MUM 



CASSEGRAIN REFLECTOR WITH CORRECTED FIELD. 



Tl- general conclusion which I lraw from S, ,,u AR/SCHILD'B investigation is that 

 unification of the two mirrora is in itsrlf not enough to give a practical solut.on . 

 tl,,- problem. We have to d.-:d with spherical aberration, coma, curvature of 

 field and astigmatism. Distort!,,,, may be set aside, Wause in itself it does not 

 vitiate the image of a point, and errors which it introduces into relative d.stano 

 nrny be computed and allowed for. We have at our disposal the figures of 

 min-i-s ,.,..1 their separation and curvatures. The last are so lock.,1 up with 1 

 kind ,,f t.-l.-s,-one which we wish to produce that they are hardly available 

 ...Ijnstment-if we want a short-focus instrument we have to take SCHWARZS, .... 

 choice and for a long-focus one the Cassegrain form. It turns out that the former 

 of these may have a flat field and the latter must have a curved field and we have 

 to rest content with that. And with respect to the figures of the mirrors , 

 within our control to say whether they shall offer themselves in our equations i 

 favourable form for removing undesired terms ; it appears from the reward 

 they appear somewhat unfavourably entailing the use of surfaces decidedly far 1 

 the sphere. It is my object to obtain a workable solution and not merely a theor 

 one and therefore I have recourse to a more complicated system, by passing the 

 beam through a definite set of lenses, the curvatures of which are more or 

 completely at our disposal. It might, at first sight, appear that this would impa 

 the achromatism of the reflector, but if a system of not less than three separa 

 lenses be made of the same glass, the two conditions for achromatism at a give, 

 plane may be completely satisfied, equally for all colours. With such a system we 

 can produce deviation in a beam, but more emphatically we can produce aberrat, 

 The details at which I arrive are given on p. 66, and need not be repeated 

 generally the plan is to replace the convex mirror by a weak convexo-concave 

 silvered at the back, and about two-thirds of the way between this and 

 of the great mirror to place a system which I call the Corrector, being a pair , 

 lenses of nearly equal but opposite focal lengths, of which the first IB double 

 with the lesser curvature first, and the latter nearly plano-convex. 



Choosing the curvatures properly a telescope is thus produced which gives, 

 strictly in the focal plane, an image free from chromatic faults, except for minute 

 chromatic residues of aberration, from spherical aberration and from coma, and 

 which points of the object are represented in the image by spots strictly circu 

 reach a diameter of 2'2 seconds at a distance of 1 degree from the centre of the 

 The givntrst angle of incidence upon any of the surfaces is 11 degrees, o 

 than alx>ut two-thirds of what is customary upon the anterior surface of 

 lens of the object glass of a refractor ; all the surfaces are spherical except 

 the great mirror which is intermediate between the sphere and paraboloid, and 

 cannot see that anywhere any serious constructional difficulty is introduced, 

 effective aperture-ratio is 1 : 14'05, or, say, about 1 : 15, allowing that 5 per . 

 more light will be lost in this construction than in other possible ones. 



