History of Astronomy Example Exam Question Hints and Suggestions

Theme 5: Rise of the Telescope

Discuss the development of the refracting telescope from 1600 to 1900. [4]
This is a straightforward test of factual recall - either you know this material or you don't. Since it is a very substantial part of the question, if you don't know the material, you should probably avoid this question.

The facts essentially fall into three categories: what happened, who did it and when it happened. The first of these is probably the easiest, since there is some logical ordering to help fill out your memory; the second and third are purely memory-based.

WhatWhoWhen
First useGalileo~1610
Convex eyepieceKeplerWidely adopted, mid 17th century
focal-plane instrumentationVarious people1660s
Better mountingsRömer (transit)1680s
Achromatic objectiveJohn Dollond1757
Better engineeringMany people, especially Fraunhofer19th century

This gives a basic skeleton. You need to flesh it out with details (what are the advantages of the convex eyepiece, or of the achromatic lens? What were the engineering improvements of the 19th century?), and there are things you could add, such as the introduction of photography in the mid-19th century and the consequent need to consider the focus for blue light (which even in an achromatic objective is not the same as the focus for red/yellow/green).

Discuss the relative advantages and disadvantages of refracting and reflecting telescopes
  • around 1700;
  • around 1800;
  • around 1900.
[3]
This is another memory question, although your answer to the previous part will help.
  • Around 1700, refracting telescopes used Keplerian optices with simple lenses. They therefore had to be very long and cumbersome to minimise chromatic aberration. They were, however, established and understood technology. Reflecting telescopes were newer (mid 17th century), and very much more difficult to make (they relied on casting a curved metal mirror, which was challenging for the technology of the day) and maintain (speculum metal tarnishes and has to be reground); the ligh gathering was also poorer for the same size of objective (speculum's reflectivity is not as good as modern mirrors). Consequently, although reflectors were known to be immune to chromatic aberration, they were not much used.
  • Around 1800, refracting telescopes were relatively small instruments, but capable of high precision and installed on good mountings. Reflectors were much bigger, so with better light-gathering power, but much more crudely mounted (usually on alt-az mounts) because of their great weight, and had some optical problems because speculum deforms easily. Therefore, at this time refractors are better precision instruments; reflectors are better for faint objects.
  • Around 1900, reflecting telescopes were beginning to become established as front-line research instruments: they were still larger than refractors, but now equally precisely mounted and with equally good-quality optics. Refractors had become very long again, owing to the desire for larger objectives, and were once more very unwieldy and difficult to use. At this point refractors had few remaining advantages over reflectors as research instruments.
In cases (ii) and (iii), identify the key technological advance that has shifted the balance between the two types of telescope. [1]
In case (ii), the key advance is the achromatic lens. This means that thicker lenses with shorter focal lengths can be used, and hence makes refracting telescopes much more compact and manageable. This essentially removes the only significant advantage of reflectors at that time, i.e. their smaller size.

In case (iii), the key advance is the development of silver-on-glass (later aluminium-on-glass) mirrors. These remove the disadvantages of speculum: they are easier to make and maintain, not so temperature sensitive, and have higher reflectivity.

Briefly discuss the technological (and other) advances between Galileo's time and 1838 that were needed to make the first measurement of stellar parallax possible. [2]
First, consider what is needed for a successful parallax measurement. The angles are extremely small, so you need high precision and stable mountings. You need to choose the right star: even now, the vast majority of stars do not have measurable parallax. Finally, you need to be able to demonstrate the reliability of your measurement, since you are dealing with such a small quantity.

The technological advances are included in the discussion above: you need good mounts (from 1680s onward, improving steadily as the Industrial Revolution improves the quality of precision engineering) and good optics (requires the achromatic objective, as invented by Dollond in the 1750s, and the improved glass technology of Fraunhofer, which made larger achromats possible). The successful measurement used a heliometer, which was a 19th century invention, though it isn't clear that this was really needed.

Choosing the right star required astronomical advances: Galileo wanted to use double stars, which he thought were chance alignments. Michell demonstrated that this was very improbable (too many close pairs to arise by chance) and Herschel later proved by observation that binary stars do orbit each other. This also showed that stars are not all equally bright, which means that using the brightest stars as targets won't necessarily work - hence the shift to more promising stars of high proper motion.

Demonstrating reliability required advances in experimental technique: Bessel's approach involved careful calibration of the instrument, and his paper is much more modern in its careful treatment of experimental error than most work of the time.

[2007 Q3.]

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