SXT has three on-chip summation modes available: full resolution (FR), half resolution (HR), and quarter resolution (QR). The pixel sizes are 2.45, 4.90, and 9.80 arcsec respectively. A factor to consider when working with images is the proper adjustments necessary when changing resolutions. The corner address saved in the index are the IDL CCD coordinates of the center of the binned pixel in the lower left corner.
Fig. 17 on page 
shows some of the
details of the CCD pixels, and how the pixels are summed on the CCD.
Due to the camera design, the FR, HR and QR images are not perfectly
aligned which each other. There is a shift of up to 3 full resolution
pixels which needs to be accounted for when inter-comparing different resolution images.
There is also the age old question of whether to refer to the center of a pixel or to the corner of a pixel. We have decided to refer to the center of the pixel since it translates much better when working with fits between different resolutions.
See SXT Calibration Note 35 for a detailed discussion of the SXT CCD pixels and all of the various coordinate systems associated with the CCD: ground-based coordinate system using the camera system test set (CSTS), CCD readout pixel coordinate system, and others.