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The GONG project will use a weighted average based upon the Modulation Transfer Function (MTF) in order to combine image data from a six-site network into a single time series of spherical harmonic coefficients for helioseismic analysis. We present an overview of the MTF merge method, including a capsule description of its theoretical basis, a summary of the method, application to merging time series of images from different sites in the network, and the limits of its applicability encountered during testing.
In order to test the method, an artificial data set of selected helioseismic modes was computed. From this "perfect" data, a degraded set of data including atmospheric, day length and instrumental effects was created to model the anticipated data from the six-site network. The results of tests comparing time series and mode frequencies from 18 days of the "perfect" data set and the set of site days merged into a single time series support the use of this algorithm for the GONG project merge.
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