Telescopes: Portals of Discovery chapter 6

Chapter 6: Telescopes: Portals of Discovery

6.1 Eyes and Cameras: Everyday Light Sensors

Goals for Learning:

  • How does your eye form an image?

  • How do we record images?

How Does Your Eye Form an Image?

  • Components of the Eye:

    • Lens: A transparent structure in the eye that refracts light.

    • Pupil: The adjustable opening in the center of the eye that lets light in.

    • Retina: The light-sensitive layer at the back of the eye where images are formed.

    • Optic Nerve: The nerve that carries visual information from the retina to the brain.

Refraction

  • Definition: Refraction is the bending of light when it passes from one substance into another.

  • The eye uses refraction to focus light onto the retina.

Focusing Light

  • Refraction can cause parallel light rays to converge to a single point called a focus.

Image Formation

  • The focal plane is where light from different directions comes into focus.

  • The image formed by a single (convex) lens is upside-down.

Digital Camera

  • A camera functions similarly to the eye by focusing light and capturing the image using a detector.

  • CCD Detectors:

    • Charge-Coupled Devices (CCDs) in digital cameras are similar to those used in modern telescopes.

Summary of Learning

  1. Your eye forms an image by using refraction to bend light rays for focus at the retina.

  2. Cameras record images by focusing light similarly and using detectors such as CCDs.

6.2 Telescopes: Giant Eyes

Goals for Learning:

  • What are the two most important properties of a telescope?

  • What are the two basic designs of telescopes?

  • What do astronomers do with telescopes?

Important Properties of a Telescope

  1. Light-Collecting Area:

    • Telescopes with a larger collecting area can gather more light in a shorter period.

    • The light-collecting area is directly related to the diameter of the telescope.

    • Formula: Area = rac{π(diameter/2)^2}{∆}

  2. Angular Resolution:

    • Larger telescopes can take images with greater detail.

    • The minimum angular separation that a telescope can distinguish is determined by the telescope’s diameter and the wavelength of light.

    • Limitations: The ultimate limit to resolution comes from the interference of light waves within the telescope, known as the diffraction limit.

    • The complexity of this is illustrated with rings observed in star images due to light interference.

Basic Designs of Telescopes

  • Refracting Telescope:

    • Focuses light using lenses.

    • Generally requires long lengths and heavy lenses.

    • Example: Yerkes refractor with a 40-inch lens and a length of 64 feet.

  • Reflecting Telescope:

    • Focuses light using mirrors.

    • Capable of achieving much greater diameters and is more common in modern usage.

Designs for Reflecting Telescopes

  1. Cassegrain Focus

    • Features a primary and secondary mirror.

  2. Newtonian Focus

    • Uses a primary mirror and a flat secondary mirror for focusing.

  3. Nasmyth/Coudé Focus

    • Also uses a primary and secondary mirror.

Amateur Telescopes

  • Examples:

    • Orion 10-inch Dobsonian: $500

    • Vixen 10.25-inch Cassegrain with Celestron CGE mount: $7,000

Telescope Mounts

  1. Altazimuth Mount

  2. Equatorial Mount

What Do Astronomers Do with Telescopes?

  • Imaging: Taking pictures of celestial objects.

  • Spectroscopy: Breaking light into its spectra to analyze properties.

  • Timing: Measuring variations in light output over time.

Summary of Learning

  • Key properties of telescopes: Light-collecting area and angular resolution.

  • Basic designs: Refracting telescopes focus with lenses, while reflecting telescopes focus with mirrors.

  • Applications of telescopes: Imaging, spectroscopy, and timing measurements.

6.3 Telescopes and the Atmosphere

Goals for Learning:

  • How does Earth’s atmosphere affect ground-based observations?

  • Why do we put telescopes into space?

Effects of Earth’s Atmosphere on Observations

  • Ideal Ground-Based Telescope Sites:

    • Calm (minimal wind)

    • High altitude (less atmosphere to view through)

    • Dark areas (far from city lights)

    • Dry regions (fewer cloudy nights)

  • Example: The summit of Mauna Kea, Hawaii, represents an ideal site for astronomical observation.

Light Pollution

  • Light pollution refers to the scattering of human-made light in the atmosphere, which poses a challenge for astronomical observations.

Twinkling and Turbulence

  • Turbulent airflow in Earth’s atmosphere distorts the view of stars, causing them to appear to twinkle when viewed from ground-based telescopes.

  • Comparison of a bright star viewed through a ground-based telescope vs. Hubble Space Telescope shows how turbulence affects image quality.

Adaptive Optics

  • Technique: Involves rapidly changing the shape of a telescope’s mirror to counteract some turbulence effects.

  • Comparison of images with and without adaptive optics highlights improvements in clarity.

Why Put Telescopes in Space?

  • Transmission in Atmosphere:

    • Only radio and visible light can easily pass through Earth’s atmosphere.

    • Space telescopes allow us to observe other parts of the spectrum that do not penetrate the atmosphere effectively.

    • No atmospheric turbulence results in sharper images.

Summary of Learning

  • Earth’s atmosphere affects observations through factors like light pollution and turbulence.

  • Space telescopes can observe wavelengths other than radio and visible light which are hindered by atmospheric interaction.

6.4 Telescopes and Technology

Goals for Learning:

  • How can we observe invisible light?

  • How can multiple telescopes work together?

Observing Invisible Light

  • A standard satellite dish functions like a telescope for detecting radio waves.

Radio Telescopes

  • Designed like giant mirrors that reflect radio waves to a focus.

  • Example: Arecibo radio telescope with a diameter of 1000 feet.

Infrared and Ultraviolet Telescopes

  • Operate similarly to visible-light telescopes but must be above the atmosphere to detect all wavelengths.

  • Examples include SOFIA and Spitzer telescopes.

X-Ray Telescopes

  • Must also be placed above the atmosphere to function.

  • Focusing Method: Requires special mirrors arranged to focus X-ray photons through grazing bounces off the surface.

  • Example: Chandra X-Ray Observatory.

Gamma-Ray Telescopes

  • These telescopes need to be in space due to the inability to focus gamma rays within Earth’s atmosphere.

  • Example: Fermi Gamma-Ray Observatory.

Looking Beyond Light

  • Astronomers collect other types of signals using various telescopes including:

    • Neutrinos

    • Cosmic rays

    • Gravitational waves

  • Example: Arial view of part of LIGO to study gravitational waves.

Working Together: Interferometry

  • Definition: Interferometry is a technique that links multiple telescopes to achieve the angular resolution of a larger single telescope.

  • This is easiest with radio telescopes but is now possible with infrared and visible-light telescopes.

  • Example: Very Large Array (VLA) in New Mexico.

Future of Astronomy in Space

  • The Moon is considered an ideal location for astronomical observations.

Summary of Learning

  • Invisible light telescopes are usually modified versions of reflecting telescopes and are often located in space.

  • Interferometry allows multiple telescopes to effectively combine their resolution power, mimicking that of a much larger instrument.