Telescopes
Telescopes gather and focus light from distant objects using lenses or mirrors, making those objects appear larger and brighter than they do to the naked eye. Understanding how they work ties together many of the optics concepts from earlier in this unit, including focal length, image formation, and the wave nature of light.
Magnification and Light Gathering
A telescope has two main optical elements working together: the objective (a lens or primary mirror) collects light from a distant object and forms a real image, while the eyepiece (a smaller lens) magnifies that image for your eye.
Magnification is the ratio of the objective's focal length to the eyepiece's focal length:
A longer focal length objective paired with a shorter focal length eyepiece gives higher magnification. For example, if and , the magnification is 100×.
But magnification isn't everything. Two other properties matter just as much:
- Light-gathering power depends on the area of the objective lens or primary mirror. Since area scales with the square of the diameter, doubling the diameter collects four times as much light (not twice). The diameter of the objective is called the aperture.
- Resolving power (also called angular resolution) is the telescope's ability to distinguish two closely spaced objects. It improves with a larger aperture and shorter wavelength of light. This is one reason the Hubble Space Telescope, with its 2.4-meter mirror observing in visible light, can resolve fine details that smaller ground-based telescopes cannot.
Refracting vs. Reflecting Telescopes
These are the two classic telescope designs, and they differ in how the objective collects light.
Refracting telescopes use a large objective lens at the front of the tube. Light passes through this lens, which bends (refracts) it to form an image near the back of the tube, where the eyepiece magnifies it. Galileo's early telescopes were refractors.
- Advantages: Good image quality; the sealed tube keeps dust and air currents away from the optics.
- Disadvantages: Large lenses are heavy and hard to support without sagging. They also suffer from chromatic aberration, where different wavelengths of light focus at slightly different points, producing color fringes. Because of these issues, the largest refractors ever built are only about 1 meter in diameter.
Reflecting telescopes use a curved primary mirror at the back of the tube. Light enters the open end, reflects off the primary mirror, and converges toward a focal point. A smaller secondary mirror then redirects the light to an eyepiece or camera. The Newtonian reflector is a common design where the secondary mirror sends light out the side of the tube.
- Advantages: Mirrors can be made much larger than lenses (the Keck telescopes have 10-meter mirrors). Mirrors also have no chromatic aberration, since light reflects off the surface rather than passing through glass. They're generally cheaper to manufacture at large sizes.
- Disadvantages: The open tube lets in air currents and dust, which can degrade image quality and require regular maintenance.
Telescopes Across the Electromagnetic Spectrum
Visible light is only a small slice of the electromagnetic spectrum. By building telescopes sensitive to other wavelengths, astronomers can observe physical processes that are invisible to optical telescopes.
- Optical telescopes detect visible light (wavelengths of about 380 to 700 nm). The Hubble Space Telescope is a well-known example.
- Radio telescopes detect radio waves (wavelengths longer than about 1 mm). They typically use large dish antennas to collect these long-wavelength signals. The Arecibo Observatory in Puerto Rico had a 305-meter dish before its collapse in 2020.
- Infrared telescopes detect infrared radiation (wavelengths between about 700 nm and 1 mm). Their detectors must be cooled to extremely low temperatures to prevent the telescope's own heat from drowning out faint infrared signals. The James Webb Space Telescope operates at infrared wavelengths.
- Ultraviolet, X-ray, and gamma-ray telescopes detect high-energy photons. Earth's atmosphere absorbs most of these wavelengths, so these telescopes must be placed in space. The Chandra X-ray Observatory orbits Earth for this reason. Focusing high-energy photons requires special techniques like grazing incidence mirrors, where photons bounce off mirrors at very shallow angles.
Different wavelengths reveal different physics. Radio waves can trace cold gas in a galaxy, while X-rays reveal extremely hot gas. Combining observations across multiple wavelengths gives a much more complete picture of any celestial object.
Advanced Telescope Technologies
Several modern technologies push telescopes well beyond what simple lenses and mirrors can achieve on their own.
- Interferometry combines signals from multiple telescopes spread across a wide area, effectively creating an aperture as large as the distance between them. The Very Large Array in New Mexico uses 27 radio antennas working together to achieve resolution far sharper than any single dish could.
- Adaptive optics uses deformable mirrors that change shape hundreds of times per second to correct for the blurring caused by turbulence in Earth's atmosphere. This allows ground-based telescopes like those at the Keck Observatory to approach their theoretical resolution limits.
- Charge-coupled devices (CCDs) are the digital sensors used in modern telescopes. They are far more sensitive than the photographic plates they replaced and make it much easier to store and process data.
- Spectroscopy splits incoming light into its component wavelengths, producing a spectrum. By analyzing the emission and absorption lines in that spectrum, astronomers can determine the chemical composition, temperature, and motion of distant objects.
- The diffraction limit sets the theoretical best resolution a telescope can achieve. It depends on both the wavelength of light being observed and the aperture size. No amount of magnification can reveal detail finer than this limit.