9.1.4 Reflecting vs Refracting Telescopes

• The two main types of aberration that affect the quality of images produced by refractors and reflectors are:
  • Chromatic aberration
  • Spherical aberration

Chromatic Aberration

• Chromatic aberration is a type of image distortion seen only in refracting telescopes, where:

Different wavelengths of light are refracted by different amounts causing the edges of an image to appear coloured

• This is due to the fact blue light has a shorter wavelength than red light, meaning blue light is refracted more by a lens than red light
  • This is because blue light has a bigger refractive index

• Consequently, different colours are brought to focus at different points
  • For example, blue light focuses closer to the lens than red light, because of this greater refraction
• The result is the image has a multi-coloured blurred edges

Chromatic Aberration of a Lens

This diagram of the Moon shows a very extreme example of chromatic aberration, it is usually more subtle than this

• Chromatic aberration cannot be eliminated entirely, but it can be significantly reduced by using a second diverging lens which refracts the light in the opposite direction
  • This allows the red light to be brought to the same focal point as the blue light

 

Correcting Chromatic Aberration

• Chromatic aberration does not happen in reflecting telescopes, because mirrors can only reflect, not refract

Spherical Aberration

• Spherical aberration is a type of image distortion which can affect both refractors and reflectors, where:

Rays of light come to focus at different points due to the spherical curvature of a lens, or mirror, causing the image to become very blurred

• The positions of the focal points depend on where the rays of light meet the lens or the mirror
  • The further the ray is from the principal axis, the shorter its focal length

Ray Diagrams for Spherical Aberration

Spherical aberration can affect both lens and mirrors

• In a refracting telescope, spherical aberration cannot be eliminated entirely, but it can be reduced by using a parabolic lens
  • However, there is a limit to how much this improves the image quality, as this further increases the size and weight of the lens

• Whereas, in a reflecting telescope, spherical aberration can be entirely overcome by using a parabolic mirror

 

Parabolic mirror

Spherical aberration is eliminated by using a parabolic mirror

Refracting Telescopes

Advantages

• Refractors require less maintenance than reflectors
• Refractors are not as sensitive to temperature changes as reflectors

Disadvantages

• Refractors are not usually the telescope of choice due to their: 

1. Size

• • It is difficult to make large-diameter glass lenses which are completely free from defects
  • Large magnifications require large objective lenses and very long focal lengths

2. Weight

• • Large-diameter lenses are heavy and tend to distort under their own weight
  • Refractors are heavy and difficult to manoeuvre so they have a slower response to astronomical events
  • Observing equipment and electronics must be mounted to the telescope which adds to the weight

3. Construction

• • Lenses can only be mounted and supported around their edges however, this is where they are thinnest and weakest

4. Image quality

• • Refractors suffer from both chromatic and spherical aberration

5. Ability to observe non-visible wavelengths

• • Refractors are only able to observe wavelengths of visible light

Reflecting Telescopes

Advantages

• Refractors are superior in their design because of their: 

1. Size

• • The diameter of a mirror can be much larger than that of a lens so greater magnifications can be achieved
  • The arrangement of the mirrors allows reflectors to be much shorter than refractors 
  • Several small mirrors can be used to form a large composite objective mirror

2. Weight

• • Large single mirrors can be made, which are light and easily supportable from behind 
  • Reflectors are lighter which allows for a more rapid response to astronomical events

3. Construction

• • Mirrors only use the front surface for reflection, which eliminates many of the problems associated with lenses
  • Mirror surfaces can be made very thin (a few nm) which allows for greater image detail

4. Image quality

• • Mirrors cannot produce chromatic aberration
  • Reflectors do not suffer from spherical aberration use if parabolic mirrors are used

5. Ability to observe non-visible wavelengths

• • Reflectors can be designed to observe wavelengths of light outside of the visible spectrum
  • Reflectors can be sent into space which eliminates light absorption due to the atmosphere 

Disadvantages

• The secondary mirror has the disadvantage of blocking some light from entering the primary mirror
• The secondary mirror and its supports will cause some diffraction which can affect the clarity of the image
• Mirrors in a reflecting telescope are exposed to air so they require regular maintenance
• Light is refracted in the eyepiece lens and therefore some chromatic aberration may be introduced at this stage