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.
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.
For more detail please visit: http://www.ilmt.ulg.ac.be/home/