The astronomical quantity of interest is an array of flux as a function of wavelength. Hence, a relationship must be established between each grating scanner position (measured by a Linear Voltage Differential Transducer (LVDT) and colloquially given the units `LVDT') as contained in the ERD and its corresponding wavelength. The relation is calculated in two steps: first the scanner position is converted to a scanner angle using a calibration file. Then the following equation can be applied to calculate the wavelength of light falling on the detectors:

where,

- is the angle from the optical axis to the entrance or light source used, as seen from the collimator,
- is the angle from the optical axis to the detector pixel,
- is the angle of incidence on the grating for a beam entering along the optical axis for the current scanner position,
*D*- is the grating constant ,
*N*- is the spectral order .

and *D* are constants, taken from calibration file
16 . The spectral order *N* is determined by calculating all the
orders that give wavelengths consistent with the transmission limits of the
band/order/aperture/filter combination. For valid observations only one order
should give a wavelength that satisfies this criterion.

The wavelength range - band/order/aperture/filter combination relationship is shown in Table 7.3.

For the Fabry-Pérot , first the grating wavelength is calculated. Calibration file 12 then transforms the nominal FP-position (an engineering parameter of no interest to the observer) into a FP-gap . This mechanical gap width must be corrected to get the optical gap width. The wavelength which passes through the FP is the one of which a whole multiple fits exactly in two times the optical gap width.

band | grating | entr. | det. | Wavelength | |

order | apert. | array | range | ||

1A | S4 | 1 | 1 | 2.38 - 2.61 | |

1B | S3 | 1 | 1 | 2.60 - 3.03 | |

SW | 1D | S3 | 2 | 1 | 3.02 - 3.53 |

section | 1E | S2 | 2 | 1 | 3.52 - 4.08 |

2A | S2 | 2 | 2 | 4.05 - 5.31 | |

2B | S1 | 2 | 2 | 5.30 - 7.01 | |

2C | S1 | 3 | 2 | 7.00 - 12.1 | |

3A | L2 | 1 | 3 | 12.0 - 16.6 | |

LW | 3C | L2 | 2 | 3 | 16.5 - 19.6 |

section | 3D | L1 | 2 | 3 | 19.5 - 27.6 |

3E | L1 | 3 | 3 | 27.5 - 29.0 | |

4 | L1 | 3 | 4 | 28.9 - 45.2 | |

FP | 5A | L3 | 1 | 5 | 11.4 - 12.2 |

5B | L2 | 1 | 5 | 12.2 - 16.0 | |

5C | L2 | 2 | 5 | 16.0 - 19.0 | |

5D | L1 | 2 | 5 | 19.0 - 26.0 | |

6 | L1 | 4 | 6 | 26.0 - 44.5 |

K. Leech with contributions from

the SWS Instrument Dedicated Team (SIDT)

and the SWS Instrument Support Team (SIST)