Hugh Allen

Hi again David,

As Andrew said, that’s a nice first spectrum. And isn’t it great that spectroscopy can be done through cloud, moonlight and light pollution! Alioth is a normal A-type giant star so the Balmer absorption lines are strong and still reasonably broad, perfect for the Star Analyser. The shallow absorption lines in your profile could be because of focus, or I suspect maybe also overexposure? The pixel map option in RSpec gives a nice way of evaluating the exposure of each pixel – if you look at the pixels around the peak brightness in your profile (near 4500Å) the brightest pixels should be no more than 80-85% of the peak ADU from your camera (so 80-85% of 65,536 if you’re using a 16-bit camera). Once you’re happy with exposure and focus so that you’re getting nice deep absorption lines I’d recommend a non-linear calibration in RSpec. The spectrum from the Star Analyser is surprisingly non-linear; here’s a comparison I made of the spectrum of Alioth (like your spectrum, not yet response corrected) with linear and non-linear calibration in RSpec.