Stellar spectroscopy

 A large fraction of my time as a working physicist involved doing neutron and gamma-ray spectroscopy of nuclear reactions.  I have always been intrigued by the possibility of trying optical spectroscopy of stars.  Fortunately, there is an easy entry into this practice for the amateur astronomer.  All it requires is an appropriate diffraction grating that can be mounted in the optical path of the telescope ahead of an eyepiece or camera.  

Tom Field, a former contributing editor to Sky & Telescope magazine, founded a company that supplies both the hardware and software required.  The starting point is Tom's website Rspec-astro.  I purchased the Star Analyzer 100 diffraction grating as well as the Rspec analysis software, which is well-written and versatile.  The software is specific to Windows, but I had no trouble installing and running this software under Crossover (Wine) for Linux.  In spite of this success, the results shown below were processed with my own software from my working days (use what you know).

Here is a an image of the diffraction grating mounted on a 2-inch extension tube that inserts into the focuser of the telescope.

 This particular grating has a line spacing of 100 lines/mm, and is housed in a standard 1.25" filter cell.  The label on the grating says "PATON HAWKSLEY, UK STAR ANALYZER 100".

The distance from the grating to the image plane was measured with a ruler as L = 140 ± 1 mm.  The exact distance doesn't matter because the spectrum calibration was obtained from the actual data. 

Light with wavelength λ is diffracted at an angle 

sin(θ) = λ/d,

where d is the line spacing (10^-2 mm) of the diffraction grating.  The wavelength of the Hydrogen-beta transition, which is a convenient reference in the blue part of the spectrum, is λ = 4861.4 angstrom. For the configuration shown here, this line will therefore appear at a displacement of  x = L*tan(θ) = 6.8 mm from the zero-order image of the star.  This displacement is well within the width (17 mm)  of the micro-four-thirds sensor of the Olympus E-M5iii.

The telescope used was an Orion 80mm f/7.5 ED refractor.  Here is an image of the star Sirius and its diffracted spectrum, ISO 1600, 1/5 sec:

 

In this image, Hydrogen absorption bands are easily visible.  The spectrum shown below was obtained from the luminosity channel of this image.  The red dotted lines correspond to the various transitions of the Balmer series in Hydrogen.  The correspondence is very good, though there may be some non-linearities that I don't yet understand (I'm just a beginner in this field). 

 The actual spectrum of the star extends well below and above the cut-off wavelengths of 3800  and 7000 angstrom seen here.  The wavelengths visible in these data are limited by the quantum response of the CMOS camera sensor and the filter stack in front of it.