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
    ascension  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.
    After the point, decimal values are assumed.

    When a projection is active, the User  coordinates  are  assumed  to  be
    projected  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. For convenience (?), LIMITS may be  specified
    in  a  so-called  ANGLE_UNIT  which  can  be  set to Seconds, Minutes or
    Degrees. Note that only the numbers you give are assumed to be  in  this
    "ANGLE_UNIT" :  they  are immediately converted to the natural projected
    unit (equivalent to Radian in case of small field) by  division  by  the
    appropriate  scaling  factor.   That  is  all what this improperly named
    ANGLE_UNIT means.

    The TYPE can be
      - NONE            Disables the  projection  system.  User  coordinates
        then  loose  their interpretation in terms of projected coordinates.
        The ANGLE_UNIT is then totally ignored.
      - 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
        distorted  than  the  Gnomonic projection. This is an inversion 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
        "projection", in which X = (A-A0).COS(D0) and Y = D-D0. The Angle is
        obviously ignored.