Published online by Cambridge University Press: 12 April 2016
Helios A was launched on December 10, 1974 into a highly elliptical orbit with a perihelion of 0.31 A.U., which was reached on March 15, 1975. The zodiacal light experiment on Helios, described in the preceding paper, worked flawlessly and provided the first observations of the zodiacal light from inside the Earth’ orbit. A typical example from the raw data of the 15° – photometer is shown in Fig.1. There is a strong intensity increase towards the sun and a remarkably flat intensity distribution at large elongations. The Milky way is superimposed on the zodiacal light at longitudes 135° to 180° and 315° to 360°. Due to the orbital motion of Helios the star background is being shifted with respect to the zodiacal light, which will facilitate its separation from the total observed intensity. The peak at the right side of Fig.1 is due to the star α CMi, which we intend to use for the calibration of the instrument in addition to the ground calibrations. A preliminary evaluation showed less than 20% difference between the two calibrations. This is typical for other stars and for the other photometers, too, and gives a safe upper limit for the accuracy of the absolute calibration. Temperature effects are comparatively small and have not been corrected so far.