Spectroscopy

 

Part III – Analysing spectra using Visual Spec

 

Updated 2017 January 14

 

Contents

 

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

 

Classification

Temperature

Max Wavelength

Color

O0

40,000 K

72.5 nm

Blue

B0

20,000 K

145 nm

Light Blue

A0

10,000 K

290 nm

White

F0

7,500 K

387 nm

Yellow-White

G0

5,500 K

527 nm

Yellow

K0

4,000 K

725 nm

Orange

M0

3,000 K

966 nm

Red

 

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

 

Colour

Wavelength (Angstroms, Ε)

Violet

2800 – 4500

Blue

4500 – 4950

Green

4950 – 5700

Yellow

5700 – 5900

Orange

5900 – 6200

Red

6200 - 7500

 

Table 2. The visible spectrum

 

Balmer Hydrogen line

Wavelength (Angstroms)

Alpha

6563

Beta

4861

Gamma

4340

Delta

4102

 

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

 

Resolution

 

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

 

Calibration 

 

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

 

Mintaka

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)

 

Measured

Actual

Element

3976

3970

Hydrogen epsilon

4108

4102

Hydrogen delta

4337

4340

Hydrogen gamma

4468

4471

Helium I

4547

4541

Helium II

4647

4647

Calcium III

4858

4861

Hydrogen beta

4930

4922

Helium I

5012

5016

Helium I

5414

5411

Helium II

5589

5592

Oxygen III

5896

5876/5889/5895

Helium I/Sodium I

6272

6300, 6280 (approx)

Oxygen I (airglow), Telluric O2

6558

6562

Hydrogen alpha

6872

6867-6944

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

 

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