15 - SPECTRAL CALIBRATION

In the following figure (red part of Vega's spectrum), the spectral profile is typical of what is available after extraction. This spectrum has been obtained with a 600 grooves/mm grating for an approximate resolution of R=1500.

An important step is to replace pixel numbers by wavelengths along the X axis. This operation is called spectral calibration. The line in the center (around pixel number 360) is the Ha line. The line in the neighbourhood of pixel number 550 originates in the terrestrial atmosphere (O2). It is in fact a molecular band. Note that fine structure can be detected with our spectrograph (read further on), whose resolution power is about 5A.

With a classical slit spectrograph, calibration is done by injection through this slit of light from a lamp containing a gas having emission spectral lines. Since those lines' positions are well known, it becomes possible to link pixels numbers to wavelengths (in first approximation the relation is a linear one). In the following figure, a spectrum from a neon lamp is shown. Such lamps are interesting for our purposes since they can be found for a low price at electrical hardware shops (they are used as indicators in industrial equipment, and their light is noticeable for its reddish tint).
3417.9035    NeI 
3472.5711    NeI 
3515.1900    NeI 
3593.5263    NeI 
3600.1691    NeI 
4488.0926    NeI 
4636.125     NeI 
4837.3139    NeI 
5005.1587    NeI 
5031.3504    NeI 
5104.7011    NeI 
5113.6724    NeI 
5144.9384    NeI 
5188.6122    NeI 
5330.7775    NeI 
5341.0938    NeI 
5360.0121    NeI 
5400.5617    NeI 
5562.7662    NeI 
5656.5664    NeI 
5689.8163    NeI 
5719.2248    NeI 
5748.2985    NeI 
5764.4188    NeI 
5804.4496    NeI 
5820.1558    NeI 
5852.4878    NeI 
5881.895     NeI 

5944.8342    NeI 
5975.534     NeI 
6029.9971    NeI 
6074.3377    NeI 
6096.1631    NeI 
6128.4499    NeI 
6143.0626    NeI 
6163.5939    NeI 
6217.2812    NeI 
6266.495     NeI 
6304.789     NeI 
6334.4278    NeI 
6382.9917    NeI 
6402.246     NeI 
6506.5281    NeI 
6532.8822    NeI 
6598.9529    NeI 
6678.2764    NeI 
6717.043     NeI 
6929.4673    NeI 
7024.0504    NeI 
7032.4131    NeI 
7173.9381    NeI 
7245.1666    NeI 
7438.899     NeI 
7488.8712    NeI 
7535.7739    NeI 
8136.4057    NeI
 

 8300.3263    NeI 
8377.6065    NeI 
8495.3598    NeI 
8591.2583    NeI 
8634.647     NeI 
8654.3831    NeI 
8655.522     NeI 
8679.493     NeI 
8681.921     NeI 
8704.111     NeI 
8771.656     NeI 
8780.621     NeI 
8783.75      NeI 
8830.907     NeI 
8853.867     NeI 
8919.5007    NeI 
9148.672     NeI 
9201.759     NeI 
9300.853     NeI 
9326.507     NeI 
9425.379     NeI 
9486.68      NeI 
9534.163     NeI 
9665.424     NeI 
10798.12     NeI 
10844.54     NeI 
11143.02     NeI
Neon Spectral Line List
 
Of course, the star's proper lines can be used as a spectral reference if they are identified beyond doubt. The following image shows the spectral signature for Arcturus (type K2III) in the blue part of the spectrum. The H et K lines for ionized calcium are easily recognized because of their mere intensity.
When there is no slit and their are few stellar lines, scaling must be done using the telluric lines available in the atmosphere. In the following figure the red part of Vega's spectrum is shown, enhanced by the main atmospheric bands in the near infrared.
Note that these atmospheric bands have a rather complex fine structure, and that one should be fairly prudent about their use. The next figure shows the resonance spectrum for O2 et H2O, extracted from a spectral databank for the earth's atmosphere.