Spectroscopy – 22nd June 2006

Jack Martin made use of slides, laser pointers, and diffraction gratings to illustrate his talk on Spectroscopy. By shining both a green and a red laser pointer through the same grating he demonstrated the spectral spreading a grating produces, and how different gratings spread the spectrum to a greater or lesser degree. The grating he uses has 200 lines per millimetre, and puts three-quarters of the light falling on it into the first-order spectrum. He uses it to produce spectra of stars on photographic film at the prime focus of his Dobsonian telescope. By allowing the star to trail in a direction perpendicular to the axis of the grating’s dispersion, a clear spread spectrum can be captured on film using exposures of between 15 and 30 seconds. He favoured Ilford Delta 400 ASA as it was sufficiently red-sensitive to capture spectral detail around the Hydrogen-alpha line. He showed some spectra captured on other emulsions with different sensitivities, and some of the spectra were very obviously incomplete. He showed us spectra of various types of star from white-hot Mintaka to dull red Betelgeuse, the blended spectra of various double stars, and the emission spectrum of Gamma Cassiopeiae. There was some discussion on the possibility of using CCD imaging, but Jack clearly still favours film.

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