10.2.1 Transformation to the Standard System

The ellipse fitting task (galfit) measures the position angle from the CCD x'-axis and counterclockwise. The standard way of measuring position angles is ``from north through east''. The task that calculates instrumental magnitudes (galmag) also attempts to transform the position angles to the standard system.

In our case, the CCD x'-axis points (approximately; see Sect.  below) towards east, and the CCD y'-axis points (approximately) towards south, so ``from north through east'' still means counterclockwise. However, instead of starting from the CCD x'-axis, we need to start from minus the CCD y'-axis. Therefore, in our case, galmag does the following transformation:

1.

${\rm PA}_{\tt galmag} = {\rm PA}_{\tt galfit} + 90^\circ$

2.

If ${\rm PA}_{\tt galmag} > 180^\circ$ then ${\rm PA}_{\tt galmag} = {\rm PA}_{\tt galmag} - 180^\circ$

${\rm PA}_{\tt galfit}$ denotes the position angles calculated by galfit (found in galaxy.prf.tab), and ${\rm PA}_{\tt galmag}$ denotes the position angles calculated by galmag (found in galaxy.mags.tab).