Microspots on IIIa-J Plates (“Gold Spot Disease”)

Abstract

The UK 1.2 metre Schmidt Telescope at Siding Spring Observatory, Australia, has taken about 8500 plates since it was commissioned in 1973. A large fraction of these plates, including those for the southern and equatorial Sky Surveys, are on Eastman Kodak IIIa-J emulsion. A significant number of these processed IIIa-J plates have developed many small gold spots similar in size to faint star images. They are most likely to occur in areas of relatively high density, such as in bright star images or on the stepwedges, and are sometimes scattered around in non-image areas (fig. 1). An alarmingly high proportion of plates taken in 1974–75 (about 50 per cent) have developed gold spots, and the most recent affected plate found so far is J6517, taken in October 1980. Four early IIIa-F plates have also been found to have gold spots. The degree to which plates are affected may be slight, with spots appearing only on one or both stepwedges; moderate, with spots appearing along one, two, or three edges of the plates; or severe, when spots appear all round the edges or all over the plate. The affected plates are divided approximately equally between these three categories. It seems that the spots take about 3 years to develop, after the plate has been exposed and processed. This makes it very difficult to establish possible causes of the problem, which is almost certainly not curable.