`ECH_DISP`

Determine the dispersion in mm for a given wavelength

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.

`CALL`

`ECH_DISP`

(`WAVE`

, M, `THETA`

, `GAMMA`

, `DEL`

, `STATUS`

)

`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 |

C.J. Hirst `UCL`

(`ZUVAD`

::`CJH`

)