Radiance calibration of the High Altitude Observatory white-light coronagraph on Skylab

The High Altitude Observatory coronagraph produced over 35,000 photographs of the solar corona over 9 mo. Images were obtained on Kodak special film type 026-02, a 3400 (Pan-X) emulsion modified by Kodak for minimal reciprocity losses at the exposure times (3 sec, 9 sec, 27 sec) used on the coronagraph. The film was processed in a specially developed chemistry to obtain the best compromise between speed, low fog, and modulation transfer function. The calibration of a nineteen-step wedge within the coronagraph is based on previously calibrated glass opal filters. The step wedge, illuminated by attenuated sunlight, is imaged on each photograph made by the instrument. Data reduction procedures employ average characteristic curves for each data set (approximately 4000 frames). It is found that the effects of radiation fog and latent image loss are negligible within these sets. The relative coronal radiance error, determined by measuring the coronal plus stray light radiance over the solar pole, is found to be le s than 8%. Based on an estimated error of 15% in the absolute calibration of the step wedge, the net absolute accuracy of a given radiance measurement is estimated to be 20%.