Following up on our previous work in an earlier post "Some fun with grisms ", we've improved our setup slightly. Before, we put two 10 degree prisms on either side of a 600 l/mm grating to make a fixed grism. This centred the green in the middle of the field of view. By adding two 4 degree prisms in combination with the 10 degree prisms, we found we could move the centre wavelength from violet in 1st order all the way to violet in the 2nd order. This is kind of a "variable grism", in the sense that the best wavelength can be tuned.
The double prism arrangement is a "beam steering" configuration used in laser optics. A pair of rotating wedge prisms can trace out a solid cone, as show in this drawing from Thorlabs:
With a pair of 10 degree and 4 degree prisms, we effectively get a continuous deflection from 6 degrees to 14 degrees. We manually rotate the prism combinations in a pair of Thorlabs 1 inch tubes. Ideally, it should be possible to have some lever to rotate the prisms (a bit like in an atmospheric dispersion compensator).
As I mentioned, we've used this to shift the centre wavelength with a 600 l/mm grating from violet in 1st order (6 degrees) all the way up into the infrared and into violet in the 2nd order (14 degrees). As it turns out, 2nd order with 600 l/mm is the same deflection as 1st order with 1200 l/mm. So the results we'll show here is with a 1200 l/mm transmission grating from Thorlabs using a fixed 14 degree prism setup. This gives very high resolution in the violet, near the Calcium K & H lines. The 1200 l/mm transmission gratings have quite high efficiency in the violet (around 40%).
Above is a photo of the setup. It is similar to our previous one, with a slightly more compact grism arrangement. We are also using the ASI 120MM camera, which turns out to be excellent for this application.
Here is our first solar spectrum using this setup. It shows the full width of the Sun (vertical height of around 1 mm as projected on the 5 micron slit) and the area near the Ca K and H lines (3934 and 3968). The theoretical resolution is 0.25 angstroms and since the H and K lines are 34 angstroms apart, that looks about right. The image has been reduced in size by a factor of two for presentation (ie it's 640 pixels wide instead of the 1280 of the sensor).
We've already done a few spectrohelioscope experiments with this simple setup and it seems to work. We'll report on that shortly. For now, I attach a single frame from a video made with the ASI 120MM at 120 frames per sec. As the apparatus is scanned across the Sun, structures appear inside the calcium lines (but not in the other lines). They look kind of ghostly now but I assume with some processing, a nice solar image may appear.
Rotating prism transmission spectroscope
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- Merlin66
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Re: Rotating prism transmission spectroscope
Well done.
You are gathering together quite a collection of interesting bits and pieces...
The early results look very promising, as does your comment about the ASI 120MM......
You are gathering together quite a collection of interesting bits and pieces...
The early results look very promising, as does your comment about the ASI 120MM......
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
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"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer
- Peter Williams
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Re: Rotating prism transmission spectroscope
What a great assemblage of bits and pieces , I love the willingness to have a go and nerdy scienciness that goes on around here.
Great results so far.
Great results so far.
Beginner amateur astronomer, keen on solar astronomy.
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Re: Rotating prism transmission spectroscope
Hi Douglas
wonderful gear. very promising results
wonderful gear. very promising results
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Re: Rotating prism transmission spectroscope
Impressive!
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