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

Fri Mar 18 07:04:22 PST 2005

```That's right.
But I suspect the underlying issue is how transformations between
coordinate systems will be specified and that is really your domain.

- Arnold

David Berry wrote:
> 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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Arnold H. Rots                                Chandra X-ray Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 67                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head.cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
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