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Old 24-06-2018, 06:50 PM
robin_astro
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Join Date: Jun 2009
Location: UK
Posts: 73
Quote:
Originally Posted by Merlin66 View Post
Guys,
Covering 50% of the grating didn't work - wishful thinking!
When you think about it, the camera lens is still producing an image of the star field whether is 50% covered or not...

The aperture stop mentioned (Astronomical Spectroscopy for Amateurs, p 182) works with the grating "in the converging beam" and isolates the target star "image" (at the grating) from the background.....
Hi Ken

Try mounting the mask some distance in front of the lens so it can form an (out of focus) image. If you use two vertical masks you can effectively form an out of focus wide slit so the spectrum of the star is projected against a dark background and the spectrum formed by the sky to the left of the target is also removed. (arrange it so the star is in the unvignetted region and the star spectrum falls in the fully vignetted region)

Do not be tempted to use a hole aperture. (use a slit instead) A hole aperture makes it impossible to effectively subtract the sky background which becomes a low resolution spectrum of the sky when a mask is used.

You can see the effect of using a slit mask (in this case placed at a focal plane) here. (Venus in daylight)

http://www.threehillsobservatory.co....troscopy_4.htm

Cheers
Robin
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