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Moreover, in long-slit spectroscopy variations in throughput appear along the slit. In order to correct for this effect, calibration images are obtained using twilight sky to obtain even illumination (Figure 9.4). At least a moderate SNR is required.

9.3.3 Wavelength calibration At this point, the spectrum's dispersion axis will be measured in pixels that should be transformed into wavelength units. The procedure used to do this involves the observation of calibration arc spectra during the observing run. The idea is to match the pixel scale of the calibration arc lamps with their known wavelengths and then to apply this scaling procedure. In practice, we first identify features in a one-dimensional spectrum of the arc, repeating the process at every spatial position, and finally we fit a surface that transforms

Figure 9.5. An arc lamp (left) and plot of the raw spectra of the same frame (right). In both images, we can see emission lines of the Cu-Ar-Ne lamp that will be used to transform pixels to wavelength. Units are as in Figure 9.2.

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