William Herschel and Herschelian Reflectors

It would not have been pardonable to neglect such an advantage, when there was a particular object in view; and wondered why it had no struck me sooner.1The astronomer William Herschel (1738-1822) was not only a great observer and theoretician, but also the undisputed master-craftsman in the construction of great reflectors, both for his own use and for sale. For the largest instruments he devised an unorthodox configuration of mirror and eyepiece, known to us as TIerschelian' . What led him to devise this arrangement, and did it serve its purpose?As seventeenth-century astronomers discovered, the image created in a simple refracting telescope suffers from 'chromatic aberration', a blurring of the colours. Newton showed that this was because the different colours, as they passed through the objective lens at the upper end of the tube, were bent ('refracted') by slightly different amounts and so came to a focus at slightly different distances from the objective. His solution was to replace the lens by a mirror of parabolic or nearparabolic shape positioned at the bottom of the tube of the telescope; the different colours were all reflected equally off the mirror, and so came to a single, consistent focus higher up the tube.Eventually it was realized that there were then alternative ways of arranging for the image thus formed to be examined in the eyepiece. In 'Gregorian' or 'Cassegrain' reflectors a second and much smaller mirror located further up the tube reflects the image back down the tube and through a hole in the primary mirror to where the eyepiece is located. But in 'Newtonian' reflectors a simple flat mirror is positioned near the top of the tube, set at an angle of 45 degrees, and this reflects the image sideways through a hole in the tube to where the eyepiece is located. In a Newtonian reflector, therefore, the observer is - somewhat surprisingly - positioned near the top of the tube and looking sideways to the direction of the heavenly body.A defect of all these configurations is that much of the light is lost in the reflection off the secondary mirror, and in 1728 the Paris instrument-maker Jacques Lemaire proposed instead that the secondary mirror be dispensed with entirely, and that the observer with his eyepiece should peer directly down the mouth of the tube.2 The disadvantage of this arrangement is that the observer's own head will obstruct some of the incoming light, and so dispensing with the secondary mirror may do more harm than good.Unless, that is, the tube and mirror are so large in diameter that the loss of light caused by the observer's head is limited and so a price worth paying for dispensing with the second reflection. Large reflectors are needed when the objects under investigation are faint; and the first observer to set his heart on investigating the distant and therefore faint nebulae and star clusters was William Herschel.3 As a musician and amateur astronomer in the English spa resort of Bath in the later 1770s, Herschel made himself Newtonian reflectors of 7-ft, 10-ft and 20-ft focal length, with mirrors of some 6, 8 and 12 inches respectively. The 20-ft was crudely slung from a pole and the observer was perched precariously on a ladder of similar length, and so it was of limited use. But the 10-ft had a stable wooden mounting, and on 28 May 1776 Herschel records that I tried a 10 feet mirror without the small one, looking in at the front of the tube and holding the eye glass in my hand. liked the method very well.4 But Herschel's head was a major obstruction for a mirror only 8 inches diameter, and there the matter rested.In September 1783, Herschel, now astronomer to King George III and resident near Windsor Castle, completed another 20-ft reflector, but this time the tube was 18 inches in diameter and mounted within a safe and stable ladder-type framework. Herschel's intention was systematically to 'sweep' the sky in the search for nebulae and star clusters, and at first he did this by standing on a nine-feet- wide observing gallery (which could be raised or lowered by ropes), and dragging the tube first to one side and then to the other, by as much as fifteen degrees in each direction

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