The user can modify directly the header variables whose names are given below. The file is modified when typing the command HEADER /UPDATE .
G_VELOFF REAL Velocity at reference pixel G_VELRES REAL Velocity increment G_RESTFRE DOUBLE Rest frequency at reference pixel G_FREQOFF DOUBLE Image frequency at reference pixel G_FREQRES DOUBLE Frequency increment G_LINE CHARACTER *12 Line name G_BII DOUBLE Galactic latitude G_LII DOUBLE Galactic longitude G_DEC DOUBLE Declination G_RA DOUBLE Right ascension G_EPOCH REAL Epoch of equatorial coordinates G_SOURCE CHARACTER *12 Source name G_COORD CHARACTER *12 Coordinate system G_UNIT CHARACTER *12 Map units G_UNIT4 CHARACTER *12 4th axis type G_UNIT3 CHARACTER *12 3rd axis type G_UNIT2 CHARACTER *12 2nd axis type G_UNIT1 CHARACTER *12 1st axis type G_EXTREMA INTEGER [ 2, 4] Position of extremax (min, max) G_MIN REAL Value of minimum G_MAX REAL Value of maximum G_BLANK REAL [ 2] Blanking and tolerance values G_CONVERT DOUBLE [ 3, 4] Pixel to user coordinate conversion G_DIM INTEGER [ 4] Dimensions G_NDIM INTEGER Number of dimensions
The pixel to user coordinate conversion for a given Axis is
Val(Pixel) = (Pixel-G_CONVERT[1,Axis])*G_CONVERT[3,Axis]+G_CONVERT[2,Axis]and the user coordinate to pixel
Pixel(Val) = NINT(Val-G_CONVERT[2,Axis])/G_CONVERT[3,Axis]+G_CONVERT[1,Axis])