrograph (HROS), a major workhorse among the first decade instruments of TMT. India is also part of the instrument development team of other first-decade instruments. Concerted efforts have been made to contribute to some of the TMT precursor instruments that will help us to maximize the scientific productivity when TMT is operational, especially in the area of exoplanet science and observations that require AO. India-TMT is part of the science team for the Keck high-resolution infrared spectrograph for exoplanet characterization (HISPEC), a precursor instrument to TMT-MODHIS. In addition, Indian Institute of Astrophysics (IIA) is participating in the science and development of Santa Cruz array of lenslets for exoplanet spectroscopy (SCALES) project for Keck, which is a direct imaging spectrograph for exoplanet studies and a precursor to the TMT planetary system imager.