The 4 m International Liquid Mirror Telescope
The technology of liquid mirror (LM) telescope is relatively simple. Three components are required:
- A dish containing a reflecting liquid metal (essentially mercury),
- An air bearing on which the LM sits, and
- 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.
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.
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.
For more detail please visit: http://www.ilmt.ulg.ac.be/home/