A scanning microphotometer with an on-line data reduction for large field Schmidt plates

The mechanical and electronic device of the microphotometer is described. The on-line data compression is performed in the domain of the HAAR transform of the measurement points. This method provides a fast treatment of the measurement data in the computer and changes the large data material sampled in equidistant steps whose width can be chosen by the seeing condition into a non-equidistant grid whose widths depend only on the characteristic length of the signal. The grain noise is eliminated. An automatical discrimination between stars and galaxies is possible. The accuracy of the photometry was tested by comparing with the photoelectrically determined B-brightness distribution of M 81.