The azimuthal orientation of the SXT CCD is mis-aligned slightly with
respect to the Yohkoh spacecraft. Thus, if the spacecraft is oriented
so that the solar north direction is parallel to the ``pitch'' axis, in
the SXT images solar north will not be exactly aligned with the y-axis
of the CCD, but rotated a small amount counter clockwise. A useful
routine for displaying a diagram of the roll angle relationships is
HELP_ROLL. The precise roll angle was not measured prior to launch.
The value has been determined from in-flight measurements and continues
to be refined. It is approximately 1 degree. The value which is used
currently in the on-line software can be checked by typing:
IDL> print,get_roll(/sxt) ; Print SXT roll-angle (degrees)
When analyzing SXT images, one should be aware that the Yohkoh
spacecraft roll orientation varies slightly. GET_ROLL by default
returns the roll angle of SXT with respect to solar north, incorporating
the CCD roll-angle and spacecraft roll angle. GET_ROLL tries to read
the ATT database files if they are available (see the reference guide
for more information). The ATT database already has the correction for
seasonal variation that exists between the spacecraft's on-board definition
and the true solar north value. If the ATT database is not available, the
routine will return values based on a sinusoidal fit to the seasonal variations,
but which does not take into account the true S/C roll.