More fun with Grisms

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thesmiths
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More fun with Grisms

Post by thesmiths »

Things seem to come full circle. I started out using the wonderful little Paton Hawksley benchtop spectroscope to look at the Sun. After many iterations of complex spectroscopes, I've tried a new approach, to make things smaller and simpler. So I asked Bob Hawksley if it's possible to buy the grism inside their spectroscope and he said he'd send me one for £20. It is 600 l/mm and 15 mm in diameter.

I inserted his grism into an M42 macro extension tube using black foam board, hot glue gun and masking tape. This is what the grating side (which faces towards the light source) looks like:
grism surface
grism surface
grism-back.jpg (54.17 KiB) Viewed 2858 times
Here is what the flat "camera" side looks like:
grism front
grism front
grism-front.jpg (66.56 KiB) Viewed 2858 times
Also using extension tubes, hot glue gun and masking tape, I positioned a Thorlabs 10 micron slit inside a 1 inch SM1 tube:
grism slit
grism slit
module-slit.jpg (63.31 KiB) Viewed 2858 times
A Pentax M42 55mm f1.8 lens acts as the collimator and is in the middle holding everything together with a 49mm to M42 macro reversing ring:
grism module
grism module
grism-module.jpg (70.44 KiB) Viewed 2858 times
The "grism module" can be attached to another camera lens of appropriate focal length depending on the size of image sensor. In the case of a DSLR, another 55mm 1.8 Pentax lens works well. Here is a photo of the grism module next to the camera:
grism camera
grism camera
module-camera.jpg (102.2 KiB) Viewed 2858 times
And finally here is the whole assembly together:
grism assembly
grism assembly
grism-complete.jpg (121.46 KiB) Viewed 2858 times
It looks a lot like a fancy macro lens set up for taking photos of insects. As a test, I pointed the instrument towards the sky and took a nice photo of the solar spectrum:
grism spectrum
grism spectrum
solar-spectrum.jpg (32.32 KiB) Viewed 2858 times
The setup shown here has a theoretical resolving power of 1,700 and resolution of 3.4 angstroms at 550 nm. It is optimised for work at around f/4. By using a 5 micron slit and a different camera, the resolution at 400 nm can be brought down to 2.0 angstroms, good enough for CaK spectroheliography.

Actually, the true motivation for this grism setup is not for solar work but for night-time stellar spectroscopy. It is actually a kind of bulked up version of the Shelyak APLY, a very compact medium resolution spectroscope. In the fact, the ALPY apparently uses a similar Paton Hawksley grism (although half the diameter, 7.5 mm) and a correspondingly shorter collimator, around 40mm.

In the next version I am working on, I will replace the camera lenses with darkroom enlarger lenses. These are made to work over a broader range of light than camera lenses (photographic paper is/was very sensitive to blue/UV). Also, enlarger lenses are built to focus a plane image (a negative) onto another plane (the paper), which is actually ideal for spectroscopy. Their resolution requirements are typically much higher than normal lenses (they need to resolve the silver crystal structure of the film). And these days, second had enlarger lenses, even very good ones, are fairly inexpensive (people are literally throwing their darkrooms away).


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Re: More fun with Grisms

Post by Merlin66 »

Very neat arrangement, well explained.
Well done!
(I realised P-H did the SA100 and the SA200 gratings and could supply grism prisms for them, but I didn't know about the 600 l/mm grism......)


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Re: More fun with Grisms

Post by marktownley »

Impressive stuff!


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Re: More fun with Grisms

Post by Montana »

Superb :bow

Alexandra


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Re: More fun with Grisms

Post by thesmiths »

Ken: the 600 l/mm 15mm diameter grism is exactly the one they use in their benchtop spectroscope. They also will sell the 600 l/mm 7.5mm diameter grating which they supply for the APLY. They apparently also sell the 43mm collimating lens from the benchtop spectroscope (I didn't order any of those).

They can also sell 50 micron and 40 micron fixed chrome coated slits for 4 pounds each (plus postage). I did order both of those and, although a bit wide for my purpose, are well made and quite reflective (although obviously not mirror quality). I have thought to try one for night-time use as a spatial filter and try to guide off of it. I discovered that with the TS off axis guider I have, I can reverse the little finger that holds the prism and point it backwards (towards the slit). I might give that a try at some point.


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Re: More fun with Grisms

Post by swisswalter »

Hi Douglas

what a wonderful setup, so neat and compact, congratulations


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Re: More fun with Grisms

Post by alex »

Really nice to see that post :) :)


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Re: More fun with Grisms

Post by Derek Klepp »

Thanks for the update


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Re: More fun with Grisms

Post by Merlin66 »

Thanks for the update...
I obviously need to check out the Paton Hawksley capabilities more often.


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Re: More fun with Grisms

Post by Merlin66 »

Re slit guiding...
Marco did your "reverse prism" arrangement....needed to add a focusing/ transfer lens....but limited success due to the extreme off axis - prism to optical axis....
If the prism can be rotated ( to throw the image of the slit towards the side) and placed as close as possible to the axis, it could be a workable solution.
(See the CAOS TRAGOS design for other ideas....)


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"Imaging Sunlight - using a digital spectroheliograph" - Springer
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