new parts of my spectro coronagraph and results

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ChrisHalpha2017
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new parts of my spectro coronagraph and results

Post by ChrisHalpha2017 »

Hello,

I've decided to upgrade my spectro coronagraph : I added a focuser to my collimator. A diffration grating can only give very sharp images if the slit is at the exact focus of the collimating lens for a given wavelength . It means the collimator has to be refocused for each wavelength you choose and the sun image on the slit should be refocused too !!

So I bought an ASI ZWO focuser :
collimateur.jpg
collimateur.jpg (324.59 KiB) Viewed 1289 times
collimateur monté.jpg
collimateur monté.jpg (320.66 KiB) Viewed 1289 times
collimateur et diaph de Lyot.jpg
collimateur et diaph de Lyot.jpg (337.45 KiB) Viewed 1289 times
3 parties du spectro coro.jpg
3 parties du spectro coro.jpg (422.92 KiB) Viewed 1289 times
détail corps du spectro coro et collimateur.jpg
détail corps du spectro coro et collimateur.jpg (349.87 KiB) Viewed 1289 times
détail tube diaph de Lyot.jpg
détail tube diaph de Lyot.jpg (468.63 KiB) Viewed 1289 times
spectro coronographe version 2023.jpg
spectro coronographe version 2023.jpg (417.48 KiB) Viewed 1289 times
To set the collimator rightly on the H alpha line I use a "finder with a focuser in wich I put a 12mm eyepiece and a H alpha filter. I aim at the cloud and set for a very sharp image.
51LmvWB-lFL._AC_UF1000,1000_QL80_.jpg
51LmvWB-lFL._AC_UF1000,1000_QL80_.jpg (42.59 KiB) Viewed 1289 times
Then I look through the collimator with the finder and aim at the image of the entrance slit. I then set the collimator to have a very sharp image and I block the focuser.

Here are the results of the today prominences ( this morning and afternoon observations , despite a very hazy sky)


600 2+++_20230610_052738.371_UT.jpg
600 2+++_20230610_052738.371_UT.jpg (343.7 KiB) Viewed 1289 times
1200 ++_20230610_053653.480_UT.jpg
1200 ++_20230610_053653.480_UT.jpg (425.46 KiB) Viewed 1289 times
1800 ++++++2_20230610_053125.272_UT.jpg
1800 ++++++2_20230610_053125.272_UT.jpg (449.8 KiB) Viewed 1289 times
1800 best 4++++++_20230610_152543.113_UT non traitée.jpg
1800 best 4++++++_20230610_152543.113_UT non traitée.jpg (56.07 KiB) Viewed 1289 times
Chris


spectro-coronographe ,Bresser 127/1200 ; Bresser 102/1350 , ASI 178MM
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Re: new parts of my spectro coronagraph and results

Post by thesmiths »

This is a fantastic project. What is the purpose of the Lyot diaphram? In the images you show, it looks like you are you using a curved slit? Of what width and radius? To me, the lowest power grating (600 l/mm) seems to give the nicest image.


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Re: new parts of my spectro coronagraph and results

Post by highfnum »

very sharp


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Re: new parts of my spectro coronagraph and results

Post by ChrisHalpha2017 »

What is the purpose of the Lyot diaphram? :

initialy Lyot (who invented the coronographe) wanted to see the solar corona. One of the major obstacle is the light created by the edges of the objective lens of the refractor because its intensity was superior to the light of the corona he wanted to observe. WHen I block the sun to observe the solar limb the light diffracted by the main lenses of the refractor creates a halo around the sun. That means that instead of having blackness aound the sun I get some hazy red light superimposed onto the prominences image. So the Lyot diaph suppresses this "parasitic" light and the sky becomes black with very bright prominences. When the sky is very clear and transparent this is what happens.

it looks like you are you using a curved slit? Of what width and radius? :

When I use a straight slit the contrast is equal all along the H alpha line wich has the same width (and shape) in the spectrum as the entrance slit. It works pretty well with the H alpha line because this strong line is quite isolated in the red portion of the spectrum. If I did that with the sodium doublet then both lines would be superimposed .

Sometimes the prominences are quite long , or just for the pleasure I want to observe a much greater portion of the sun to watch the spicules or other tiny eruptive events like macrospicules . In that case I use a circular slit. In fact its almost impossible to make a good circular slit by hand so the trick I use is to use a rounder cutter blade of 18 mm for one jaw and a standard straight razor blade for the other jaw. Thats the best option I found. I dont want the round blade to match the sun's image too well because then keeping the sun hidden at all time with require motorization ( wich i dont have) and tremendous precision when I do the following by hand!!. So instead I prefer using a disk that is between 15mm to 30mm and then I scan the edge of the sun little by little.

To me, the lowest power grating (600 l/mm) seems to give the nicest image. :

Yes it does and there is a reason for that : the Doppler effect and the diffraction grating dispersion. If the dispersion is too strong then the light coming from a point of the image of the prominence on the entrance slit will appear as a LINE at the focus of the spectroscope on the CCD sensor. SO the image is blurred because not all parts of a prominence has the same velocity. When using a genuine narrow bandwidth H alpha filter of 0.5A for example , you see only the part of the prominence that isnt off-band. Thats why photos of prominences are dimmer and can show the fine inner structure. When I use a 2400 lines/mm with my 200mm focal lenght collimator the dispersion is high and the image is even more blurred and elongated.

Chris.
Last edited by ChrisHalpha2017 on Mon Jun 12, 2023 5:13 am, edited 1 time in total.


spectro-coronographe ,Bresser 127/1200 ; Bresser 102/1350 , ASI 178MM
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Re: new parts of my spectro coronagraph and results

Post by thesmiths »

Thank you for your explanation. I have been intrigued by this technique ever since I found some pages in the Adam Hilger catalogue some years ago. I attach the relevant pages. He mentions using a curved slit. Since you only need low power refraction, the idea of using prims is interesting.

By the way, the address (75a Camden Road) is very close to where I live. But, unfortunately, I believe they now only sell pizza there.

Pages from Adam Hilger Catalogue 1920.pdf
Solar Spectroscopes
(927.14 KiB) Downloaded 48 times


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Re: new parts of my spectro coronagraph and results

Post by ChrisHalpha2017 »

thanks very much :) its very interesting indeed.

At the moment I use mainly the 1800 lines because the sky is hazy most of the time above Paris. But I find the 1200 lines a good compromise. Indeed the 600 lines gives the best definition but the image is very bright as the diffused light from the sky background is much less spread by the dispersion. But when using the camera instead of directi vision it gives very detailed images as you noticed. I even suspect that a 300 lines might even be better or i should divide the collimator lens by 2 for same results I guess. I'll give a try but of course NO DIRECT vision with such a low dispêrsion.


spectro-coronographe ,Bresser 127/1200 ; Bresser 102/1350 , ASI 178MM
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Re: new parts of my spectro coronagraph and results

Post by ChrisHalpha2017 »

By the way to make easily a good spectroscope you can simply use 2 finders like the one I bought : in the 1st finder you insert the entrance slit that you can focus precisely and the other finder you serve to observe the spectrum with any eyepiece you choose or even put the camera.

This particular model of finder is great because you can extend the tube to mach the different focal length of the different eyepiece or even for a camera. I bought it on amazon.


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Re: new parts of my spectro coronagraph and results

Post by Montana »

Superb detail in the prominence :bow :hamster:

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Re: new parts of my spectro coronagraph and results

Post by thesmiths »

ChrisHalpha2017 wrote: Mon Jun 12, 2023 5:12 am thanks very much, its very interesting indeed.
By the way, the reason prisms are still interesting is that they do not lose light to higher order diffraction. All light goes into a single spectrum (not +/- and not into 2nd, 3rd, etc order). So for visible use you get much higher brightness. Professional astronomers who use an echelle will typically use a prism to sort the orders so they don't waste any photons.


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Re: new parts of my spectro coronagraph and results

Post by ChrisHalpha2017 »

yes indeed but the big problem is that the dispersion isnt linear and prisms still absorbs light. Also prisms are very heavy and fragile compared to a single grating ( especially for a prominence spectroscope). I found notes of Secchi about the fact that a Rowland grating was the equivalent of 5 or 6 prisms ( around 600 lines/mm) so its not advantageous for a prominence spectroscope . Also if you dont use prisms at the minimum deviation angle you have anamorphosis like with a spectroscope in classical configuration : so it means that the image of prominences is squashed unlike the Littrow config that I use. And I forgot to say that prisms are very expensive !


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