The measurement of axial aberrations of telescopic systems of small magnification

Abstract

The aim of this work is to give a method for the measurement of axial aberrations (spherical and chromatic) of a telescopic system of small magnification. The dioptric expression for an axial aberration is given and the special advantages of such a dioptric expression for the experimental measurement of aberrations are pointed out. The principal peculiarity of a dioptric expression for axial aberration is the fact that the shape of a curve of spherical aberration remains unchanged if the focussing is varied. The arrangements used in measuring axial aberrations are described. The method of the measurement consisted in getting two out of focus photographs of a slit or pinhole placed in the focus of a collimating object-glass and illuminated by monochromatic light. The object-glass of a system to be measured was covered by a diaphragm with slit or round hole apertures. The distance between the two photographic plates and the distance between the traces for two definite slits or holes on both the plates having been measured, it was possible to get the value of the angle between a given ray and the axis of the system. By means of this angle and the exit height (in the exit pupil) we get the distance of the intersection of the exit ray with the axis of the system from the centre of the exit pupil. The values of these distances, as expressed in dioptres, for the series of rays for different entrance or exit heights and for different wave-lengths being known, the curves of spherical aberration can be traced. The aberrations of a number of binocular telescopes were measured to test this method. The results of the measurements for three telescopes are given in the paper. The data for one of these telescopes are given in more detail to show the attainable accuracy of the measurements. The results obtained by measurements with a round hole and round apertures were compared with those obtained by means of a slit diaphragm. Also the results obtained in focussing the system O D (infinity) and - 4 D were compared dagger. The discrepancies in both cases lie within the limits of error. The measurements of aberration of one of the telescopes were compared with the aberration curve obtained by means of computations for the other telescope of the same binocular. Two telescopes showed so-called axial astigmatism and axial coma. The presence of axial astigmatism and axial coma is caused by insufficiently careful adjustment and centering of the optical system of these telescopes.