Amateur astronomical spectroscopy

Tuesday 2nd December, 2014

Usually we invite a professional astronomer to come and tell us about their latest research but sometimes the so-called amateurs are also experts in their own field. For our December talk we asked John Strachan to give us the benefit of his experience on the subject of "Amateur Astronomical Spectroscopy".

He began by explaining that astronomical spectroscopy is the analysis of light in order to understand the properties of celestial objects such as stars and galaxies. It can be used to find out various properties of stellar bodies including their chemical composition, temperature, density, mass and even their relative motion. Professional spectroscopes attached to large telescopes can analyse all the three major bands of radiation (i.e. light) — optical, radio and x-rays. However, amateur astronomers only work with optical radiation that is more commonly known as visible light. In everyday experience these colours can be seen smeared out in the spectrum of a rainbow.

Spectroscopy as a science is thought to have begun in 1666 with Sir Isaac Newton's optics experiments that involved shining white light through a prism and obtaining bands of colours that he named a "spectrum". Later, in the early 1800s Joseph von Fraunhofer replaced the prism with a diffraction grating which allowed him to examine the spectrum in much more detail. Using this grating that was made up of thousands of parallel tiny slits he went on to make a detailed study of the dark lines in the Sun's spectrum. Following on from his work, it was later discovered that these dark lines were caused by atoms in the Sun's photosphere absorbing certain wavelengths of light.

John mentioned that there are everyday instances of a diffraction effect. For example, a CD or DVD held at an angle to a bright light source reflects a spectrum back to the observer's eye and is known as a reflective diffraction grating. Another example is a transmissive diffraction grating. This can occur where bright sunbeams (such as from parked cars) passes through net curtains which split up the light into numerous spectra due to the small mesh in the material.

Nowadays, amateurs mainly use transmissive gratings, along with their telescope and CCD camera, to take spectra from stars or other bright light sources. The data is then processed on a computer and can be uploaded to relevant websites dedicated to the subject. He mentioned that there is not much more outlay involved if an astronomer is already set up to do astrophotography and recommended a "Star Analyser" grating to begin with.