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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 ​technology ​of ​liquid ​mirror ​(LM) ​telescope ​is ​relatively ​simple. ​Three ​components ​are required:

  1. ​A ​dish ​containing ​a ​reflecting ​liquid ​metal ​(essentially ​mercury),
  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. ​The ​mirror ​being parabolic ​in ​shape ​requires ​an ​optical ​corrector ​to ​get ​a ​flat ​focal ​surface ​of ​about ​27 ​arcminute in ​diameter. ​All ​these ​elements ​are ​mechanically ​coupled ​by ​an ​external ​structure ​and ​a ​spider.

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.

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).

Access ​to ​ILMT ​data ​archive ​will ​be ​provided ​through ​a ​dedicated ​arXiv ​[link ​will ​be ​provided here] ​after ​pre-processing ​of ​the ​data. ​Data ​will ​be ​provided ​to ​all ​the ​astronomical ​community, through ​a ​policies ​framed ​by ​partner ​institutes ​after ​the ​first ​light.

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.

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