The Photography of Groups of Galaxies

Abstract

The method of combining several negatives together is at last coming into use. Forty years ago this method was used for the first time by Leontovsky in Leningrad. Putting together many negatives he was able to do photometry of M31 to fainter magnitudes than was possible photoelectrically. The method of preliminary baking of emulsions is more effective when it is done in a nitrogen atmosphere and still better in hydrogen. Spectacular results with the last method were obtained by A. Smith (1977). With a 70m exposure on IIIa-J plates there are no traces of the planetary nebula the Helix. After hydronization the same exposure reveals the overexposed image of this object. Arp and Lorre (1976) obtained striking results on IIIa-J emulsions using the process of deconvolution which improves the negatives. By means of some kind of filtration, removing faint stars and those of medium brightness etc., they improved the resolution of the extended images and obtained a better contrast. The authors published their photographs of the Stefan group and of the jet in M87. As a result of deconvolution it becomes evidence that NGC 7320 is much nearer to us than its apparent neighbouring galaxies because it is clearly resolved into HII regions and star clusters.