DEFECTS OF SURVEYING TELESCOPES

Abstract

Now that the internal-focusing telescope is almost exclusively fitted to modern surveying instruments, there appears to be some misgiving on the part of many who, using older instruments, are inclined to believe that they are further handicapped by the obsolescent form of draw-tube, or external-focusing, telescope. Little contention can be raised against the claims of the makers, who fairly summarize the merits and demerits of the modern form, clearly stating the relatively small loss of light through the additional lens and the slight deviations from anallactism in the case of stadia observations. Thus, many of the younger generation of engineers and surveyors are aware of a “transition period” with a lack of more intimate knowledge of the original form of telescope; and, in this connexion, it should be remembered that the construction of draw-tube telescopes was carried to so high a degree of perfection that only the exigencies of lightness and compactness rendered possible by precision methods led to the adoption of the internal-focusing pattern. Wherefore, it appears that a few notes and observations might be interposed expediently at this belated stage, not from the designers' office, with detailed technical knowledge and experience at hand, but from the field engineers' point of view.