Echelle spectrograph images -- in colour

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thesmiths
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Echelle spectrograph images -- in colour

Post by thesmiths »

Some years ago, we did some some experiments with an echelle spectrograph viewtopic.php?f=8&t=8103. We decided to revisit this topic in a more compact setup, as has become the norm these days with recent spectroheliograph designs. The advantage of the echelle spectrometer is the ability to view a large spectral range with high resolution. Our current echelle setup is shown below:

current echelle spectrometer setup
current echelle spectrometer setup
1285c.jpg (263.4 KiB) Viewed 574 times

The aperture consists of the 30 micron wide, 3 mm long stainless steel slit masked off to a length of approximately 0.25mm. The image of this aperture is "de-magnified" by a 200mm collimating lens and a 100mm camera lens (both vintage Pentax M42 lenses). The echelle grating is a 25mm x 50mm 79 l/mm 75 degree blaze from Thorlabs. Cross dispersion is created by a Star Analyser 100 and a 3.8 degree prism, both from Paton Hawksley.

The image below is an example using "white light" from an LED flashlight. The camera in this case was a Canon 100D DSLR, which as a large sensor, although a limited spectral range due to its built-in cut-off filters:

white light spectrum
white light spectrum
5593a.jpg (119.21 KiB) Viewed 574 times

The dim line at the bottom is the "un-dispersed" output of the echelle grating. The slanted colour lines above are the "cross-dispersed" output of the Star Analyser (which is a 100 l/mm transmission grating, very strongly blazed in the first order). The purpose of the 3.8 degree prism is to put the output lines more in the middle of the camera sensor.

The next image is of compact florescent bulb, which has a mixture of a few narrow spectral lines and several very broad florescent lines:

compact florescent spectrum
compact florescent spectrum
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Next is a neon bulb which has a large number of narrow lines, mainly in the red and yellow:

neon spectrum
neon spectrum
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After these initial tests, we then turn to sunlight as the light source. We simply let direct sunlight fall onto the aperture. We tried a variety of cameras: the Canon 100D DSLR, a ZWO ASI 174MM and an ASI 224MC. The most pleasing results came with the colour astronomy camera, although the sensor on the 224MC is rather small. We therefore needed to add a 0.5 focal reducer in front of the camera to fit the whole solar spectrum onto a single image:

solar spectrum
solar spectrum
20211121_121137a.jpg (263.67 KiB) Viewed 574 times

Visible in this image are the "magnesium triplet" in the green, the "sodium doublet" in the yellow" and H-alpha in red. Above H-alpha are the various infrared absorption lines, mainly due to oxygen in the atmosphere. The echelle grating in this last image was set at a fairly high angle in order to achieve high dispersion and therefore good resolution (in the previous test images, the angle of echelle was set lower, in order to achieve higher intensity).
Last edited by thesmiths on Tue Nov 28, 2023 2:05 am, edited 1 time in total.


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Re: Echelle spectrograph images -- in colour

Post by Merlin66 »

Doug,
That's a very impressive outcome from your "MiniEchelle".
Well done!!


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Re: Echelle spectrograph images -- in colour

Post by Montana »

Wow!! that last image is an absolute beauty :bow :bow :bow

Alexandra


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