A Nifty New Telescope

The latest Earth based telescope, the ILMT, is pretty cool.

Nestled in the mountains of Northern India, is a 4-metre rotating dish of liquid mercury. Over a 10-year period, the International Liquid Mirror Telescope (ILMT) will survey 117 square degrees of sky, to study the astrometric and photometric variability of all detected objects. . . .  

Baldeep Grewal, et al., "Survey of Variables with the ILMT" arXiv:2311.05620 (November 8, 2023). 

There is another description of the new telescope at arXiv:2311.05615 which explains the benefits of using a liquid mirror:

A perfect reflective paraboloid represents the ideal reference surface for an optical device to focus a beam of parallel light rays to a single point. This is how astronomical mirrors form images of distant stars in their focal plane. In this context, it is amazing that the surface of a  liquid rotating around a vertical axis takes the shape of a paraboloid under the constant pull of gravity and centrifugal acceleration, the latter growing stronger at distances further from the  central axis. The parabolic surface occurs because a liquid always sets its surface perpendicular to the net acceleration it experiences, which in this case is increasingly tilted and enhanced with distance from the central axis. The focal length F is proportional to the gravity acceleration g and inversely proportional to the square of the angular velocity ω. In the case of the ILMT, the angular velocity ω is about 8 turns per minute, resulting in a focal length of about 8m. Given the action of the optical corrector, the effective focal length f of the D=4m telescope is about 9.44m, resulting in the widely open ratio f/D∼2.4. In the case of the ILMT,  a thin rotating layer of mercury naturally focuses the light from a distant star at its focal point located at ∼8m just above the mirror, with the natural constraint that such a telescope always observes at the zenith. 

Thanks to the rotation of the Earth, the telescope scans a strip of sky centred at a declination equal to the latitude of the observatory (+29◦21′41.4′′ for the ARIES Devasthal observatory). The angular width of the strip is about 22′, a size limited by that of the detector (4k×4k) used in the focal plane of the telescope. Since the ILMT observes the same region of the sky night after night, it is possible either to co-add the images taken on different nights in order to improve the limiting magnitude or to subtract images taken on different nights to make a variability study of the corresponding strip of sky. Consequently, the ILMT is very well-suited to perform variability studies of the strip of sky it observes. While the ILMT mirror is rotating, the linear speed at its rim is about 5.6km/hr, i.e., the speed of a walking person.

A liquid mirror naturally flows to the precise shape paraboloid shape needed because it is a liquid under these conditions. And, since its surface is always dynamically readjusting itself to this shape, rather than being fixed in place just once as a solid mirror would be, any slight imperfections in its surface that do deviate from its paraboloid shape don't stay in exactly the same place. Instead, distortions from slight imperfections in the shape of the liquid mirror average out over multiple observations of the same part of the sky, to an average shape at any one location that is much closer to perfect than a solid mirror cast ultra-precisely just once. Thus, a liquid mirror reduces one subtle source of potential systemic errors that can arise from slight imperfections in the mirror's shape at particular locations that recur every time a particular part of the sky is viewed when a solid mirror is used.

The inauguration of the 4m International Liquid Mirror Telescope (ILMT) took place in Devasthalm, Uttarakhand, India on March 21, 2023. The observatory is situated in the Kumaon Himalayas at an altitude of 2450 meters (8,038 feet). 

The coordinates are 29°21′42″N 79°41′06″E, which matter in astronomy, because the latitude impacts which part of the sky it can see.