Unique Liquid-Mirror Telescope to Observe Astronomical Objects Commissioned
A unique International Liquid-Mirror Telescope (ILMT) has recently been installed at the Devasthal Observatory campus of the Aryabhatta Research Institute of Observational Sciences (ARIES), an autonomous institute of Department of Science and Technology (DST), Government of India, in Nainital, Uttarakhand.The telescope has been designed to monitor the strip of the overhead sky to identify transient or variable objects such as supernovae, gravitational lenses, space debris, asteroids, etc. It employs a 4-meter-wide rotating mirror comprising a thin film of liquid mercury to collect and focus light. Liquid-mirror (LM) telescopes are based on the fact that the surface of a rotating liquid takes a natural parabolic shape ideal for focusing light. The earth’s rotation causes the images to drift across the camera, but the camera compensates for this motion electronically. This mode of operation increases the efficiency of observation and makes the telescope particularly sensitive to faint and diffuse objects.Although such telescopes have been constructedin the past, this is the foremost sizeable one to be mounted at a highaltitude of 2450 metres.
The ILMT is the result of a collaboration between India (ARIES), Belgium (the University of Liège and the Royal Observatory of Belgium), Poland (Poznan Observatory), Uzbekistan (Ulugh Beg Astronomical Institute of the Uzbek Academy of Sciences, and the National University of Uzbekistan), and Canada (University of British Columbia, Laval University, University of Montreal, University of Toronto, York University, and the University of Victoria). The telescope was designed and constructed by the Advanced Mechanical and Optical Systems (AMOS) Corporation and the Centre Spatial de Liège in Belgium.
The ILMT is made up of three components:
- A primary mirroris formed by a rotating container with a highly reflective liquid (mercury). The surface of the rotating liquid takes the shape of a paraboloid, which is ideal for focusing light under the constant pull of gravity and the centrifugal acceleration, which grows more robust with the distance from the central axis. A thin transparent film of mylar protects the mercury from the wind. The reflected light passes through a sophisticated multi-lens optical corrector that produces sharp images over a wide field of view. A large-format electronic CCD (charge-coupled device) camera (4096×4096 pixel) located at the focus records the images.
- An air compressor operates an air bearing onwhich the LM sits.To avoid any interruption of the mirror rotation, the installation of two parallel air systems that can deliver free air at pressure as high as 10-13 bar is preferred.
- A drive system.
The ILMT is a promising instrument that will be entirely dedicated to a photometric/astrometric direct imaging survey. Liquid mirror telescopes are zenithal pointing telescopes capable of viewing only a small field around the zenith (point along the local vertical direction).Unlike conventional telescopes, LM telescopes are not capable of titling and hence cannot track celestial objects. Thus, in ILMT, tracking is done electronically through auniquereal-time imaging CCD readout technique known as Time Delayed Integration (TDIor Scan Dual Mode)which uses a 4K × 4K thinned out, backside-illuminated CCD detector that tracks by electronically stepping its pixels.Taking an image with a CCD camera is usually done in two steps: First, the sensor pixel matrix is exposed to the light of the source, which is imaged. During the exposure, photoelectrons are generated and stored in the sensor's pixels. The number of photoelectrons generated in a pixel is proportional to the flux of light arriving in this pixel during the exposure. In the second process, known as the CCD readout, the number of photoelectrons in each pixel is counted. Classical read out of a CCD is done by successively drifting each column of the sensor pixel matrix to an electronic device, the register located just next to the sensor, which counts the number of photoelectrons in all the pixels of each column. In the TDI mode, the drift of the columns is slowed down. In the case of the ILMT, as a star goes through the field of view of the telescope, its image crosses the sensor. The drift of the columns is slowed down in such a way that photoelectrons generated by the star are drifted over the sensor at the same speed as the star's image travels through the focal plane.
Consequently, as soon as a star gets out of the field of view, the number of photoelectrons it has generated is counted. At each moment, the ILMT creates an image of the field of sky passing at the zenith.Integrating images with LMTelescope is possible due to theearth’s rotation.The telescope scans an area of constant declination equal to the observatory’s latitude. Because anLMTelescope observes the same region of the sky night after night, it is possible either to co-add the images taken on different nights to improve the limiting magnitude attainable with an LMTelescope or to subtract images taken on different nights to make a variability study of the corresponding strip of sky.
The Devasthal observatory is located (latitude +29° 22’ 26”)ideally in Uttarakhand, India,to access the northern galactic pole. From this site, a deep 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 (new quasars, supernovae, clusters, gravitational lenses, space debris, and asteroids).The information will be stored on disks so the night observations can be co-added with a computer, leading to long equivalent integration times. The image subtraction technique will also detect transient objects (supernovae, gamma-ray bursts, micro-lensing events). Its small (~2) F/D (focal length to aperture size) ratio makes it an ideal instrument for detecting faint surface brightness objects. It can also contribute to a census of faint nearby objects such as white dwarfs and brown dwarfs, basedon accurate measurements of their trigonometric parallaxes and proper motions.
The ILMT achieved first light in the 2nd week of May 2022 andwill be ideal for continuous photometric and astrometric variability monitoring for over five years.The ILMT will generate a wealth of data (approximately 10 GB every night) which will be analysed to reveal variable and transient stellar sources.The objects observed with the ILMT will be classified by implementingBig Data and Artificial Intelligence/Machine Learning (AI/ML) algorithms. After pre-processing the data,access to the ILMT data archive will be provided through a dedicated arXiv to the entire astronomical community.