A NEW AUTOCOLLIMATOR

BY F. E. WRIGHT

In the Gauss method for autocollimation light reflected by a glassplate placed back of the cross hairs in the eyepiece of a telescope istransmitted through the telescope system to a reflecting mirror whenceit is returned through the telescope. With this arrangement the crosshairs and their image appear as dark lines in an illuminated field.Autocollimation is attained when the cross hairs cover their image.Experience has shown that more accurate settings can be made ifthe two fields are not superimposed, but, as in the range finder of thecoincident, self-contained type they are placed side by side in contact.The accuracy of a single setting under these conditions is about fivetimes that obtainable where the fields are superimposed. In theautocollimator described below this factor has been taken into account.

The arrangement of the several parts is illustrated in Figs. 1A (topview) and B (view of field as seen through the eyepiece). The lightfrom a small electric bulb, N1, after passing through the ground glass,G1, and through a narrow adjustable horizontal slit, A, is brought tofocus by the lens, L1, on the telescope axis, EO. -Half the beam isreflected by the silvered surface of the cemented prism, P. the left halfof whose diagonal face is silvered, indicated in Fig. 1A by the heavyline. The reflected beam passes along the telescope axis through theobjective, 0 (distant its own focal length from the axis of the lenses,L1L2), and emerges as a parallel light beam to the reflecting surface, R,whose angular position is to be determined, thence back to the objective,0, into the telescope where it is imaged in the vertical plane throughthe axis of the lenses, L1L2; it is there viewed through the eyepiece, E,as a fine, horizontal bright line on a dark ground in the left half of thefield (Fig. B). The second half of the beam from N1 and L1 passesthrough the prism, P, to the lens, L2 (distant its own focal length fromthe telescope axis, EO), whence it emerges as a parallel beam and isreflected back by the front surface of the lens, L3, into L2 by which it isimaged on the rear side of the silvered surface of P where it is viewedthrough the eyepiece, E, as the horizontal stationary line of referencein the field (Fig. 1B). A scale, S, illuminated by a second small electriclamp, N2, and ground glass, G2, is imaged by the lenses L2L3 on therear side of the silvered surface of P where it is seen together with thereference line.

187

[J.O.S.A. R.S.I., 9

The field produced by this arrangement is shown in Fig. 1B. It isessentially a dark field on the left hand side of which is the fine brighthorizontal line which moves up and down with the movements of thereflecting surface, R. On the right hand side is the bright narrow line ofreference and a scale consisting either of dark lines on a light ground or,vice versa, of light lines on a dark ground. The contact between thehalves of the field is sharp (dge of silvered reflecting surface of prism,P) and accurate settings can be made as in a range finder. The scale,S, can be graduated so as to read directly to 10", or 5", or less, of angle

S

L3L2

-m A

A

ON, BFIG. 1A. Plan of new atocollimiator showing arrangement of optical parts.

FIG. B. Field of view as seen through eyepiece, E, of autocollimator.

of rotation of the reflecting surface, R, depending on the focal lengthof the objective employed. In case the autocollimator is used with amirror suspended vertically the arm, LIL2, of the telescope is mountedin a vertical position.

This form of autocollimator was devised for use on a new gravitymeter and has been found to answer the purpose well. On the crystalgrinding goniometer it is an improvement over the special autocolli-mating system employed by the writer some years ago.' In place of theadjustable slit, Al, a circular aperture or a signal, such as is in commonuse on the crystal goniometer, is preferable in this case; the scale, S,is unnecessary.

GEOvIYsICAL. LABORATORY,

CARNEGIE INSTITUTION OF WASHINGTON,MAY, 1924.

I J. Wash. Acad. Sci., 5, pp. 3541; 1915.

188 F. E. WRIGHT