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Spectroscopic Atlas for Amateur Astronomers 180

32 Terrestrial Light Sources

32.1 Spectra of Gas Discharge and Calibration Lamps

Gas discharge lamps play a key role for astronomers. They can be useful, for example to
calibrate the spectral profiles - but also represent a disturbing source e.g. by light pollution
from the road- and municipal lighting. For beginners they are also useful exercise objects to
study the spectra – particularly during cloudy nights. Further spectra and info to calibration
light sources see [32], [34], [35], [508].

Unfortunately, gas discharge lamps are operated with relatively high voltage. This requires,
especially for outdoor operation, ie minimal electrical knowledge of relevant safety meas-
ures such as isolation transformers, GFCI devices, or DC/AC Power Inverters (ie 12V
DC/230V AC).

Table 101: Neon glow lamps

The orange glowing neon glow lamps are used as indicator lights, e.g. for stoves, flat irons,
connection plug boards etc. They produce a large number of emission lines, mainly in the
red region of the spectrum. Their wavelengths are known with high accuracy. These proper-
ties make them very popular as calibration lamps among the amateurs. The disadvantage is
that the intense lines are limited to the red region of the spectrum. Further they can only be
operated with mains voltage of 230V, therefore posing a safety risk and requiring specific
electrotechnical safety measures. With very long exposures, especially in the green area
further emissions will appear, but coupled with the disadvantage that the most intense
neon lines in the red range of the spectrum become oversaturated at low to moderately
high-resolution spectrographs. Such lines can therefore no longer be used for calibration
purpose within the same profile. For the calibration of broadband spectra (red to blue) or
higher-resolved profiles within the blue range, low cost solutions according to Tables 106 –
108, or even better [35] are preferred. In the professional sector or on the senior amateur
level, relatively expensive hollow cathode lamps are used, producing a fine raster of eg
iron-argon or thorium emission lines.

Table 102: Energy saving lamp ESL Osram Sunlux

Amateurs often use ESL as a complement to the neon lamps, which are limited to the red
part of the spectrum. These lamps contain several gases and substances performing differ-
ent tasks – among other things, fluorescent substances, usually so called rare earth metals.
The mixture depends largely on the colour, the lamp should produce. Anyway for calibration
purposes useful are only the intense lines of the auxiliary gases, e.g. Argon (Ar), Xenon (Xe),
and Mercury (Hg). Unfortunately, some of these line positions are located very close to-
gether and are therefore difficult to distinguish, such as Ar II (6114.92) and Xe (6115.08).

Table 103: Xenon strobe tube

Better-suited for the broadband calibration of profile graphs, is the spectrum of a xenon
strobe light, e.g. kit K2601 from Velleman. This kit is designed primarily as position lamp
for model aircraft as well as for lighting effects on stages, dance floors, in shop windows
etc. The flicker frequency for the calibration purpose must be adjusted to the maximum and
the lamp requires some 15 seconds warmup time to produce accurately the specified lines.
This lamp generates some 50 useful lines, distributed over the entire range from the near
infrared to violet (about λλ 8,000 – 3,900). In the infrared domain further emissions are
available, but not documented here. Most emission lines are produced by xenon. However
the shortwave-end of the spectrum is dominated by lines of rare earths. The xenon tube
gets very hot during operation. Further it requires also mains voltage of 230V. Therefore an
appropriate housing is needed.