Investigation of wavelength calibration for an echelle cross-dispersion spectrometer

Abstract

The effects of ambient temperature and relative humidity on the positions of spectral lines on a two-dimensional focal plane were investigated. When environmental conditions change, a spectral line can move in both the horizontal (x) and vertical (y) directions. To characterize these changes, a position-shift vector for each spectral line was defined, and its magnitude in the horizontal Δλ_x and vertical Δλ_y directions determined as temperature (T) and relative humidity were systematically varied over a range of 24–34 °C and 31–51%, respectively. Spectral lines were examined that spanned a range from 250–570 nm. Overall, Δλ_x declines with increasing ambient temperature, whereas Δλ_y becomes larger. A change in relative humidity does not influence Δλ_x, but raises Δλ_y in an almost linear fashion. Both Δλ_x and Δλ_y were found to change with wavelength non-linearly. Position-shift equations involving Δλ_x and λ show they are related to the grating constants and the incident angle α of the echelle, and these involving Δλ_x and T are governed by the wavelength of a spectral line, the grating constant, the focal length of the spectrometer, thermal expansion coefficients of the grating and spectrometer, the ambient temperature and the diffraction angle of the echelle.