Spectroscopy

Spectroscopy used to be the domain of professional astronomers using very expensive equipment; this is not the case anymore. Using relatively cheap software and a diffraction grating filter you can use ordinary backyard telescopes to see methane absorbtion ( dark ) bands in Neptune’s spectrum and the spectral type, composition and temperature ( measured in Kelvin ) of any star above your horizon.

Stars are classified by spectral type and given letters ( O,B,A,F,G,K and M remembered by the mnemonic Oh Be A Fine Girl/Guy Kiss Me ). Each class is subdivided into 10 subclasses, numbered from 0 to 9.

The equipment used is the same as for lunar or planetary imaging with the addition of the diffraction grating in front of the camera.
I use a Paton Hawksley Star Analyser 100 grating. It’s a 1 1/4 inch size filter that screws into a camera nosepiece. There is a mark on the side of the filter to allow correct orientation between the grating filter and camera sensor.


The software used is RSpec, from Field tested systems. It is very easy to use and comes with instructional videos for every aspect of the software. There is free software available (Visual Spec from Astrosurf ) but the learning curve to use it is very steep so I prefer to use RSpec.
The camera I use is a ZWO ASI120mm mono ( more sensitive than colour cameras ), it allows me to fit both the zero order and first order spectrum on the sensor across its width. This means I don’t have to rotate the image in the software that may cause artifacts.


This is used with my main imaging scope that has an aperture of 8 inches and gives a good resolution of 7.8 Angstroms per pixel. You can see the elements in your spectra by using the Elements library.

Elements library
Elements library

Amateurs like myself can estimate a star’s type and temperature by comparing your spectrum to the software’s built in reference library. It has a list of star types ( the Pickering library ) and Planck curves and is accurate to a couple hundred degrees Kelvin.

For example, the star Vega is a type aOv star and has a professional estimated temperature value of 9600° K +/- 200° K: my estimated value is 9500°K, pretty close.

Wolf Rayet stars often called WR stars, are particularly interesting as they are evolved massive stars that have lost ( thrown off ) their outer Hydrogen and are fusing Helium or heavier elements at their cores and show strong emission ( bright ) lines.

Recommended books are-

Keith Robinson – Spectroscopy The Key to the Stars_Reading the Lines in Stellar Spectra, published by Springer

Ken M Harrison – Astronomical Spectroscopy for Amateurs, published by Springer

Richard Walker – Spectroscopic Altas for Amateur Astronomers.

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