Star spectra

The spectrum emitted by a star is key to its composition and luminance.  Anyone with a telescope and webcam/CCD can obtain the spectrum of a star by using a suitable diffraction grating in a filter cell.  Using software such as VSpec (see link at left), the spectrum can be analysed. 

Larger scopes and more sensitive cameras can obtain the spectra of dimmer objects - my small telescope and basic webcam set-up only enables me to access the brighter stars in the sky, perhaps to mag +4 or so.  With a Canon 400D I can get to about mag 8.

Star Analyser diffraction grating:

Paton Hawksley


Visual Spec software:



Stellar classification:



Amateur spectroscopy:



UPDATE, April/May 2011:
The recurrent nova T Pyxidis was detected in outburst on 14 April 2011 after a quiescence lasting over 44 years.  By 23 April it had risen to about mag 7.5 and I was able to get a rough, low-resolution spectrum using a piggybacked Canon 400D at 200mm zoom and a Star Analyser diffraction grating. The Balmer line emissions H-alpha, H-Beta and H-gamma are all visible in the spectrum.  On 8 May with T Pyx brighter at about mag 7.1, I managed to obtain a stronger response in the main hydrogen lines, and responses in FeII lines which were reported to have strengthened.  These are marked in the calibrated presentation below:
  Click on images to enlarge...
May 8 2011:
By way of contrast, here is the poorer spectrum obtained 23 April:
Don't forget to click on the images that open in the new page, to expand them to full size.
The spectrum is nothing to write home about, but it does demonstrate that with my limited equipment I can identify the characteristic spectral signature of bright novae (say, brighter than mag 8). 
UPDATE, October 2009:

On a 55mm widefield image of the Large Magellanic Cloud, I noticed a bright blue & fainter red spot, representing the spectrum of the Tarantula Nebula which was effectively a point source at that scale.  I zoomed in to 200mm, where the emissions should show as repeated 'images' at the relevant wavelengths, and that worked OK, particularly for the brightest OIII emission.  Was able to calibrate the 'spectrum' and identify new lines.

Then turned to Orion as a check, and it seemed to work well, confirming calibration in rough terms anyway (in the absence of a 'slit').  Differences in emissions were also evident, with Tarantula emitting strongly at OIII while Orion emitted strongly at Ha.
Best outcome in terms of the exercise was identifying three Balmer lines from Hydrogen emissions.  Now if only I can find time to get my head around VSpec!!!!

Click on image to enlarge...

Don't forget to click on the image that opens in the new page, to expand it to full size.


  UPDATE, September 2009:

Still not much progress, but I did make a small adaptor for fitting the Star Analyser diffraction grating to the 55-200mm Sigma lens on my Canon 400D DSLR.  First test shots were promising, and with this set-up I should be able to obtain the spectra of much fainter stars than with the webcam.

So here is Antares, taken at 200mm, 3.2 sec, Large image size (3888 x 2592 pixels).  Stack of 3, actual pixel size composite:



Just starting this journey!  So far, all I have done is:

(a) Proven that I can obtain a useable spectrum from brighter stars with my set-up (4.5" f8 reflector, NexImage webcam);

(b) Created bar spectra from the raw image;

(c) Created a basic graph in VSpec;

(d) Calibrated my images at at 17Å (1.7nm) per pixel, based on the Mg line(s) in the Betelgeuse spectrum.


Here is a raw image from Celestron NexImage webcam through 4.5" f8 reflector, using a Star Analyser diffraction grating:


The following is a bar spectrum obtained by rotating above image to make spectrum horizontal, cropping a 2-pixel wide strip across the image, reducing the height to 1 pixel, then increasing the height to 30 pixels:


Finally, this is a graph created in VSpec.  Sharp dips in the graph represent absorption lines of various component elements of the star (NB no atmospheric correction or other adjustment):


There is much, much more to it than this though, and it will be a process of carefully working through the VSpec manual.  Stay tuned!