Wavelength Calibration and Extraction

Wavelength Calibration

Now we have the following frames remaining in a temporal sequence

• 1 Ne = Ne1
• 1 ThAr = ThAr1
• some objects
• 1 Thar = ThAr2

We do wavelength calibration with ThAr1. After marking some peak in the central line with the approximate wavelength in Angstrom all other rows are searched for peaks and each row is calibrated in wavelength. As a result the ThAr lines are now straight and strictly upright. This calibration is silently applied to the other frames and all frames are rebinned to the uniform dispersion 0.05 Å/px (original disp is around 0.11 Å/px).

So we are done with wavelength calibration.

And what to do with the other calibration frames ThAr2 and Ne1, Ne2?
ThAr2 is needed for detect bending of the telescope mounted spectrograph. I do a crosscorrelation of ThAr1 to ThAr2 to see the drift in wavelength during recording the science frames. This drift is with my Lhires3 e.g. 0.04 Å.

And the neon frames? I like to play and find differences between different calibrations. The neon frames are calibrated with ThAr1 and not with themselves. I wonder how good this calibration is. To see this the wavelengths of the lines Ne1 are measured with a gauss fit and compared with the tabulated wavelength from NIST or whenceever. The wavelengths differ about ±0.03 Å. The rms-error of the eleven used lines by themselves in ThAr1 equals 0.007 Å.

Extraction

Now we dial only with the object frames. To get the true ADUs in the trace of spectrum we have to remove any background signal. This background contains light from the night sky, street lamps if you observe in the midst of a village like me, accumulated dark signal. This is found below and above the trace of the spectrum. A parabola is fitted in this regions to model the finally removed background signal.