Measurement of stray radiance in the High Altitude Observatory’s Skylab coronagraph

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Abstract
The stray radiance present in the field of view of the High Altitude Observatory’s Skylab coronagraph has been measured from results obtained in orbit with two techniques. First, employing images of the lunar disk made near the time of solar eclipse, a comparison of the apparent contrasts recorded by the coronagraph of maria and highlands with those of previous studies, allows the determination of the magnitude of the stray radiance and its variation along a radius. Next, the azimuthal (position angle) variation was found through measurement of the change in the net radiance of a point in the coronal image, as the image (coronagraph) was rotated. From these measurements the stray radiance was found to be a maximum near the inner limit of the instrument field of view, variable in azimuth from 1.4 to 5.6 × 10−9B⊙, where B⊙ is the mean solar disk radiance. Further, the stray radiance was found to be a steep, decreasing function of distance along a radius, falling to a nearly uniform, azimuthally independent value of ~3.5 × 10−10B⊙ in the outer field. In this latter region, the coronagraph stray radiance is well below a typical midtotality eclipse sky radiance of ~1.8 × 10−9B⊙. Finally, the radiance of lunar maria and highlands (in units of 10−10B⊙) was determined to be 2.2 and 4.5, respectively, for the 30 June 1973 eclipse and 1.4 and 3.0, respectively, for the 24 December 1973 eclipse, in agreement with other observers.