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

The ​4m ​International ​Liquid Mirror ​Telescope(ILMT)

The ​technology ​of ​liquid ​mirror ​(LM) ​telescope ​is ​relatively ​simple. ​Three ​components ​are required:  ​(i) ​a ​dish ​containing ​a ​reflecting ​liquid ​metal  ​(essentially ​mercury), ​(ii) ​an ​air ​bearing ​on which ​the ​LM ​sits, ​and ​(iii) ​a ​drive ​system. ​10.2bar ​Air ​Compressors.

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.

                                             Fig: ​Compressor room ​with ​the ​two ​air compressors ​and ​air tanks

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  ​angstrom ​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 ​24 ​arcminute in ​diameter. ​All ​these ​elements ​are ​mechanically ​coupled ​by ​an ​external ​structure ​and ​a ​spider.

                                                                                Fig: Fish ​eye ​view ​of ​the ​ILMT

The ​rotation ​of ​the ​Earth ​induces ​the ​motion  ​of ​the ​sky ​across ​the ​detector ​surface. ​The ​CCD detector ​works ​in ​the ​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, ​allowing ​an ​integration ​as ​long ​as ​the ​target ​remains ​inside ​the ​detector ​area  ​(about

102 ​seconds). ​The ​CCD ​detector ​is ​cooled ​down ​to ​a ​temperature ​near ​-110C, ​in ​order ​to ​reduce as ​much  ​as ​possible ​the ​dark ​current.

ILMT​ ​observations:

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 ​90 ​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.).

The  ​first ​light ​of ​ILMT ​is ​expected ​before ​the ​monsoon ​of ​2018. ​After ​first ​light, ​the ​continuous zenith ​monitoring ​within ​a ​narrow ​24 ​arcminutes  ​strip ​of ​sky ​at ​latitude ​of ​~+29  ​22’ ​26” ​will ​be carried  ​out ​for ​a ​duration ​of ​at ​least ​next ​five ​years ​(or ​more).  ​The ​key ​features ​of ​the instrument/detector ​are ​as ​below:

Detector:              4k x4k ​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).

ILMT​ ​Data​ ​access:

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.

Collaborations: ​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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