7.5 Wavelength calibration

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:

(7.4)

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 3.1.

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.