10.2.2 Correction for the Slight CCD Misalignment

We found in the astrometry section (Sect. , p. ), that the CCD coordinate system was turned $+0.29^\circ$ with respect to the ideal plate coordinates. Let us denote this angle by $\gamma$, i.e. $\gamma = +0.29^\circ$. Because of this misalignment, galmag did not quite get us to the standard system, we are still $0.29^\circ$ off.

Figure  shows the setup. On the figure, an angle $\theta'$ is shown. $\theta'$ is measured from minus the CCD y'-axis counterclockwise, i.e. just as the position angles that galmag outputs. It can readily be seen, that the angle $\theta$, measured from the true north axis and also counterclockwise, can be calculated as

$$\theta = \theta' + \gamma \enspace .$$ (10.1)

Therefore, all the PA-profiles of the galaxies from all nights where corrected for this mean $+0.29^\circ$ CCD misalignment as

$${\rm PA}_{\rm corr} = {\rm PA}_{\tt galmag} + 0.29^\circ \enspace .$$ (10.2)