Observation by David Swan: Gamma Cassiopeiae: spectrum

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David Swan


David Swan


2018 Jan 18 - 20:28


2018 Jan 18 - 21:10



  • 200mm SCT @ f/6.3
  • ASI178MM
  • Star Analyser
  • FireCapture and RSpec

Stack of 100 x 100ms frames


Tynemouth, UK

Target name

Gamma Cassiopeiae


Gamma Cassiopeiae: spectrum

About this image

A very clear hydrogen alpha emission line from this well-known Be star.

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Hugh Allen
Hugh Allen, 2018 Jan 18 - 21:28 UTC

That's very nice David!

David Swan
David Swan, 2018 Jan 18 - 21:45 UTC

Thanks very much Hugh. Focusing is quite a challenge isn’t it? For example, I found that when I achieved a sharp tapering where the first order spec starts (in visible light of course!), the Halpha line for gamma cass wasn’t as clear as it was at a slightly different focus. What do you recommend? David

Andy Wilson
Andy Wilson, 2018 Jan 20 - 11:18 UTC

Great spectrum David! As well as the hydrogen emission you can also see absorption by the Earth's atmosphere, telluric lines, a good example between H-alpha and 7000 Angstroms.

Hugh Allen
Hugh Allen, 2018 Jan 21 - 21:29 UTC

Hi David,

As I showed in the forum conversation I adjust the focus based on the Bahtinov mask pattern formed by the zero order star image. I experimented with adjusting the focus until I got the sharpest A-type star spectrum and then used the appearance of the Bahtinov mask image to act as a reference for future focus setting. I always set the focus to give the same Bahtinov pattern. Does that make sense?



David Swan
David Swan, 2018 Jan 23 - 22:22 UTC

Thanks Hugh - I was a bit slow on the uptake, but get the idea now. A clever system. I might buy one of these masks soon.

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