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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, is a result of an international collaboration between many institutes from multiple countries. The telescope is designed to perform deep photometric and astrometric variability surveys to detect transients. It achieved the first light in April-May 2022 and, following a series of tests and corrections, was formally inaugurated on 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.

To ​operate ​the ​air ​bearing ​an ​air ​compressor ​is ​needed. ​However, ​in ​order ​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 speed ​of ​rotation. ​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), ​will be ​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 do. The tracking is done artificially by using a technique called Time Delay Integration (TDI), 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 will monitor a strip of the sky of approximately 22 arcminutes centered at the declination of +29o21'40". It will have access to ~40 square degrees of sky every night. The TDI technique provides for an effective exposure time of 102s which allows the ILMT to reach down to a limiting magnitude of 22mag in i' band in a single scan. The exposures from multiple nights can be coadded together to probe even fainter objects. In contrast, image subtraction techniques will also be applied in order to detect transient objects (supernovae, gamma-ray bursts, micro-lensing events, etc).

Top view of the ILMT showing the liquid mercury mirror covered by a thin mylar film.

The ILMT achieved first light in the 2nd week of May 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. To highlight the features of galaxies and other stellar objects, the green colour has been slightly enhanced in the image. NGC 4274 Galaxy can be seen in the top right corner.

Colour composite image obtained from first light observations of ILMT.

The ILMT was inaugurated on March 21st 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 treaks 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 ​in ​order ​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 ​approximately ​cover ​50 ​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 going to be used for a ​continuous zenith ​monitoring ​within ​a ​narrow ​27 ​arcminutes ​strip ​of ​sky ​at ​latitude ​of ​~+29° ​22’ ​26” ​for ​a ​duration ​of ​at ​least the ​next ​five ​years ​(or ​more). ​The ​key ​features ​of ​the instrument/detector ​are ​as ​below:

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 can be downloaded using this link.
  2. ​Authors are requested to mention 4m ILMT in 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 first commissioning phase (October - November 2022) 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.