PRECISE DETERMINATION OF THE COORDINATE SYSTEMS FOR THE YOHKOH TELESCOPES AND THE APPLICATION OF A TRANSIT OF MERCURY

The Yohkoh solar X-ray observatory carries two telescopes that require coalignment at a level better than the minimum pixel size of 2.45 ''. This coalignment is needed both internally within Yohkoh and for many scientific applications involving data from ground-based radio and op tical observatories. We describe the methods successfully developed fo r this purpose and now incorporated in the Yohkoh software. Soft X-ray observations of the 1993 transit of Mercury across the solar disk pro vided key information for the calibration of the coalignment procedure s.