Astronomy and Astrophysics – Astrophysics
Scientific paper
Feb 1986
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986a%26a...155..265g&link_type=abstract
Astronomy and Astrophysics, Vol.155, NO. 2/FEB(I), P. 265, 1986
Astronomy and Astrophysics
Astrophysics
2
Scientific paper
Our first paper established a general formula linking the aceuracy of the determination of the angular position of a star to the noise affecting the measurement. On applying this result to the situation where ouly the noise arising from the detection of source radiation modulated by the focal grid is present (Fig. I.1), we found that for the CERGA photoelectric astrolabe the measurement accuracy achievable with this first kind of noise represents an ultimate limit which can usually not be reached because of atmospheric turbulence. In this second part, we propose a study of this main deterioration of the signal. The analytic difficulties are circumvented by the use of some approximations which do not affect the physical significance of the results. In Sect. 3, devoted to weak turbulence, we successfully separate the effects of scintillation and those, notably more important, of agitation. In the expression for the latter we have found the contribution of the very low temporal frequencies whose effects have been suitably called "accidental refraction" by observers. Then, in Sect. 4, we examine the other extreme that is very strong turbulence. The theory developed explains the homogeneity of results noted by observers when this situation exists. There is no manageable analytic theory for the intermediate case. By simply connecting the two asymptotic situations, we show in a last paragraph where the atmospheric effects fall with respect to the effects of the detection noise previously studied. Finally, we deduce the criteria which should allow us to make the best choice for the focal grid of an astrometric instrument.
Gay Jean
Pochet Jean-Marie
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