### ECH_XDISPFS

Determine the cross-dispersion in mm for a given wavelength

##### Description

Calculate the refractive index at the given wavelength. Then calculate it at the point where the cross-dispersion is by definition zero. Knowing the focal length of the camera, calculate the cross-dispersion in mm on the assumption that the light enters and leaves the prisms at the same angles of incidence (a first order approximation is used).

ECH_INIT must be called before calling this routine.

##### Invocation

CALLECH_XDISPFS (WAVE, GAMMA, DEL, STATUS)

##### Arguments
 READ WAVE REAL Wavelength of point at which cross-dispersion is to be calculated (Angstroms) READ GAMMA REAL Echelle gamma angle corresponding to the current configuration (radians) WRITE DEL REAL Cross-dispersion (mm) READ, WRITE STATUS INTEGER Global status value

None known.

##### Authors

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