The Scientific Legacy of William Herschel by Clifford J. Cunningham

Author:Clifford J. Cunningham
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

Fig. 4.39Close-up showing the aperture stops used by John Herschel on the 20-ft front-view during his observations in the 1830s in Southern Africa. On left is what appears to be a set of concentric circles; to the right of them a stop in the shape of an equilateral triangle (Image reproduced by permission of the Harry Ransom Center, University of Texas at Austin)

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The accompanying text (quoted from Warner ) is as follows:

The diaphragms . The performance of telescopes on different nights and under different atmospheric circumstances is so unequal that it is indispensable to have constantly ready at hand and to be familiar with the application of every means of bringing on distinctness of vision, even at the sacrifice of some portion of the light afforded by a large aperture; when, for example it is required either to separate a close double star – to perform some careful micrometrical measurement, or to examine with attention and minuteness any particular object for which perfect definition is required and whose light is powerful enough to bear some sacrifice. It has been my practice therefore to have constantly within reach (suspended on the outside of the gallery railing, as seen in the perspective view) a set of circular diaphragms , ring within ring, fitting into one another and finally into the aperture of the tube, by which the aperture might be contracted successively to 15, 12, 9 and even to 6 inches as occasion might require. Besides these were also provided two other diaphragms in the form of equilateral triangles of 9 in and 6 in respectively, in their sides. Such a form of aperture, area for area , is much more effective in the division of double stars than a circular form since it reduces their spurious discs to a smaller apparent diameter without destroying their circular shape, and gives them a neatness and insulation by converting their whole system of interferential appendages into six hair-breadth rays…which is extremely advantageous in such observations: though of course fatal to distinct vision if applied to Planets or objects of a sensible diameter. The appearance of a bright single star with such an aperture is…[a]…very small and perfectly circular [disk], the rays of surprising length, delicacy and straitness [sic], and the intervals between them totally devoid of all irradiation. In consequence a much better chance is afforded of detecting a very minute or very close companion than when (as in the case of circular apertures) the disc is surrounded by a series of dark and bright rings which are almost never at rest. 227

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