The optical axis of the X-ray telescope has been determined from pre-launch measurements of the effective area as a function of off-axis angle. The analysis is described more fully in SXT Calibration Note 36 (R. Fuller, Jan-94). The relative offset between the X-ray optical axis and the optical axis in the Wide band or Narrow band filters was determined by Tom Metcalf (22-Oct-92). The combined results are summarized below:
Optical Axis (CCD Full-Res Pixels)
x y
X-ray 515 633
Narrow Band (NaBan) 514.64 634.47
Wide Band (WdBan) 515.24 634.26
The values can be quickly acquired on line using GT_SXT_AXIS. The next
three lines will reproduce the values in the table above. Note that the
routine also accepts an index or roadmap data structure and that the filter
will be determined from the SXT structure.
IDL> print,gt_sxt_axis(0) ; 0=X-ray
IDL> print,gt_sxt_axis(1) ; 1=NaBan
IDL> print,gt_sxt_axis(2) ; 2=WdBan
IDL> print,gt_sxt_axis(1,/rel) ; NaBan relative to X-ray
The sign convention can be understood if you remember that the NaBan image is
Northeast of the X-ray image.
The X-ray optical axis has a somewhat large uncertainty, since the effective area function is broad near the center of the field of view of the telescope. The relative precision between the optical and X-ray axes has been determined to better than 0.4 Full-Res pixels (see the Instrument Guide).
The /rel_xray keyword option can be used to return the relative offset to the X-ray axis.