Skip to main content

The ​4 m ​International ​Liquid Mirror ​Telescope

View​ ​of​ ​the ILMT​ ​compressor room,​ ​control​ room and​ ​the​ ​main​ ​ILMT buildings.

The ILMT, located in the Devasthal Observatory campus of ARIES, results from an international collaboration between institutes from multiple countries. The telescope is designed to perform deep photometric and astrometric variability surveys to detect transients. It achieved the first light on the 29th of April 2022 and, following a series of tests and corrections, was formally inaugurated on the 21st of March 2023. The telescope is now ready for science observations.

The ​technology ​behind ​liquid ​mirror ​(LM) ​telescopes ​is ​relatively ​simple. The primary mirror of the telescope is a rotating container with a highly reflective liquid (mercury) in it. The surface of the spinning liquid takes the shape of a paraboloid. ​Three ​components ​are required:

  1. ​A ​dish ​containing ​a ​reflecting ​liquid ​metal,
  2. ​An ​air ​bearing ​on which ​the ​LM ​sits, and
  3. ​A ​drive ​system.

An air compressor is needed to operate the air bearing. ​However, ​to ​avoid ​any interruption ​of ​the ​mirror ​rotation ​(cf. ​during ​the ​maintenance ​of ​the ​compressed ​air ​system), ​it is ​best ​to ​have ​two ​parallel ​air ​systems. ​Therefore, ​we ​have ​installed ​two ​Air compressors, ​both of ​which ​are ​of ‘High-quality ​rotary ​screw ​air ​compressor’ ​with ​full ​PLC ​controller ​and ​can deliver ​free ​air ​at ​pressure ​as ​high ​as ​10-13 ​bar, ​needed ​to ​operate ​our ​air ​bearing ​systems.

The ​mercury ​mirror ​of ​the ​ILMT ​has ​a ​4-meter ​diameter ​with ​an ​aperture ​of ​f/2 ​defined ​by ​the rotation speed. ​A ​4Kx4K ​CCD ​camera ​manufactured ​by ​'Spectral ​Instruments' ​and ​which ​can operate ​over ​the ​4000 ​to ​11000 ​Å ​spectral ​range ​(SDSS ​filters ​g', ​r', ​i' ​are ​available) is positioned ​at ​the ​prime ​focus ​of ​the ​ILMT ​at ​about ​8m ​above ​the ​mirror. As liquid mirror telescopes cannot be tilted, they cannot track as conventional telescopes. Tracking is done artificially using Time Delay Integration (TDI) mode, in which the CCD electronically transfers its charge from one column to the next. The mirror, being parabolic in shape, requires an optical corrector to get a flat focal surface of about 22 arcminutes in diameter. ​All ​these ​elements ​are ​mechanically ​coupled ​by ​an ​external ​structure ​and ​a ​spider.

The ILMT monitors a strip of the sky of approximately 22 arcminutes centered at the declination of +29o21'40". It accesses about 40 square degrees of sky every night. The TDI technique provides an effective exposure time of 102s, allowing the ILMT to reach a limiting magnitude of 22mag in the i' band in a single scan. The exposures from multiple nights can be combined to probe even fainter objects. In contrast, image subtraction techniques are also applied to detect transient objects (supernovae, gamma-ray bursts, micro-lensing events, etc).

The top view of the ILMT shows the liquid mercury mirror covered by a thin mylar film.

The ILMT achieved first light on the 29th of April 2022. Using the first light observations through the g, r and i Sloan filters, a colour composite image (shown below) of a small portion of the sky was prepared. The green colour has been slightly enhanced in the image to highlight the features of galaxies and other stellar objects. NGC 4274 Galaxy can be seen in the top right corner.

Colour composite image obtained from first light observations of the ILMT.

The ILMT was inaugurated on the 21st of March 2023 by the Honourable Minister of State for Science & Technology and Earth Sciences, Government of India, Dr. Jitendra Singh, in the gracious presence of the Honourable Governor of Uttarakhand, Lt. Gen (Retd.) Gurmeet Singh at Devasthal. Many Belgian and Indian dignitaries were also present on this momentous occasion. Glimpses from the inaugural function:

Honourable Minister of State for Science & Technology and Earth Sciences, Government of India, Dr. Jitendra Singh, speaking during the inaugural function.
Honourable Governor of Uttarakhand Lt. Gen (Retd.) Gurmeet Singh speaking during the inaugural function.
Unveiling of the inaugural plaque by Prof. Dipankar Banerjee, Director, ARIES, on behalf of the Honourable Governor of Uttarakhand.
A colour composite photograph of a small portion of the sky observed with the ILMT through the g, r and i Sloan filters. Seven positions of the asteroid 3548 Eurybates are seen in this picture. The blue streak is due to passing space debris (ILMT, October 2022)
A colour composite photograph of a small portion of the sky observed with the ILMT through the g, r and i Sloan filters. The colourful streaks are caused by Meridian 3 (37212), an old Russian satellite (ILMT, October 2022).
A colour composite photograph of a small portion of the sky observed with the ILMT through the g, r and i Sloan filters. A nice nebula is seen in this picture. It represents the ashes of a dead star (ILMT, October 2022).

Given ​the ​zenith ​observing ​mode ​of ​a ​liquid ​mirror ​telescope ​and ​to ​access ​the ​northern galactic ​pole, ​the ​Devasthal ​observatory ​is ​ideally ​located ​in ​latitude ​(near ​+29° ​22’ ​26”). ​From this ​site, ​a ​deep ​(i' ​= ​22 ​mag) ​survey ​will ​cover approximately ​40 ​square ​degrees ​at ​high ​galactic latitude, ​which ​is ​very ​useful ​for ​gravitational ​lensing ​studies ​as ​well ​as ​for ​the ​identification ​of various ​classes ​of ​interesting ​extragalactic ​objects ​(cf. ​new ​quasars, ​supernovae, ​clusters, ​etc.).

Having achieved ​first ​light, the ILMT is used for continuous zenith ​monitoring ​within ​a ​narrow ​22 ​arcminutes ​strip ​of ​sky ​at a ​latitude ​of ​~+29° ​22’ ​26”. The survey is expected to ​last at least ​five ​years ​(or ​more) since the first light. ​The ​key ​features ​of ​the instrument/detector ​are ​as ​follows:

Detector: 4k x 4k CCD camera
Manufacturer: Spectral ​Instruments
Operational ​wavelength: 4000 ​to ​11000 ​Å ​spectral ​range
Filters: SDSS ​filters ​g', ​r' ​, ​i' ​
Integration ​mode Time ​Delayed ​Integration ​(TDI) ​mode, ​i.e. ​it ​tracks ​the ​stars ​by electronically ​stepping ​the ​relevant ​charges ​at ​the ​same ​rate ​as ​the ​target drifts ​across ​the ​detector ​(integration ​time ​about ​102 ​seconds).
  1. ​The raw data and astrometric calibrated data (up to June 2023) can be downloaded using this link.
  2. ​Authors are requested to mention 4m ILMT in the title or abstract of the research publications.
  3. ​The following paper should be cited while describing the telescope.

    Surdej J., Hickson P., Borra H., et al. 2018, Bulletin de la Societe Royale des Sciences de Lie`ge, 87, 68

  4. Acknowledgment: The 4m International Liquid Mirror Telescope (ILMT) project results from a collaboration between Aryabhatta Research Institute of observational sciencES (ARIES), India, the Institute of Astrophysics and Geophysics (University of Liège, Belgium), the Universities of British Columbia, Laval, Montreal, Toronto, Victoria and York University. It is a zenith sky survey telescope run and managed by ARIES, an autonomous Institute under the Department of Science and Technology, Government of India.

    The ILMT team involved in the data obtained during the commissioning phase is composed of Bhavya Ailawadhi (1,2), Talat Akhunov (3,4), Ermanno Borra (5), Monalisa Dubey (1,6), Naveen Dukiya (1,6), Jiuyang Fu (7), Baldeep Grewal (7), Paul Hickson (7), Brajesh Kumar (1), Kuntal Misra (1), Vibhore Negi (1,2), Kumar Pranshu (1,8), Ethen Sun (7), Jean Surdej (9).

    1. Aryabhatta Research Institute of Observational sciencES, Nainital, India
    2. ​Deen Dayal Upadhyay Gorakhpur University, Gorakhpur, India
    3. ​National University of Uzbekistan, Tashkent, Uzbekistan
    4. ​Ulugh Beg Astronomical Institute, Tashkent, Uzbekistan
    5. ​Laval University, Quebec, Canada
    6. ​Mahatma Jyotiba Phule Rohilkhand University, Bareilly, India
    7. ​University of British Columbia, Vancouver, Canada
    8. ​University of Calcutta, Kolkata, India
    9. ​Liège University, Belgium

    The ILMT team is thankful to Hitesh Kumar, Himanshu Rawat and Khushal Singh for their assistance in the operations of the telescope.

  5. The next release of data will be in July 2024.

The ​4m ​International ​Liquid ​Mirror ​Telescope ​(ILMT) ​project ​results ​from ​a collaboration ​between ​Aryabhatta ​Research ​Institute ​of ​Observational ​Sciences ​(ARIES, ​India), the ​Institute ​of ​Astrophysics ​and ​Geophysics ​(Liege ​University), ​the ​Canadian ​Astronomical Institutes, ​University ​of ​Montreal, ​University ​of ​Toronto, ​York ​University, ​University ​of ​British Columbia ​and ​Victoria ​University.

For ​more ​detail ​please ​visit:

A digital brochure giving an overview of the ILMT is available here. A short film on the ILMT is given below.