Reflecting vs Refracting Telescopes

Chromatic aberration

• Chromatic aberration occurs because blue light has a shorter wavelength than red light, which means blue light has a bigger refractive index and is refracted more by a lens.
• The consequence is that different colours are brought to focus at different points along the principal axis. Blue light focuses closer to the lens than red light.
• This produces a multi-coloured, blurred fringe around the edges of the image.
• Chromatic aberration can be reduced (but not eliminated) by using an achromatic doubletA combination of a converging (crown glass) lens and a diverging (flint glass) lens cemented together. The diverging lens refracts light in the opposite direction, bringing different colours to approximately the same focal point.: a second diverging lens that refracts the light in the opposite direction.
• Crucially, chromatic aberration does not occur in reflecting telescopes, because mirrors reflect light rather than refract it, so all wavelengths are treated equally.

Spherical aberration

• Spherical aberration affects both refractors and reflectors.
• The key part is that rays hitting the outer edge of a spherical lens or mirror are brought to focus at a different point than rays hitting nearer the centre. The further a ray is from the principal axis, the shorter its focal length.
• In a refracting telescope, spherical aberration can be reduced by using a parabolic lens, though this increases size and weight.
• In a reflecting telescope, spherical aberration can be entirely eliminated by using a parabolic mirror. This is a major advantage of reflectors.

Advantages of refracting telescopes

• Refractors require less maintenance than reflectors (the lenses are enclosed).
• Refractors are less sensitive to temperature changes than reflectors.

Disadvantages of refracting telescopes

• Size: It is difficult to make large-diameter glass lenses free from defects. Large magnifications require very long focal lengths, making refractors impractically long.
• Weight: Large lenses are heavy and tend to distort under their own weight. They can only be supported around their edges, which is where they are thinnest and weakest.
• Image quality: Refractors suffer from both chromatic and spherical aberration.
• Wavelength range: Refractors can only observe wavelengths of visible light.

Advantages of reflecting telescopes

• Size: Mirrors can be made much larger than lenses. Several small mirrors can form a large composite objective mirror. The folded light path also makes reflectors much shorter.
• Weight: Large single mirrors are light and easily supportable from behind.
• Construction: Mirrors only use the front surface for reflection, so internal defects in the glass do not matter. Mirror surfaces can be made very thin.
• Image quality: Mirrors cannot produce chromatic aberration, and parabolic mirrors eliminate spherical aberration.
• Wavelength range: Reflectors can be designed to observe non-visible wavelengths and can be sent into space to eliminate atmospheric absorption.

Disadvantages of reflecting telescopes

• The secondary mirror and its supports block some incoming light and cause some diffraction, slightly reducing image clarity.
• Mirrors are exposed to air and require regular maintenance.
• Some chromatic aberration may be introduced at the eyepiece lens stage.