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C.G. Toner (NSO/GONG)
In order to combine data from the six instruments in the GONG network the alignment of all of the images must be known to a fairly high precision (~0\arcdeg.1 for GONG classic and ~\arcdeg.01 for GONG+). The relative orientation is obtained using the angular cross-correlation method described by Toner and Harvey (1998). To obtain the absolute orientation the Project periodically records a day of drift scans, where the image of the Sun is allowed to drift across the CCD repeatedly throughout the day. These data are then analyzed to deduce the direction of Terrestrial East-West.
The transit of Mercury on Nov. 15, 1999, which was observed by three of the GONG instruments, provided an independent check on the current alignment procedures. Here we present a comparison of the alignment of GONG images as deduced from both drift scans and the Mercury transit.
The Global Oscillation Network Group (GONG) is managed by the National Solar Observatory, which is operated by AURA, Inc. under a cooperative agreement with the National Science Foundation.