Create a wavelength scale

The following requires a spectrograph with a turnable grating.

By changing the angle of the grating to the optical axis of the collimator the wavelength range is adjustable. The angle of incidence must be adjustable for you by a control device. In the simplest case with a long pointer to the rotational axis and an arbitrary circular arc angular scale, as I had realized it on my "mice Villa". More comfortable and stable that manages a micrometer screw, which rotates on a suitable mechanism, the grating sensitivly and reproducibly (eg LHIRES III).

The calibration is done with a calibration lamp. The common used neon glow lamp costs a few cents, is operated on 230 V and emits a faint orange-red light. These lamps do you know for example from the old 230-voltage detectors (within the gripping section of a screwdriver).

On the left side see the neon spectrum, that I still use as a standard.

An Excel spreadsheet with all Neon emission lines you can get by click on the link. There are, however, not all lines in the emission spectra of glow lamps visible! Diagram left side in practice so far is to use better.

The neon lines can be also in VSpec identified very easy.


The slit of the spectrograph will light upwith the neon glow lamp. For slitless spectrographs must be used a pinjole in the focus of the collimator (artificial star). The pinhole should be in slitless spectrograph as possible have the size of the star picture in the focus of the telescope (or smaller) so that the achieved resolution is not lower as in the measurements of star spectra. I have used a 5 µm pinhole in my "mice Villa". With this artificial star in the focus of the collimator, I was able to get very sharp neon points on the CCD.

We obtain in this manner the adjacent shots. The emission lines of neon gas in the calibration lamp appear as sharp peaks with a half maximum (FWHM), which corresponds in the case of a Littrow spectrographs because of its 1:1 mapping to the diameter of the pinhole or slit (expressed in pixels). On the basis of the known emission spectrum of such Neon glow lamps we can assign the pixel numbers of the individual lines (peaks) the concrete wavelengths. This happens as a normal routine with a data reduction method of a software like VSpec or MIDAS. In the left side shown spectrum a dispersion of 0.384 angstroms / pixel is found. The CCD camera was a Audine with KAF 401ME CCD chip, which has 9um x 9um pixels.

To cover the wavelength range from 5800 to 7000 angstroms, produce many of these recordings, where the grating angle is adjusted so that "adjacent" spectra show about 1 / 3 overlap. Obove is shown an example from my early practice with a crude home-made "pinhole" in the form of a pierced aluminum foil. So you can move hand over hand the lines of the neon spectrum. With a little practice you will recognize fastly the pattern of lines (distance and intensity right) .

Having thus established a connected series of calibrated neon spectra, I filled out an Excel spreadsheet that represents me graphically illustrated the range of wavelengths and the dispersion in each case as a function of wavelength. If you want to use this Excel sheet, you can download it here.

The adjacent calibration graph for the 2400 lines / mm grating and a CCD chip with KAF 401ME (Audine) shows the following:
The two parallel lines represent the first and last pixels in the dispersion direction (the gap is about 100 angstroms wide (wavelength window)). The orange line refers to the right scale, is therefore the function of grating position (expressed in mm reading on the micrometer screw of the LHIRES III), and the measured dispersion.
What do I do with this graph? An example: I want to measure the Na doublet D1, D2 at about 5900 angstroms. The 5900 position corresponds to a grating position of of 17.8 mm (on the micrometer screw). II adjust the micrometer screw and already.... I have the desired lines on the CCD chip. The expected dispersion amounts to 0.132 Angstr. / Pix. . This graphs (depending on grating and CCD), I use so often that I always had it in the observatory at hand. When changing the grating, the CCD or other optical components of the spectrograph, a new calibration must always be carried out.

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