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Fun with a Diffraction Grating, David McClain

Fun with a Diffraction Grating

Description

My buddy Ray had a 1-inch diffraction grating that we placed into a cardboard mask and taped over the Canon 200mm lens. At this focal length the dispersion was ridiculously high, 1.7 Angstroms per pixel on the ATIK 490 sensor. (0.4 Angstrom/arcsec) How do you tell if you are in focus?

At any rate, top to bottom, we have examples of increasing stellar surface temperature. The top spectrum belongs to Betelgeuse, type M2. You can see how the red end is emphasized over the blue end.

At the bottom is the opposite situation, where Gamma Ori (Bellatrix - the other shoulder star of Orion), early type B2 star. It shows the blue end getting emphasized over the red end.

And in the middle we have Beta Aur, an A2 type star. And you can just barely make out the Hydrogen Alpha and Beta absorption lines.

The stars were separated from their spectra by more than 3 deg. My sensor on the Canon 200mm lens only has a 3 deg FOV. So the stars weren't even on the sensor when I pointed directly at them. And in order to get the complete spectrum in the frame, I had to further offset another deg away from the star.

Astrobin doesn't ask about my equipment for this category of picture, so I'll tell you myself.

Mount = Paramount MYT with TheSkyX

Sensor = ATIK 490 OSC (3.69 micron square pixels)

Lens = Canon 200mm F/2.8 running at F/4, but with the mask containing the grating, more like F/8

Image Processing in PixInsight

No Guiding, just the mount doing a great job of tracking.

Grating = 1.25 inch diam, 100 lines per mm, blazed to emphasize 1st order diffraction.

Integration times ranged from 300s for Beta Aur, to 100s for Alpha Ori and Gamma Ori. None of the images saturated at these exposure durations, despite these all being very bright stars - we had so much dispersion.

Oh yeah... the weird modulation in the amplitude of the spectra? - caused by the light going through my IDAS LPR light pollution filter on the front of the lens, which takes out the Mercury and Sodium line regions. Looks like the filter is doing what it is supposed to do. Knowing the graph of the filter transmission allowed us to estimate the dispersion and locations for the Hydrogen lines. So it was helpful, and didn't really hide anything of interest, other than a bit of yellow color.

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Fun with a Diffraction Grating, David McClain