Part III – Analysing spectra using Visual Spec


Updated 2017 January 14




Spectroscopy, Part I – Using the SSON Transmission Grating Spectrograph


Spectroscopy, Part II – Generating a profile using Visual Spec


Spectroscopy, Part IV - Resources


Spectroscopy, Part V – The Miles catalogue of spectra


The Visual Spec website at http://www.astrosurf.com/vdesnoux/ provides access to tutorials on all aspects of using Visual Spec. A User Manual is available via the ‘?’ on the Menu tool bar.


The objectives of this tutorial are to;

- identify spectral lines

- measure resolution

- determine the temperature of a body (star in this case);

- export data to an Excel spreadsheet

- import data

- work through an exercise to identify lines in a spectrum of Mintaka (Delta Orionis) imaged with the SSON telescope (image ID 401419 which is available here)

  on 2015 January 14

- using a standard star


Identification of spectral lines


Aids to identification


Spectral types


Figure 1. Spectral type. Credit Wikipedia




Max Wavelength



40,000 K

72.5 nm



20,000 K

145 nm

Light Blue


10,000 K

290 nm



7,500 K

387 nm



5,500 K

527 nm



4,000 K

725 nm



3,000 K

966 nm



Table 1. Spectral classification. Credit Astronomy Education at the University of Nebraska-Lincoln



Wavelength (Angstroms, Ε)


2800 – 4500


4500 – 4950


4950 – 5700


5700 – 5900


5900 – 6200


6200 - 7500


Table 2. The visible spectrum


Balmer Hydrogen line

Wavelength (Angstroms)










Table 3. Fraunhofer lines


Telluric lines


Figure 2. Tellluric lines. Credit Spectroscopic Atlas for Amateur Astronomers by Richard Walker


Spectral types II


Figure 3. Typical stellar spectra. Credit Spectroscopic Atlas for Amateur Astronomers by Richard Walker


Identification of spectral lines


Image used is 1_330900 of Theta Aurigae obtained on 2015 November 5 using the SSON robotic telescope and obtainable from here. Drag the cursor across the line of interest and then right click the mouse. Repeat for the other (Hydrogen in this case) lines. In each case the wavelength of the line in Angstroms is displayed adjacent to the line, Figure 4. Table 3 lists the relevant lines.



Figure 4. Hydrogen lines labelled


Using the Elements library


The lines in the spectrum can be identified by using the elements library;

- open a wavelength calibrated profile

- select Tools/Elements

- in the Elements section in the bottom right hand corner of the Elements window scroll down to and select H(ydrogen) and then Sort. Data for the Hydrogen lines is

  shown in Figure 5.


Figure 5. Elements


We know from comparison of this profile with that in the SSON TGS User’s Guide which the Hydrogen lines are. Moving the cursor to the line just to the left of the peak of the profile and clicking the left mouse button highlights the relevant line in the list. In this case the Hydrogen Beta line at 4861 angstroms, Figure 6.


Figure 6. Selected Hydrogen line


Generating a synthetic spectrum


Having selected the Hydrogen lines in the Elements window click on Export and a synthetic spectrum will be overlaid on the original, Figure 7.


Figure 7. Synthetic spectrum overlaid on original


Planck temperature


The outward appearance of stars depends more strongly on the underlying continuous spectrum coming from the inner parts of a star than the absorption at its surface. Continuous spectra for stellar interiors at different temperatures are described by Planck Curves shown in the Figure 8. It can be seen that as the temperature increases the total amount of light energy produced (the area under the curve) increases and the peak wavelength (the color at which the most light is produced) moves to shorter, more energetic wavelengths.


Planck temp


Figure 8. Planck curves. Credit Astronomy Education at the University of Nebraska-Lincoln


To determine the temperature of a body (star in this case);

- open the corrected profile (23 Andromedae)

- select Radiometry/Planck

- vary the temperature in the window until the slope of the two profiles match (the vertical scales may need adjusting), Figure 9

- the chosen temperature is that of the body observed – 7600 K in this case which would appear to be about right



Figure 9. Measurement of Planck temperature




The true image of a star is a pinpoint of light but telescope optics and atmospheric seeing spread this into a disk. Similarly a spectral line may only be less than an angstrom in width but this is spread into a Gaussian like curve of, in this example, 50+ Angstroms in width at half maximum.


Resolution, R, is the ability to detect differences between very close features (lines) in a spectrum. It is defined as R = λ/Δλ where;

- λ is the wavelength

- Δλ is the minimum distance between two features which are resolved


The Full Width Half Maximum (FWHM) of a line is equal to Δλ and can be measured using VSpec. To measure the FWHM

- drag the cursor across the line in question

- select Spectrometry/Gaussian fit

The line centre wavelength and FWHM for the two lines on the left (blue end) of the profile are displayed in the Infos window - Figure 10. The Resolution is thus;


R = 4359/57 = 76

R = 4847/56 = 87


Resolution should not be confused with the plate scale or dispersion which is the number of Angstroms per pixel – 10.3494 in this case. The FWHM in terms of number of pixels equals, for example 57/10.3 = 5.5.



Figure 10. Measurement of Resolution


Export and import


Exporting data to an Excel spreadsheet


Export and Import are covered in Tutorial – lesson 7 at http://www.astrosurf.com/vdesnoux/tutorial7.html


Files can be exported in txt or dat format. The procedure is;

- open a profile that has been calibrated and corrected for camera response (1_330900_corrected.spc in this example)

- select File/Export txt and a text file with the same file name but with a txt file type is saved in the same location as the spc file (1_330900_corrected.txt)

- alternatively a dat file can be saved by selecting File/Save as… and saving the file as file type dat (1_330900_corrected.dat)

- open Excel, select Data/Import External Data/Import Data

- navigate to the location of, in this example, the txt file and open that file

- in the Text Import Wizard select Delimited and Start import at row 16

- select Next twice and then Finish and Existing worksheet as the location of the imported data

- in Figure 11 column B is the wavelength in Angstroms and C is the intensity at that wavelength


Excel import


Figure 11. Data imported into Excel


To display the spectrum;

- select Insert/Chart, Chart type XY(Scatter) and then Next in Step 1 of the Chart Wizard

- in Step 2 remove all series except series 1and then select Series

- in the resulting Source Data window select column B for the X values and column C for the Y values by clicking on the icons to the right of the relevant boxes, Figure 12.


Source data


Figure 12. Source data selection


- select Next and add labels for Chart Title, and X and Y axis labels and then Next again

- display the chart in a new sheet and format as required, Figure 13


Figure 13. Spectrum of Theta Aurigae


Importing data


Export and Import are covered in Tutorial – lesson 7 at http://www.astrosurf.com/vdesnoux/tutorial7.html


To import one of the one of the spectra from Visual Spec’s library;

- select File/Open profile

- navigate to the folder Visual Spec\LibSpec

- select .dat format to list the available spectra in dat format

- open the desired profile, Figure 14



Figure 14. Library profile of an a0i spectral type star


Worked example - Mintaka (Delta Orionis), spectral type O9, 2015 Jan 14, 401419, O9


This example will cover;

- multiline calibration

- identification of lines in the Mintaka spectrum




After following the procedure documented in Spectroscopy, Part II – Generating a profile using Visual Spec the flattened spectrum shown in Figure 15 was obtained.


Identification of spectral lines

The lines were labelled and compared with the spectrum in Figure 16 taken from the Spectroscopic Atlas for Amateur Astronomers.


Figure 15. Mintaka Spectrum showing the synthesised spectrum above



Figure 16. Spectra of Mintaka. Credit Spectroscopic Atlas for Amateur Astronomers by Richard Walker


From comparison of the obtained spectrum with data from the Spectroscopic Atlas for Amateur Astronomers I believe I may have identified the lines listed in Table 4 below.


Spectral line (Angstroms)







Hydrogen epsilon



Hydrogen delta



Hydrogen gamma



Helium I



Helium II



Calcium III



Hydrogen beta



Helium I



Helium I



Helium II



Oxygen III



Helium I/Sodium I


6300, 6280 (approx)

Oxygen I (airglow), Telluric O2



Hydrogen alpha



Telluric O2


Table 4. Lines identified in the spectrum of Mintaka


Using a standard star


A standard star in this context can be any star for which you have a reliable reference spectrum i.e. you know what the spectrum should look like (The equivalent of a comparison star in photometry) This can used to calibrate the instrument (in wavelength and spectral response) and correct for atmospheric extinction. They are normally hot (typically main sequence A) stars where the continuum is clear and the Balmer lines can be used for wavelength calibration.  In this example Vega, an A0v type star, is used as described by Robin Leadbetter in his presentation (pp 34-43) to the BAA Variable Star Section. Image 1_40729a is obtainable from here.


The processing procedure is described in Spectroscopy – Part II so only the screen shots of the various stages are shown here.


Figure 17. Calibrated profile


Figure 18. Calibrated profile overlaid with library spectrum of A0V type star


Figure 19. Profile corrected for camera response overlaid with library spectrum to confirm calibration