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Figure 2.8: The ROSAT spacecraft coordinate frame. This virtual
right-handed frame is fixed to the spacecraft and the 
axis is aligned with the optical axis of the XRT in the
pointing direction, while the 
axis is perpendicular
to the solar panels in the anti-sun direction
[Hasinger1990a].
The aspect solution for a given observation is referenced to the
ROSAT (S/C) cartesian coordinate system ( 
).
This virtual right-handed frame is fixed to the S/C
(Fig. 2.8)
with the 
-axis aligned with the optical axis of the XRT
(and parallel to that of the WFC) in the pointing direction,
and the 
-axis perpendicular to the solar panels in the
anti-Sun direction.
In orbit, the ROSAT coordinate system is defined by the GSOC attitude
solution derived from the measurement of stars in the star tracker
FOV\
(Sect. 1.2.5).