by Rex Parker

Spectroscopy has always been an interesting and appealing branch of optics and chemistry ever since I made my first diffraction grating spectroscope for a junior-high science fair project. Of course spectroscopy has many scientific applications today, especially in fields like biochemistry and biophysics that occupy my day-time hours. So for a few years I’ve been looking for an affordable spectroscope for amateur astronomy, but I was not that impressed with the readily available Rainbow Optics star spectroscope for visual application with telescopes. Then I came upon the Paton Hawksley Star Analyser 100, designed for amateur astronomical spectroscopy using CCD or DLSR cameras with telescopes. The first order spectrum of this 100 line/mm blazed diffraction grating, mounted in a standard 1.25-inch filter cell, is dispersed to just the right dimensions for small CCD chips if it is positioned correctly in the telescope light path.

I recently acquired the new SBIG ST-i camera to use as external guide camera with my ST10-XME for narrow-band filter and RGB imaging of deep sky objects. The lightweight ST-I monochrome CCD camera (about the size and weight of a 30-mm Plossl eyepiece!) seemed a good match for the Star Analyzer 100. So a couple weeks ago I gave my new equipment first light using my 3-inch Tak refractor. What better comparison to make in this situation than the spectra of two contrasting giant stars of winter significance – Betelgeuse the red supergiant, and Rigel the blue supergiant, both in Orion. Included in my setup is the specialized spectroscopy software program, RSpec, which helps to display the absorption lines in the spectra and calibrate using known stars spectra files. RSpec also has a nifty color synthesizer to give a color spectrum based on the monochrome CCD image readout, after proper calibration. The data below, however, have not yet been corrected for CCD wavelength-dependent sensitivity response. That will come later on the learning curve. As you can see by comparing the blue vs red regions of the spectra for these two super giants, there is a large difference in intensity (energy) and absorption bands indicating differences in composition and temperature, which is consistent with their positions in the Hertzsprung-Russel diagram for luminosity/temperature correlation.

If you’re interested in getting involved in stellar or deep sky spectroscopy, please catch me at an upcoming AAAP meeting, where we can chat about equipment, software, and interpretation!

CCD spectroscopy images and calibrated spectral data for Betelgeuse (left) and Rigel (right).