Stage 3: Wavelength Calibration

You'll have noticed that the data allready have a wavelength scale. This is approximate - it's from an estimation based on the cgs4 motor positions. You should now do an accurate wavelength calibration using the arc spectra you took.

Look at the nightlog file and find the _wce frame for the arc-lamp observation you're going to use. This will normally have been taken shortly before the 1st standard star at that confiuration. If several are availiable, use the one at closest telescope position to where your target was observed, to minimise effects from instrument flexure.

Extract a spectrum from the arc frame, using the same rows you used for the target frame main-beam. If you did an optimal extraction, use the same profile as you did for the main-beam.

Use the arc-lamp maps on the web to identify the lines in your arc spectrum. Use the wavelength calibration routine in your data reduction software. The principle of these routines is that you tell the software the exact wavelength of a number of lines. The software measures the position of the peaks of these lines and comes up with a function to relate the wavelength to pixel number. You should use as low-order function as gives a reasonable fit. A 3rd order polymonial is usually sufficient, though 5th order is sometimes useful, especially if you have a large number of identified lines in your arc spectrum.

Apply the calibration you've just generated to the target and standard star spectra you extracted earlier.