The Reduction of Radio Source Positions to Standard Epoch

Abstract

The usual method of calculating corrections for precession nutation and aberration involves the use of the precession constants M, N, m, n and the Day Numbers A, B, C, D, E, J, J’. For the computer analysis of radio source data obtained with the Molonglo Telescope it seemed desirable to compute the corrections directly from the basic formulae, thus avoiding the trouble and possibility of accidental error in copying out the Day Numbers for each day’s observation. In this paper we quote in full the equations that are used in the analysis of the Molonglo data. These equations are abstracted from The Explanatory Supplement to the Astronomical Ephemeris and Smart’s Spherical Astronomy, and expressed in a form which parallels the organization of the computer routine. In addition to the observed position, the only other data required to compute the precession nutation and aberration corrections is the Julian date (J.D.). This basic parameter in the equations is very simply calculated to the required accuracy of a few minutes from the time and date of the observation. The computer routine that computes the corrections occupies 1350 words of programme in the KDF9 computer and takes 200 msec to compute one position. If a large number of computations are being carried out on continuous data, then the average time per computation is only 5 msec as a full computation of the coefficients does not have to be made for each point separately.