### ECH_DISP

Determine the dispersion in mm for a given wavelength

##### Description

Use the grating equation to calculate the echelle beta angle at the given wavelength in the given order. Then calculate it at the point where the dispersion is by definition zero. Knowing the focal length of the camera, calculate the dispersion in mm.

ECH_INIT must be called before calling this routine.

##### Invocation

CALLECH_DISP (WAVE, M, THETA, GAMMA, DEL, STATUS)

##### Arguments
 READ WAVE REAL Wavelength of point at which dispersion is to be calculated (Angstroms) READ M INTEGER Order number in which this wavelength lies (it doesn’t have to be within the free spectral range) READ THETA REAL Echelle theta angle corresponding to the current configuration (radians) READ GAMMA REAL Echelle gamma angle corresponding to the current configuration (radians) WRITE DEL REAL Dispersion (mm) READ, WRITE STATUS INTEGER Global status value

None known.

##### Authors

C.J. Hirst UCL (ZUVAD::CJH)