Telescope
From the humble beginnings of Galileo’s Galileoscope to the awe-inspiring modern telescopes, our quest to understand the universe has been intertwined with the evolution of these remarkable instruments. Telescopes have played a pivotal role in expanding our view of the cosmos, revealing its mysteries, and pushing the boundaries of human knowledge. In this blog post, we embark on a journey through time and space to explore how telescopes have helped us unlock the secrets of the universe.
Bigger is Better: The Power of Aperture
One of the fundamental principles governing telescopes is the concept of aperture, the telescope's diameter.
“And if thou gaze long into an abyss, the abyss will also gaze into thee.” — Friedrich Nietzsche
In the case of telescopes, the larger the aperture, the more light they can collect and the sharper the images they can deliver. Through this mechanism, telescopes grant us more profound insights into the cosmos.
Over the centuries, telescopes have grown in size, leading to groundbreaking discoveries. As we delve into the history of optical telescopes, we encounter the Palomar 200-inch telescope, which stood as the world’s largest for several decades. However, a renaissance in telescope building in the 1980s and 1990s ushered in a new era of colossal instruments. The advent of 8- and 10-meter telescopes expanded our vision of the universe, allowing us to gaze further and delve deeper into its intricacies.
Beyond the Visible
Telescopes not only enable us to observe the universe in visible light but also extend our reach into the invisible realms of ultraviolet, infrared, and radio wavelengths. Telescopes equipped with specialized detectors and instruments help us decipher the cosmic symphony played by these invisible wavelengths.
In this regard, space-based telescopes occupy a special place. While the Hubble Space Telescope might not rank among the largest ground-based telescopes, it has revolutionized our understanding of the universe. However, the astronomical community eagerly awaited the arrival of the James Webb Space Telescope, a successor to Hubble, which promises to bring space astronomy to new heights. With its six-and-a-half-meter mirror stationed a million miles from Earth, the James Webb Space Telescope aims to rival the capabilities of its ground-based counterparts.
The Cost of Exploration: Pushing the Limits
Astronomy does not come cheap these days.
The pursuit of knowledge and the quest for ever-larger telescopes necessitate substantial financial investments. Throughout history, the cost of major ground-based facilities has remained remarkably consistent. Even the upcoming 20 and 30-meter telescopes are estimated to reach the billion-dollar mark. However, the price of venturing into space is even steeper. The Hubble Space Telescope, with its awe-inspiring achievements, has come with a price tag ranging between six and eight billion dollars.
The Exponential Progression: Logarithmic Gains
Telescope development has witnessed an exponential progression since Galileo’s time. As we examine the history of telescopes, we observe a logarithmic growth in size and capabilities over the past 400 years. However, this remarkable curve is accompanied by a harsh cost curve. The larger telescopes become, the more expensive they are to construct and maintain.
To overcome these financial challenges, astronomers and engineers have resorted to innovative approaches. Understanding the limitations imposed by Earth’s atmosphere, they have devised ingenious methods to enhance the performance of ground-based telescopes. By deploying adaptive optics and active control systems, astronomers mitigate the blurring effects caused by atmospheric conditions, further refining the resolution and quality of their observations.
Reflecting on Design: The Triumph of Reflectors
In the world of telescopes, there are two classic designs: refractors and reflectors. While refracting telescopes, like Galileo’s original device, employ lenses to form images, they suffer from chromatic aberration and size limitations. Refractors are ill-suited for frontier research in astronomy due to their inability to focus different wavelengths of light at the same point.
Reflecting telescopes, on the other hand, have emerged as the design of choice for major ground-based observatories. The concept of reflecting telescopes traces back to Isaac Newton, and its basic design has remained largely unchanged. Light enters the telescope, reflects off a primary mirror, and is redirected by a secondary mirror to a focal point below the telescope. This folded design offers several advantages, including compactness and improved stability. Major observatories worldwide rely on this design to unlock the secrets of the cosmos.
The Keck Telescopes: Unveiling the Giants
Among the giants of the telescope world, the Keck telescopes stand tall as exemplars of human ambition and dedication to scientific exploration. Funded by the visionary founder of Superior Oil, these twin 10-meter telescopes reside atop the dormant volcano Mauna Kea in Hawaii. Comprising a mosaic of 36 hexagonal mirror segments, each 1.8 meters across, these colossal instruments delve into the depths of the universe, revealing its wonders with unparalleled precision.
The Power of Aperture: Collecting Area and Angular Resolution
A telescope’s power and quality hinge on its aperture or diameter. The collecting area, which scales with the square of the aperture, determines the amount of light a telescope can gather. Simultaneously, the resolving power, or angular resolution, defines the smallest detail an instrument can discern. Angular resolution plays a crucial role in observing fine details on planetary surfaces and exploring crowded regions like galaxies and star clusters.
When it comes to angular resolution, the larger the telescope, the finer the details it can unveil. Additionally, shorter wavelengths enhance angular resolution, providing more intricate and detailed observations. However, ground-based telescopes face limitations imposed by Earth’s atmosphere, which induces blurring effects. Overcoming these constraints has been a major focus for astronomers striving to refine their observations.
Observing Limitations
As we continue our exploration of telescopes and their impact on our understanding of the universe, we must confront the limitations imposed by our terrestrial environment. These limitations, rooted in light pollution, atmospheric blurring, and atmospheric opacity, present significant challenges to ground-based astronomy.
The Dark Skies Fading: Light Pollution
In today’s world, most people reside in urban or suburban areas, where the night sky is obscured by a phenomenon known as light pollution. The glow of artificial lights impedes our ability to observe the distant reaches of the universe, limiting our view of the cosmos. As we examine a nighttime image of the United States, we observe a stark difference between regions. Urban and suburban areas in the eastern part of the country are bathed in continuous artificial light, rendering astronomical observations challenging.
While some areas in the western United States still provide suitable conditions for astronomy, light pollution continues to encroach upon pristine skies. The consequences of light pollution extend beyond the scientific community, affecting everyone, including young people who may have never experienced the awe-inspiring sight of the Milky Way. Nonetheless, there are a few sanctuaries for astronomical research, such as Mauna Kea in Hawaii and various sites in Northern Chile, where dark skies, clear air, and excellent infrastructure converge to support the advancement of astronomy.
The Blurring Effect: Atmospheric Turbulence
Another significant limitation imposed by Earth’s atmosphere is the blurring, or twinkling, of starlight caused by turbulent motions in the upper atmosphere. This blurring affects the depth and sharpness of images obtained through ground-based telescopes. Astronomers refer to this phenomenon as “seeing.” A comparison between images captured by the Hubble Space Telescope and a ground-based telescope of the same aperture reveals the dramatic impact of atmospheric blurring. The Hubble’s clear and crisp images underscore the need to mitigate the effects of blurring on ground-based observations.
The Veil of Opacity: Atmospheric Extinction
The Earth’s atmosphere acts as a barrier to most wavelengths across the electromagnetic spectrum. Schematic representations of the electromagnetic spectrum reveal that only a limited range, including the visible spectrum, parts of the radio spectrum, and segments of near-infrared radiation, can penetrate the atmosphere. Ultraviolet, X-ray, gamma-ray, and millimeter waves, crucial for studying celestial objects, are essentially blocked by Earth’s atmosphere.
Escaping Earth’s Grasp: The Need for Space
To overcome the limitations imposed by Earth’s atmosphere, astronomers must venture into space. While ground-based telescopes remain vital, exploring the invisible and uncharted realms of the universe requires space-based observatories. These observatories, free from atmospheric interference, can access wavelengths that are otherwise obscured by our planet’s protective blanket.
The High Cost of Expansion: A Costly Pursuit
As astronomers strive for bigger and more powerful telescopes, the associated costs soar. The larger the aperture, the more significant the structural requirements and expenses become. A rule of thumb suggests that the cost of a major telescope increases as the two-and-a-halfth power of its diameter. This steep cost curve poses challenges as we envision even larger facilities in the future. Innovative approaches and clever tricks will be necessary to overcome the financial barriers and ensure the continued growth and progress of astronomical research.
Confronting the Limitations: A Balancing Act
Despite these limitations, astronomers persist in their pursuit of knowledge, finding ways to navigate the challenges and maximize the potential of ground-based telescopes. Seeking out remote, high-altitude locations with pristine skies, such as the Andes and Hawaii’s extinct volcanoes, provides opportunities to escape light pollution and atmospheric turbulence. These regions have become focal points for the construction of major telescopes, pushing the boundaries of what we can observe and understand.
“The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars.” — Carl Sagan.
Overcoming Telescope Limitations
It becomes evident that substantial progress has been made in overcoming the limitations that once impeded our exploration of the universe. One of the most significant historical limitations was the size of the telescope mirror. However, through innovation and ingenuity, astronomers have triumphed over this obstacle and pushed the boundaries of what was once thought impossible.
A Giant Leap in Mirror Size
In 1948, the Palomar 200-inch telescope, boasting the largest mirror ever built at the time, was commissioned in Southern California. This monumental achievement appeared to mark the pinnacle of telescope mirror size. However, the University of Arizona shattered expectations about a decade ago by constructing an even larger mirror. Surprisingly, these newer and larger telescopes are housed in smaller buildings, thanks to compact telescope designs.
Progress in mirror size has been substantial, with mirrors surpassing the five-meter or 200-inch mark. One noteworthy example is the mirror for the Large Synoptic Survey Telescope (LSST), which was cast under the stands of our football stadium at the University of Arizona. Not only is this mirror large, but it is also phenomenally accurate. To visualize its precision, imagine expanding this mirror, larger than most people’s living rooms, to the size of the continental United States. The imperfections on this mirror would be bumps less than one inch high, making it the most accurate mirror surface ever created.
Telescopes with such large mirrors require physically substantial detectors. While the underlying technology remains similar to the charged coupled devices (CCDs) found in cameras and cellphones, these detectors are much larger. The CCD in a camera or cellphone may be the size of a little fingernail, but the detector in the LSST is immense. As a result, a massive amount of data is generated. With a data rate of approximately 20 terabytes per night, astronomers will face the challenge of managing this fire hose of information.
A Journey of Mirror Expansion
The journey toward larger mirrors began when the multiple-mirror telescope replaced six smaller mirrors with a single six-and-a-half-meter mirror. Located on Mount Hopkins in southern Arizona, this telescope represented the first step toward constructing larger mirrors. Encouraged by this success, astronomers proceeded to produce one mirror approximately every 18 months.
Roger Angel, a pioneering figure in this field, played a crucial role in developing innovative methods to construct large and lightweight mirrors. Professor Angel, based at the University of Arizona, paved the way for significant advancements. The Twin Magellan Telescopes in Chile, situated in one of the darkest locations on Earth, were equipped with two 6.5-meter mirrors. These mirrors are truly awe-inspiring, both in terms of their enormous size and their exquisite precision.
The next leap forward was achieved with 8.4-meter mirrors — the largest size possible with the current technique. These mirrors were installed in the Large Binocular Telescope on Mount Graham in southern Arizona, creating an effective aperture of over 11 meters. This monumental achievement solidified its position as the largest telescope on Earth. Witnessing the twin mirrors inside the telescope dome is a humbling experience, as these colossal pieces of hardware combine both immense size and extraordinary precision.
The ongoing dream and aspiration of the astronomical community is the Giant Magellan Telescope (GMT), slated to be located in Chile on an adjacent mountaintop to the existing twin 6.5-meter mirrors. The GMT will utilize seven of the 8.4-meter mirrors, arranged like flower petals around a central mirror, resulting in the equivalent of a 22- or 23-meter telescope — the largest in the world. The project aims to reach completion within six years, with the first light expected in approximately seven years.
Unlocking the Secrets: The Art of Mirror Creation
The realization of such large mirrors seemed implausible for decades until breakthroughs pioneered by Roger Angel and his team at the Mirror Lab in Stewart Observatory. The secret lies in the construction of honeycomb mirrors — mirrors composed of a honeycomb structure rather than solid blocks of glass. Borosilicate glass, an uncommon sand, is used for its creation.
The honeycomb structure serves two vital purposes. Firstly, it significantly reduces the weight of the mirror compared to a solid glass block of the same thickness. Secondly, a lightweight mirror allows for better temperature regulation, minimizing temperature differentials between the mirror and its surroundings. Temperature differentials can degrade image quality. The honeycomb mirrors maintain close temperature equilibrium with the surrounding structure, with variances kept within a tenth of a degree Celsius.
The process of creating these extraordinary mirrors begins with the placement of ceramic fiber boxes, shaped like hexagons, in a large tub. Each box is unique and collectively forms the desired parabolic contour. Next, 18 tons of borosilicate glass blocks, meticulously selected for their purity, are loaded on top of the preformed hexagons. The lid of the oven is then closed and sealed, initiating a controlled temperature cycle. Reaching temperatures of up to 1160 degrees Celsius, the glass melts and fills the spaces between the hexagons, leaving a one-inch faceplate with the perfect parabolic shape. Subsequent slow cooling, known as annealing, takes three months to prevent the formation of bubbles and minimize the risk of fracturing.
Following the annealing process, the mirror moves to a stressed-lap polishing machine. Over a period of several months, the machine grinds away the surface down to millionth-of-a-meter levels, ensuring a perfectly optical shape. When the mirror is finally completed, its level of precision is extraordinary. If we were to imagine the mirror leaning back, we could read a newspaper from a distance of five miles.
Challenges of Transportation and Maintenance
Once the mirrors are crafted to perfection, the challenge of transporting them to remote mountaintops awaits. Given the massiveness of these mirrors, even the most advanced military lift helicopters cannot transport them to high mountaintops with thin air. Instead, they must be carefully transported via road, often requiring the construction of customized routes to navigate hairpin turns. The vehicles used for transportation employ cantilever weights that swing in and out to prevent the trailer from toppling off the mountainside. This daring process takes around a week to ten days to transport a single mirror from Tucson to the nearby mountaintops.
Once the mirror reaches its designated telescope cell, it remains fixed in position. Unlike older telescopes, which required the periodic removal of mirrors for re-aluminization, modern telescopes cannot risk moving these fragile mirrors. Re-aluminization is now carried out in situ, every couple of years, to maintain a clean mirror surface. Thin layers of aluminum are sputtered onto the mirror’s surface, ensuring its pristine condition. Although telescopes operate in a natural environment susceptible to dust and moisture, regular re-aluminization ensures optimal performance.
As we continue to push the boundaries of telescope design and mirror construction, we unveil the marvels of the universe and embark on a journey of discovery that transcends the limitations of the past. Through the dedication and expertise of astronomers, scientists, and engineers, we continue to unlock the secrets of the cosmos, expanding our knowledge and pushing the frontiers of human understanding. The future holds immense possibilities as we strive to peer deeper into the universe and unravel its mysteries, armed with ever more powerful and precise telescopes.
“The universe is not only stranger than we imagine; it is stranger than we can imagine.” — Sir Arthur Eddington.
