6.2 Modern Day Telescopes

1. Evolution of Reflecting Telescopes

  • Since Newton's time, reflecting telescopes have significantly increased in size.

    • The 1948 Palomar Mountain telescope in Southern California featured a 5-meter (200-inch) diameter mirror, remaining the world's largest visible-light telescope for decades.

    • Today's primary mirrors are typically 8 to 10 meters in diameter, with even larger ones under construction.

2. Modern Visible-Light and Infrared Telescopes

  • The period starting in 1990 marked an unprecedented rate of telescope construction globally.

  • Technological advancements made it possible and cost-effective to build telescopes significantly larger than the 5-meter Palomar telescope.

  • New technologies are also designed to perform well in infrared wavelengths, not just visible light.

2.1 Largest Visible-Light and Infrared Telescopes (Learning Objective: Recognize the largest visible-light and infrared telescopes in operation today)

  • European Extremely Large Telescope (E-ELT):

    • Aperture: 39 meters

    • Location: Cerro Armazonas, Chile

    • Estimated First Light: 2025

  • Thirty-Meter Telescope (TMT):

    • Aperture: 30 meters

    • Location: Maunakea, HI

    • Estimated First Light: 2025

  • Giant Magellan Telescope (GMT):

    • Aperture: 24.5 meters

    • Location: Las Campanas Observatory, Chile

    • Estimated First Light: 2025

  • Keck I and II (two telescopes):

    • Aperture: 10.0 meters each

    • Location: Maunakea, HI

    • Completed: 1993–1996

  • Very Large Telescope (VLT):

    • Aperture: 8.2 meters (four telescopes)

    • Location: Cerro Paranal, Chile

    • Completed: 2000

2.2 Technological Advancements in Telescope Design

  • Palomar vs. Gemini North Comparison:

    • The Palomar 5-meter telescope: A massive steel structure to counteract the sagging of its 14.5-ton mirror.

    • The Gemini North 8-meter telescope: Weighs 24.5 tons (less than twice the Palomar mirror's weight) with a mirror only 8 inches thick, despite being larger.

  • Active Control:

    • Modern computers measure mirror sag many times per second.

    • Forces are applied at 120 different locations on the back of the mirror to correct sag, maintaining the mirror's precise shape.

  • Segmented Mirrors (e.g., Keck Telescopes):

    • Instead of a single large mirror, each 10-meter Keck telescope combines light from 36 separate hexagonal mirrors, each 1.8 meters wide.

    • Computer-controlled actuators constantly adjust these individual mirrors to act as a single, perfectly shaped reflecting surface.

2.3 George Ellery Hale: Master Telescope Builder

  • Hale initiated projects for what became the world’s largest telescopes four times throughout his career.

  • He was instrumental in securing funding from wealthy benefactors for these ambitious projects.

  • Notable Projects:

    • Yerkes 40-inch refractor (1897): Remains the largest refractor in the world.

    • Mount Wilson 60-inch reflector (1908).

    • Mount Wilson 100-inch Hooker Telescope (1917): Used by Edwin Hubble to confirm the existence of other galaxies.

    • Palomar 200-inch (5-meter) Hale Telescope (dedicated 1948): Funded by the Rockefeller Foundation.

  • Hale predicted that significantly larger telescopes would need to be reflectors due to the aperture limits of refractors.

3. Picking the Best Observing Sites (Learning Objective: Discuss the factors relevant to choosing an appropriate telescope site)

  • Investment Justification: The high cost (around 100100 million) of modern telescopes necessitates optimal site selection.

  • Ideal Site Characteristics:

    • High Altitude: Minimizes atmospheric filtering, especially water vapor absorption in the infrared.

    • Dry: Essential for infrared observations due to less water vapor.

    • Dark: Far from city lights to avoid light pollution, which scatters glare and limits visibility of faint stars. Observatories prefer sites at least 100 miles from large cities.

    • Stable Air (Good Seeing): Air turbidity (turbulent air) causes light to bend and twist, resulting in blurred star images. Best sites have minimal atmospheric blurring.

    • Clear Weather: Sites should have clear skies as much as 75% of the time, free from clouds, wind, and rain.

  • Preferred Locations:

    • Andes Mountains of Chile (e.g., Cerro Paranal for VLT)

    • Desert peaks of Arizona

    • Canary Islands in the Atlantic Ocean

    • Maunakea in Hawaii (a dormant volcano at 13,70013,700 feet or 42004200 meters) — known for air stability due to long flow over water.

  • Light Pollution: A growing concern, impacting both professional astronomy and general enjoyment of the night sky, with new threats from large satellite swarms reflecting sunlight.

4. The Resolution of a Telescope and Adaptive Optics (Learning Objective: Define the technique of adaptive optics and describe the effects of the atmosphere on astronomical observations)

  • Resolution Defined: The precision of detail in an image, referring to the smallest distinguishable features.

  • Effect of Earth's Atmosphere on Astronomical Observations:

    • Turbulence: Earth's atmosphere is turbulent, containing small cells of gas at varying temperatures.

    • Light Bending: Each gas cell acts like a tiny lens, bending (refracting) light rays slightly.

    • <strong>Blurred</strong><strong>Blurred</strong> Images: As air cells move, the path of light constantly changes, resulting in blurred and distorted images.

    • "Twinkling" of Stars: This atmospheric effect causes stars to appear to vary in brightness.

    • Resolution Limit: Traditional ground-based telescopes are limited to resolutions of several tenths of an arcsecond due to atmospheric blurring.

  • Adaptive Optics Technique:

    • Mechanism: Employs a small, flexible mirror placed in the telescope's light beam.

    • Sensor: Measures atmospheric distortions in the image.

    • Correction: Instructions are sent to the flexible mirror up to 500 times per second.

    • Shape Change: The mirror continually changes shape to compensate for the atmosphere-induced distortions.

    • Effectiveness: Most effective in the infrared spectrum with current technology.

    • Resolution Improvement: Ground-based telescopes with adaptive optics can achieve resolutions of 0.10.1 arcsecond or better in the infrared, matching the Hubble Space Telescope's resolution in visible light.

    • Example: The Very Large Telescope in Chile used adaptive optics to produce one of the clearest ground-based images of Jupiter.

5. How Astronomers Really Use Telescopes Today

  • Modern Astronomy Practices:

    • Most astronomers do not live at observatories; they work from universities or laboratories.

    • Direct observation at telescopes is infrequent; typically, a week per year.

    • Focus is on measuring and analyzing data acquired through collaborations and surveys.

    • Many astronomers use radio telescopes or work on theoretical problems using supercomputers.

    • Remote Observation: Electronic detectors permanently record data, and observations can often be made remotely from thousands of miles away.

    • Telescope Time Allocation: Time on major telescopes is competitive; astronomers submit proposals, which are ranked by a committee before time is assigned.

  • Shift from Traditional Observation: While some nostalgia for long, cold nights of direct observation remains, modern astronomy is largely conducted in warm rooms with teams of observers working with computers.