Radio vs. Optical Telescopes: A Comparative Guide

This article compares radio telescopes and optical telescopes, outlining the differences in their operation and applications.

Radio Telescope

Radio telescopes capture radio frequency waves from space.

Figure 1: Radio Telescope Block Diagram

Here’s how a radio telescope works:

• The telescope gathers radio frequency (RF) waves using a large dish.
• The dish concentrates the RF waves and reflects them toward an RF receiver.
• Concentrating the RF waves improves the definition and resolution of the received signal.
• The captured RF signals from distant radio sources are sent to a control room for further processing.
• The control room houses the receiver, filters, computer, recording devices, display, and analysis systems.
• Radio telescope electronics emit RF waves, which can interfere with observations. To minimize this, the receiver is cooled using coolants like liquid nitrogen or liquid helium.
• Radio telescopes can be used individually or in arrays where multiple antennas are electronically linked.
• Radio telescopes can operate day and night.

Optical Telescope

Unlike radio telescopes, optical telescopes collect light. Their design leverages several properties of light:

• Law of Reflection:
  • Angle of Incidence = Angle of Reflection
• Law of Refraction:
  • Light bends towards the normal when entering a medium with a higher index of refraction.
  • Light bends away from the normal when entering a medium with a lower index of refraction. (“Normal” refers to the ray path perpendicular to the mirror’s surface.)
• Inverse Square Law:
  • Light intensity decreases with the square of the distance from the source.

When light passes from one medium (e.g., air) to another (e.g., glass or water), its speed changes, causing it to bend or refract. The index of refraction quantifies this bending:

$Index \space of \space refraction \space (n) = \frac{speed \space of \space light \space in \space vacuum}{speed \space of \space light \space in \space medium}$

There are two primary types of optical telescopes: refractor telescopes and reflector telescopes. Unlike radio telescopes, optical telescopes are typically used during the daytime.

Figure 2: Refractor Optical Telescope

Refractor Telescopes:

• Use lenses to focus light to a point.
• Provide the sharpest views of planets and the moon.
• Are generally the most expensive for a given aperture.
• Often have long tubes, though shorter designs are now available.
• Inexpensive models may suffer from chromatic aberration (color fringing).

Figure 3: Reflector Optical Telescope

Reflector Telescopes:

• Use mirrors to focus light to a point.
• Require periodic alignment of the mirrors.
• Are the least expensive for a given aperture.
• Available in both long and short tube designs.
• Generally do not suffer from chromatic aberration.
• The angular resolution ($\theta$) of a reflecting telescope depends on the wavelength of light being viewed ($\lambda$) and the diameter of the primary mirror ($D$):

$\theta = 1.22 * \frac{\lambda}{D}$

A third type, the compound telescope, utilizes both mirrors and lenses to focus light. It offers a moderate cost for a given aperture and is the most portable type.

Large Optical Telescopes

Larger telescopes are preferred because they:

• Collect more light than smaller telescopes. The amount of light collected depends on the mirror area: $\pi * (\frac{D}{2})^2$.
• Produce brighter images.
• Detect fainter objects.
• Increase the sharpness of the image, allowing for finer details to be distinguished.
• Offer greater magnifying power, resulting in a larger image in the field of view.