`ECH_XDISPFS`

Determine the cross-dispersion in mm for a given wavelength

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.

`CALL`

`ECH_XDISPFS`

(`WAVE`

, `GAMMA`

, `DEL`

, `STATUS`

)

`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 |

C.J. Hirst `UCL`

(`ZUVAD`

::`CJH`

)