# Position angles (Re: STC V1.21 posted as PR

David Berry dsb at ast.man.ac.uk
Fri Mar 18 06:58:23 PST 2005

```Alberto,

> A question about the position angle (as you said, it is source of
> confusion:-) ):
> By saying "North" does that mean that the pos angle is the angle
> that goes from North to the "Y" axis of the detector (projected onto
> the sky going through East) or to the "X" axis?
> I vote for the first one, in which case pos_angle = 0 means North == Y.
> Just to make sure...

A position angle is a way of describing the orientation of a line within a
coordinate system. You ask about "the angle that goes from North to the
"Y" axis of the detector" - this would be the position angle of the
detector Y axis within the "North/East" latitude/longitude coordinate
system (assuming you use a Reference attribute of "North").

My understanding is that if

1) Reference=North: The position angle is the angle from north to the line
of interest, measured positive in the same sense as rotation from north to
east.

2) Reference=X: The position angle is the angle from the X axis (i.e. the
first axis) of the associated coordinate system to the line of interest,
measured positive in the same sense as rotation from X to Y.

3) Reference=Y: The position angle is the angle from the Y axis (i.e. the
second axis) of the associated coordinate system to the line of interest,
measured positive in the same sense as rotation from Y to X.

Is that right Arnold?

David

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