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Analysis and Interpretation of Astronomical Spectra 37
8.12 Absolute Flux Calibration [erg s-1 cm-2 Å-1] F
As a final step of the radiometric correction, the intensity axis Fλ
could be absolutely calibrated in physical units for the spectral
flux density – normally [erg s-1 cm-2 Å-1]. This calibration is very [erg s-1 cm-2]
challenging, time consuming and just needed by some special
sectors of the professional astronomy. It is relatively common for λ1 λ2 λ
spectra, recorded by space telescopes, which of course at least
remain clean of any atmospheric influences [301].
For amateur applications an acceptable accuracy of the results
is usually prevented already by the inadequate quality of the ob-
servation site. Thus even in the professional sector, such abso-
lutely flux-calibrated spectra can be found rather rarely.
The total flux of an emission line, here in a simplified form di-
rectly drawn on the wavelength axis, corresponds to their area,
[302]. The unit for the total flux of the line is [erg s-1 cm-2].
If the emission line is superimposed on a continuum, the continuum flux must
be subtracted from formula . However, there exists no easy way to determine the real
original undamped flux of an emission line (sect. 8.14).
This process is based on the comparison of the absolute calibrated radiation flux of a stan-
dard star. However, many additional data are required here, such as the exposure time of
the spectral recordings. Further, the recording usually requires a large slit width to measure
really the total flux of the object.
8.13 Intensity Comparison between Different Spectral Lines
The intensities of two different lines can be compared in normalised profiles with
their equivalent widths . However if the widths of the lines are not too different, a rough
comparison is also feasible with the peak intensities .
8.14 Reconstruction of the Original Emission-Line Intensities
Most of the emission nebulae (sect. 22), don’t show any evaluable continuum. But even in
spectra with existing continuum (eg P Cygni), the original values of the emission lines
in , can never be reconstructed with the already presented, proportional-radiometric
procedures according to sect. 8.10 to 8.12. At least for amateurs here just remains the ar-
ithmetical scaling of the individual, attenuated emissions, proportionally up to the known
line ratios of the undampened Balmer-Decrement for the hydrogen emissions (sect. 20 and
21). These corrected intensitiy ratios allow now to perform the appropriate procedures, eg
for plasma diagnostics (sect. 22), with higher accuracy. Anyway with the extinction at
, according to sect. 21.1, formula , the original, absolute values of the -
emissions could roughly be estimated.
