Observational Errors. Instrumental Equation

Abstract

With the Sproul telescope the differential method i. e. central relative to reference stars, yields a precision of about 1μ or 0″.02 for the average of two exposures. By taking up to five exposures per plate and up to four plates on any one night a precision of 0.5 μ or 0″.01 may be reached. And by combining several nights within one year into a normal point an accuracy of 0.3 μ or 0″.005 — by a supreme observational effort as low as 0.1 μ or 0″.002 — may be approached. It appears difficult if not impossible to reduce the error any further; the law of diminishing returns indicates the existence of a ‘year error’ of about 0.1μ or 0″.002 (Chapter 5).