Tutorial- lesson 6 - Spectral measurements

Getting spectrum profiles correctly calibrated and corrected is only the first step. Spectroscopy gives access to astrophysical data and measurements which complete the spectral analysis.

To get the width of the line at mid-heigh (FWMH) you have to select the line and get into the computation dialog box

Among the different computation available, select the FWMH
The results will be displayed in the infos... window - you can save the content in a text file named infos.txt
The best example of the utility of such computation is the measurement of the speed expansion of the shell in a Nova spectrum.
The doppler shift is produced by the shell expansion, and the velocity of expansion is then related to the FWMH by the doppler equation:

delta(lambda)/lambda ref * c = V

c=speed light, 300 000km/s

lambda ref = 6562.63 angströms

with delta(lambda) = FWMH

The above equation gives:

V=2004km/s with an error of 100 km/s

At the same time, a UAI note indicated a speed expansion of 2050km/s...

Another set of computation can be useful to assess the quality of the spectrum by computing the Signal to Noise ratio
In this example, the values of the snr clearly confirms that the first spectrum has a much better quality with a greater SNR than the second one.
In spectroscopy, to measureand compare the line force, spectroscopists use the Width Equivalent which represent the power of the line. Again, the computation requires you to select the line yo want to measure
If you study spectra profile over time, you can display the header data then the WEQ (or LEQ) in the infos window, then save the results and build a graph that shows the WEQ evolution over time.
As most of the line selection is made manually, some errors can be driven by a non homogenous zone selection when comparing values from different spectra. Vspec has a console with some clasical commands which allows you to perform the same computation and indicates the exact wavelength domain where you want to make the measurement.
To get the list of the possible command and their parameter description, please refer to the Vspec user manual.

The help ? will display the command list

Below is shown a command sequence to compute the WEQ on different spectra with no mouse usage