Presenter: Lavanya Saindane, Core Member of Antariksh.
Date: 19th Oct 2024, Saturday.
Introduction:
Episode 7 of the Internal Talk Series featured an enlightening presentation by Lavanya Saindane, one of our dedicated core members at Antariksh. The focus of this episode was the International Liquid Mirror Telescope (ILMT). Lavanya provided an in-depth look into the history, development, and significance of liquid mirror telescopes, tracing the evolution from early concepts to the establishment of the ILMT.
The International Liquid Mirror Telescope (ILMT):
Situated in the Himalayas, Uttarakhand, at an altitude of 2450 meters in Devasthal, the ILMT is India’s first and Asia’s largest operational liquid mirror telescope. It is one of its kind worldwide, a result of a joint effort by ARIES (India), the University of Liège (Belgium), and the University of British Columbia (Canada). Lavanya emphasized the collaborative international effort that made this groundbreaking project possible, showcasing the global interest and investment in cutting-edge astronomical research.
Advantages of Liquid Mirror Technology:
Lavanya put a strong emphasis on why liquid mirrors are a revolutionary choice for telescopes, delving into an in-depth analysis of the following points:
1. Cost-Effectiveness: Liquid mirror telescopes are significantly cheaper to construct compared to traditional glass mirror telescopes. The use of liquid mercury as the reflective surface eliminates the need for expensive and complex mirror grinding and polishing processes.
2. Self-Healing: One of the most remarkable properties of liquid mirrors is their ability to self-heal. The liquid surface naturally smooths out any imperfections, ensuring a consistently high-quality reflective surface without the need for constant maintenance and adjustments.
3. Easy to Maintain: Maintenance of liquid mirror telescopes is relatively straightforward. The liquid nature of the mirror surface allows for easy cleaning and adjustments, reducing downtime and operational costs. This makes liquid mirror telescopes not only efficient but also practical for long-term astronomical observations.
Principles of ILMT:
Before diving into the core principles of the ILMT, Lavanya engaged the audience with an intriguing question: "What happens if you rotate a liquid-filled vessel?" She then explained the principle behind it:
Formation of a Parabolic Shape: When a liquid is poured into a circular plate and allowed to rotate, the centrifugal force exerts pressure outward, directing the liquid outward. In contrast, the remaining liquid in the center points downward, forming a parabolic shape. If the liquid is reflective and shiny, it can replace mirrors or lenses.
Mathematical Aspect: This formula illustrates the height of the slope as a function of radial distance.
Why Mercury?
Lavanya then addressed the crucial question of why mercury is used as the liquid in ILMTs. She highlighted the following reasons:
1. High Reflectivity: Mercury has a high reflectivity, making it an excellent choice for creating a reflective surface in a telescope.
2. Low Vapor Pressure: Mercury’s low vapor pressure ensures that it remains in a liquid state under various conditions, minimizing evaporation.
3. Liquid at Room Temperature: Mercury is liquid at room temperature, which simplifies the construction and maintenance of the liquid mirror.
4. Low Cost and Inexpensive: Mercury is relatively inexpensive, reducing the overall cost of constructing and maintaining the telescope.
Construction of the ILMT:
Lavanya explained the construction of the ILMT, detailing the key components:
1. Rotating Liquid Disc:
- The ILMT features a rotating liquid disc of 4 meters in diameter, rotating at a linear speed of 5.6 km/hr.
- The disc is filled with 50 liters of liquid mercury, weighing around 700 kg.
2. CCD & Filters:
- A CCD (Charge-Coupled Device) is attached 8 meters above the disc, perpendicular to it, to capture images.
- An optical corrector is used to correctly align the converged light for accurate mapping.
3. Optical Corrector: The optical corrector ensures that the light collected by the rotating mercury mirror is correctly focused for imaging.
4. Filters: The ILMT uses several filters to observe specific bands of the light spectrum:
- g filter: 400-550 nanometers
- r filter: 550-750 nanometers
- i filter: 750-800 nanometers
These filters allow the ILMT to observe light rays in specific spectral bands, enhancing the quality and detail of the captured images.
Image Mapping:
Lavanya provided an easy-to-understand analogy to explain image mapping:
Strip Scanning:
- The ILMT scans a strip of the sky around 22 arcminutes wide.
- This allows the telescope to access 47 square degrees of the sky every night, providing extensive coverage and enabling detailed astronomical observations.
Lavanya provided an enlightening explanation of the ILMT's image mapping capabilities, using clever everyday analogies to make complex astronomical concepts more accessible.
Image Mapping Fundamentals:
Lavanya explained how the ILMT scans the night sky, using a relatable hand gesture demonstration. Just as we might measure distances with our fingers, the telescope measures angular distances in the sky. The visual aid showed different finger positions marking 1', 5', 10', 15', and 25' against a dark blue sky background, making arcminutes—a rather abstract astronomical measurement—instantly understandable to the audience.
Key Technical Specifications:
- The ILMTsky system scans a strip approximately 22 arcminutes wide.
- It covers an impressive 47 square degrees of sky each night.
- For perspective, she showed that the full moon appears as 31 arcminutes and 0.2 square degrees.
Image Processing Techniques:
The presenter compared the image subtraction process to a "spot the difference" game, where comparing images from different nights reveals new or changing objects. By combining multiple exposures, like stacking photos in low light, the telescope can detect fainter objects and reduce background noise.
ILMT Observations and Achievements:
Lavanya showcased some remarkable images captured by the ILMT, highlighting its capabilities:
1. Asteroid Tracking (October 2022):
- Demonstrated the telescope's ability to track moving objects.
- Showed seven consecutive nights of observations.
- Yellow circles marked the asteroid's position each night, showing its movement against the fixed star background.
2. NGC6834 Cluster (May 2022):
- Revealed a stunning star cluster.
- Showcased the telescope's ability to capture detailed images of deep sky objects.
- Demonstrated excellent resolution in distinguishing individual stars within the cluster.
3. Galaxy Observations:
- A spiral galaxy capture showing detailed structure and composition.
- An impressive image of two interacting galaxies, highlighting the telescope's capability to observe galactic interactions and dynamics.
These images demonstrated ILMT's ability to study both nearby and distant celestial objects. The systematic approach of ILMT's observations, combined with its sophisticated image processing capabilities, enables astronomers to make significant contributions to our understanding of various celestial phenomena, from near-Earth asteroids to distant galactic interactions.
Conclusion:
The episode concluded with a Q&A session where Lavanya addressed various questions from the audience, further elucidating the concepts and importance of liquid mirror telescopes. The talk was an enriching experience, providing valuable knowledge and sparking curiosity among the attendees about the fascinating field of liquid mirror astronomy.
