`ECH_FORMAT_CENTRE`

Determine the appropriate spectrograph settings

Firstly, calculate the prism system rotation and position. The required echelle gamma angle is not quite the same as the prism rotation angle. This is because of the 12 degree separation between the incoming ray to to the echelle grating and the outgoing ray to the camera - the prism rotation angle has to be multiplied by an empirical correction factor of 1.011 (I am not sure of the complete justification for this).

Having determined the appropriate echelle gamma angle, the cross dispersion is now correct and it is necessary to determine the appropriate echelle theta angle to give the correct wavelength in the centre of the field. If the order number was not explicitly specified, assume the order within which the given wavelength lies with the free spectral range. Theta is determined simply by stepping along the order and, once having stepped past the required wavelength, binary chopping until convergence.

Finally, determine the appropriate collimator to use and the necessary slit assembly rotation in order to give vertical slit images on the detector.

`ECH_INIT`

must be called before calling this routine.

`CALL`

`ECH_FORMAT_CENTRE`

(`WC`

, `MCEN`

, `THETA`

, `GAMMA`

, `COL_COLOUR`

, `PRISM_POS`

, `SLIT_ANGLE`

,
`STATUS`

)

`READ` | `WC` | `REAL` | Desired central wavelength (Angstroms) |

`READ` | `MCEN` | `INTEGER` | Desired central order. If |

`WRITE` | `THETA` | `REAL` | Echelle theta (radians) |

`WRITE` | `GAMMA` | `REAL` | Echelle gamma (radians) |

`WRITE` | `COL_COLOUR` | `CHARACTER` | Collimator ( |

`WRITE` | `PRISM_POS` | `REAL` | Prism position (mm) |

`WRITE` | `SLIT_ANGLE` | `REAL` | Slit angle (radians) |

`READ` , `WRITE` | `STATUS` | `INTEGER` | Global status value |

F. Diego `UCL`

(`ZUVAD`

::`FD`

)