PROJECTION

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PROJECTION

            [GREG2\]PROJECTION [A0 D0 [Angle]] [/TYPE Ptype]

        Define a projection of the (celestial) sphere  from  point  of  (A0,D0),
        (which  are  Longitude and Latitude respectively) of the specified type.
        Angle is the angle between the Y axis and the North pole.  The  previous
        values  are kept if no argument is specified. All angles are in degrees,
        except if the SYSTEM is EQUATORIAL in which case A0 is the right  ascen-
        sion  and  must  be  specified  in  hours.  Formats  like -dd:mm:ss.s or
        hh:mm.mmm in sexagesimal notation up to the point field are allowed. Af-
        ter the point, decimal values are assumed.

        When a projection is active, the User coordinates are assumed to be pro-
        jected coordinates of the sphere, and hence in the case of  small  field
        of  view  where distortion are negligible, correspond to angular offsets
        MEASURED IN RADIANS. The field of view of the projection is  defined  by
        command LIMITS.

        The TYPE can be
          NONE          Disables  the  projection  system. User coordinates then
                        loose their interpretation in terms of projected coordi-
                        nates. The ANGLE_UNIT is then totally ignored.

          CARTESIAN     Cartesian projection with linear coordinates in both di-
                        rections, with a possible projection angle.

          GNOMONIC      Radial projection on the tangent plane. Being  R  and  P
                        the  (angular)  polar  coordinates  from  the projection
                        point (tangent point),  the  projected  coordinates  are
                        given by X = Tan(R).Sin(P) and Y = Tan(R).Cos(P) .

          ORTHOGRAPHIC  View   from   infinity.   X  =  Sin(R).Sin(P)  and  Y  =
                        Sin(R).Cos(P)

          AZIMUTHAL     Spherical offsets  from  the  projection  center.   X  =
                        R.Sin(P) and Y = R.Cos(P).

          STEREOGRAPHIC Uses  Tan(R/2)  instead of Tan(R), and is thus less dis-
                        torted than the Gnomonic projection. This is  an  inver-
                        sion from the opposite pole.

          LAMBERT       Equal    area    projection.   Projected   distance   is
                        2*Sin(R)/Sqrt(2*(1+Cos(R)).

          AITOFF        Equal area projection. Angle and D0 are ignored.

          RADIO         The standard radio astronomy single dish  mapping  "pro-
                        jection",  in  which X = (A-A0).COS(D) and Y = D-D0. The
                        Angle is obviously ignored.

          SFL           Sanson-Flamsteed projection, similar to the  RADIO  pro-
                        jection  with  one  exception:  X = (A-A0).COS(D-D0) and
                        Y =D-D0. Beware that an Euler rotation of the  celestial
                        sphere  is necessary as an additional step, i.e. (A0,D0)
                        and (A,D) here are not the  absolute  celestial  coordi-
                        nates (see Calabretta & Greisen 2002).

          MOLLWEIDE     Equal  area  projection.  Angle  and D0 are ignored. The
                        projection trades accuracy of angle and shape for  accu-
                        racy  of  proportions in area, and as such is used where
                        that property is needed, such as maps  depicting  global
                        distributions.

          NCP           North  Celestial Pole. Projection to a plane perpendicu-
                        lar to the pole. Used by the WSRT.