On-line analysis and compression of spectra-spectroheliograms

The on-line algorithm used by the NASA/National Solar Observatory (NSO) Spectromagnetograph (SPM) for analysis of long-slit spectrum-line profiles is presented with a view towards future application to processing large volumes of data from spacecraft and other remote instruments. At every spatial position, the procedure finds continuum intensity, equivalent width, and wavelength position and intensity of the central extremum of absorption or emission. The wavelength position of line center is found from the zero-crossing of the convolution of the line profile with a fixed anti-symmetric kernel. As currently used with the spectromagnetograph, images based on the line parameters computed at every spatial position are saved, but the original data are discarded. This paper explores the possibility of preserving the data in compressed form by saving, in addition to the derived line parameters, the differences between the data and model profiles synthesized from the on-line analysis.