Building a basic spectroheliograph

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taratasy
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Building a basic spectroheliograph

Post by taratasy »

Greetings everyone !

I am a geology student and long-time astronomy enthusiast. I grew a strong interest in observing and imaging the sun, now with my faithful lunt LS50THa. The h-alpha line is marvelous ! Everyday I check the GONG H-alpha monitor in hope to see an odd-looking prominence and twisted filaments.
Recently I fell in love with the Ca-K line :oops: thanks to many images posted on the net ; unfortunately I can't afford any new expensive equipement to image it and fullfill my passion.

Then, I learned about the spectroheliograph. Man, what a fine piece of instrument !
Being able to image the sun in ANY visible wavelenght is outstanding, it sure caught my attention.
I found this web page http://www.astrosurf.com/cieldelabrie/sphelio.en.htm which made me think that I could possibly build one myself ! I am no expert in optic and DIY but I am a quick learner and enthusiastic.

I found many websites such as http://www.astrosurf.com/spectrohelio/index-en.php which helped me a lot to understand the theory of spectroheliography.
In the end the construction of an amateur spectroheliograph seems fairly difficults with many mathematics and optical elements involved such as the telescope, the collimator, the camera lens, the sensor, the grating, the slit... But it doesn't scare me and I have all the time I need.

However I dont think I could do it all by my own, therefor I registered in Solarchat and posted here in hope to get comments and advice from you all :D

For recap, I would like to build a small spectroheliograph prototype that will do the trick, not necessarily an high quality construction.
At this point I own some elements that might be usefull for the construction :

- a webcam creative live ultra with a ICX098AK CCD sensor (1/4" and 5.6μm pixel size)
- a 500mm/f8 mirror telephoto lens
- a 135mm/f2.8 macro lens
- a 50mm/f1.8 lens

However I am not very sure if I can obtain enough spectral resolution (or dispersion ???) with those optics. I would at least get some proms in h-alpha.. (that means I need to get ~0.8 angstrom per pixels right ?).
I need to make some calculation but I lack some informations. Fortunately I will receive the book "Imaging Sunlight Using a Digital Spectroheliograph" shortly, It will surely provide everything I need :)

I considered to build a Y-shapped instrument similar to this one http://www.astrosurf.com/buil/us/stage/calcul/spec.jpg as it seems to be the easiest to assemble.

So, that is where I am ! I hope you don't find my idea too farfetched :)

Looking forward to reading from you all.

Taratasy

PS : I use google translat occasionally, forgive me if some sentences are weird.


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Re: Building a basic spectroheliograph

Post by Merlin66 »

Welcome on board, mate!
There are a few members who have already built their SHG's and I'm sure will be ready to assist.
When you read the book I'm sure everything will be much clearer.
I also have a spreadsheet which will assist you to validate your design and estimate the performance.
It's currently available over on the files area of the Astronomical Spectroscopy group - look for SimSpec SHG. If you have problems drop me a PM or email for a copy.
Other than the fast frame camera (mono preferred) there are only three things to build and assemble:
The telescope
The entrance slit
and the spectrograph.


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Re: Building a basic spectroheliograph

Post by taratasy »

Thanks Merlin, the spreadsheet and the book are a great help !

I think I found a viable SHG configuration using the elements I mentioned above :

- I will use the 50mm/F8 mirror lens with a barlow x2 as the telescope (1000mm focal lenght in total)
- The 135mm/f2.8 macro lens will be the collimator and the 50mm/f1.8 lens will be the camera lens
- I will use a 12.5x25mm reflection grating at 2400 grooves/mm
Now, the collimator/camera angle would be 20° and the camera/grating distance 120mm
Considering the lenses dimension and d1, everything should fit together whithout any problem !

In the end :
- The sun height should cover almost all the sensor's length.
- Spectral resolution at 6563 Å : 0,95 Å (Not as much as I wanted but hey, you can't achieve miracles with so little to beggin with). It will be enough for Ca-K though.

Well I find all of that very exciting, I hope it works in practice !


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Re: Building a basic spectroheliograph

Post by Merlin66 »

Tara(?)
Caution.....
With no ERF etc. the mirror lens may not be a good choice for the telescope, the build up of the solar energy on the secondary mirror/ corrector may give problems.
A cheap and cheerful 60/900 achromatic refractor would achieve the same job.


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Re: Building a basic spectroheliograph

Post by taratasy »

I got a 50/500mm (f10) achromatic and redesigned the whole construction, things got wayyy easier :) !

But I have a little concern : I need to use a collimator with 200mm focal length, fortunately I own a small telescope finder 50/200mm (f4) which would do the job. However the SHG spreadsheet tell me that my collimator must work at f10. Can I just put an opaque mask with a 20mm hole in front of the lens to achieve the correcte aperture/focal ratio or should I get a real 200mm length f10 lens at SurplusShed or any 200mm telephoto-lens with a f10 stop ?

EDIT : in the book, chapter 7.1, it says as an exemple that the apperture of the lens should be stopped down if the focal ratio is smaller than the telescope's one. Well I guess I will simply use an opaque mask. :mrgreen:

Many thanks.


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Re: Building a basic spectroheliograph

Post by Merlin66 »

Tara,
No real need to fit a physical aperture stop to the collimator....
The incoming f10 beam from the telescope through the entrance slit will automatically generate a f10 beam at the collimator and act as an aperture stop.
Hope this helps.


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Re: Building a basic spectroheliograph

Post by taratasy »

Although I waiting for my diffraction grating, it does not stop me from using my setup :

Quick shot of the moon :
hourq.jpg
hourq.jpg (41.25 KiB) Viewed 10488 times
The slit made from two razor blade seems to work ! Focusing is a bit harsh but I will surely get better at it.

Next step : The Sun !
More to come in two or three weeks :D


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Re: Building a basic spectroheliograph

Post by highfnum »

hey another moon image!!


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Re: Building a basic spectroheliograph

Post by taratasy »

My SHG is almost finished ! I made a try today, I can see clearly the spectral lines with my webcam.
I tried to take a picture in CaK but the result is pretty bad :
derp.jpg
derp.jpg (14.32 KiB) Viewed 10386 times
I find it pretty difficult to have the sun on focus on the slit, is there a way to tell or do we have to guess and try ?
Plus my slit is dusty..

I should greatly improve in the next few days.


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Re: Building a basic spectroheliograph

Post by marktownley »

You're getting there!


Image
http://brierleyhillsolar.blogspot.co.uk/
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Re: Building a basic spectroheliograph

Post by Merlin66 »

Slit focus - a bit of trial and error.
Look at the limb of your image - it's not crisp and tight - this should be as clear and crisp as possible.
(See "Imaging Sunlight", p143)


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Re: Building a basic spectroheliograph

Post by taratasy »

Almost there !

Ha
16_17_56d_000 - copie.jpg
16_17_56d_000 - copie.jpg (72.66 KiB) Viewed 10357 times
CaK
16_57_33_000 - copie.png
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Re: Building a basic spectroheliograph

Post by highfnum »

getting there


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Re: Building a basic spectroheliograph

Post by Merlin66 »

Do you know what slit gap you're using?
Just needs a little more practice......
Onwards and Upwards.


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Re: Building a basic spectroheliograph

Post by taratasy »

Merlin66 wrote:Do you know what slit gap you're using?
No I don't, I tried to make the gap the thinnest possible by eye. Now I got a laser pointer so I will get the right width.


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Re: Building a basic spectroheliograph

Post by Merlin66 »

I think the colour webcam chip will become an issue...
The size of the solar image v's chip size I think will need a bit more processing.
All in all you're doing well....keep it up.


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Re: Building a basic spectroheliograph

Post by taratasy »

Thanks, I keep you updated.
I will soon get a Zwo ASI120MM-S mono !


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Re: Building a basic spectroheliograph

Post by Merlin66 »

Sounds good!
Next time upload a couple of images of your SHG, it will be interesting to others....


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Re: Building a basic spectroheliograph

Post by Montana »

I think the camera may be causing a few problems but it is working, I see proms and plage :hamster:

Alexandra


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Re: Building a basic spectroheliograph

Post by taratasy »

Here is the beast :

IMG_6875 - copie.jpg
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IMG_6876 - copie 2.jpg
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IMG_6877 - copie.jpg
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It is not a work of art.
The telescope and collimator are held in place by two sets of 3 screws mounted on PVC tubes, it greatly helps to align the optics.
The camera lens is held in place by... rubber band. This is the best I can do for the moment but it works well.
The diffraction grating is glued on a wood piece, which is glued on a rotative platform made of an old lens cap.

There is several things I need to improve :

- Find another way to held the grating on the platform
- Find another way to held the camera lens
- Protect the whole thing from parasitic lights
- I need to align correctly all the optical components, it is kind of wonky at the moment (as you can see in the third pic).


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Re: Building a basic spectroheliograph

Post by Merlin66 »

Looks promising!
I'd suggest a couple of rings to support and align the camera lens similar to the ones you use on the telescope/ collimator.
You can use a laser collimator to align all the optics and grating. Just mount it central and square to the telescope objective....
Place a wire (ex coathanger?) loop over the grating area and then drape the instrument with a black cloth to reduce the parasitic light.
(I don't see the blaze arrow on the top of the grating??? This arrow should point towards the imaging lens.)


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Re: Building a basic spectroheliograph

Post by taratasy »

Thanks for the tips !
The arrow is present but it does not appear on the photo I don't know why.

I am worrying because vertical lines started to appear on the grating. I manipulate it with great care so I don't really understand what causes this.


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Re: Building a basic spectroheliograph

Post by Merlin66 »

Taratasy,
Can you explain further??
Are you seeing "defects" in the grating surface??


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Re: Building a basic spectroheliograph

Post by taratasy »

I bought it from edmund optics, optometrics was out of stock at the time. It was clean and without defects when I unboxed it.

Here is a picture : I first noticed those lines when they where smaller than that, the next day they had grown in lenght and number. Fortunately they seem to have stopped !
IMG_6879 - copie.jpg
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Re: Building a basic spectroheliograph

Post by Merlin66 »

Hmmm
Yes, I've seen similar grating faults...usually they do not have a major impact on the performance and can be caused during the manufacture of the grating.
It's unusual to see them change over time...
The epoxy substrate may be causing the problem, but as I say, it's not something I've seen before.


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Re: Building a basic spectroheliograph

Post by taratasy »

Tadaaa ! I'm getting better at this !

CaH sun, treated with photoshop thanks to Wah's awesome tutorial ;) (couldn't get ride of the horizontal lines...)
I seriously need to dump my webcam. The resolution is atrocious and even the minimum exposure is too high -> EVERYTHING is overexposed except the CaH and CaK lines.

This is my first "not so bad" picture !
16_42_36_000 - copie copie.jpg
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Re: Building a basic spectroheliograph

Post by Merlin66 »

Taratasy <name?>
Certainly improving!
The horizontal banding must be related to camera artifacts....
Onwards and Upwards.


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Re: Building a basic spectroheliograph

Post by Montana »

:hamster: fabulous! keep up the good work :hamster:
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Re: Building a basic spectroheliograph

Post by taratasy »

Many thanks, all of you guys are a great support !

I put the project on standby untill I get my new camera, I can't get anywhere with my current webcam :roll:


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Re: Building a basic spectroheliograph

Post by highfnum »

much much better !!!


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Re: Building a basic spectroheliograph

Post by highfnum »

Once your done with this project
Start monochromator!


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Re: Building a basic spectroheliograph

Post by taratasy »

taratasy wrote:I put the project on standby untill I get my new camera
In fact, no.
I made some improvement :

- a new holder for the grating, I am very proud of it :mrgreen:
IMG_6996 - copie.jpg
IMG_6996 - copie.jpg (125.1 KiB) Viewed 10202 times
- a new slit made with cutter blades, more rigid than razor blades -> it makes it easier to set the correct slit width. I didn't tested it yet, it could cause horizontal lignes, I hope not.
IMG_6997 - copie.jpg
IMG_6997 - copie.jpg (109.76 KiB) Viewed 10202 times
- a new imaging lens which will better fit the new Zwo ASI 120MM camera. This is a Vivitar 75-205mm F3.8, it will be set around 160mm for a good fit of the sun's heigt on the sensor. I think it will also be nice to visualize a wider portion of the spectrum at 75mm :)
(yeah still haven't got rid of rubber bands)
IMG_6998 - copie.jpg
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If the SimSpec excel sheet is right (I'm sure it is) I will achieve a resolving power of 13600 and a dispersion of 0,23Å/pixel (binning on x axis at 2), that's not bad at all for what I hoped to achieve !

I just have one question for the correct slit width :

The pixels are 3.75μm and I need 2 pixels for recording the best signal -> thereby my slit should be 3.75x2 = 7,44μm wide
However I have a reducing factor of 160/200 = 0.8, so the slit gap should be adjusted at 7,44x0,8 = 6μm wide.
Am I correct on this ? 6μm is very tiny :shock:

Thanks all !


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Re: Building a basic spectroheliograph

Post by Merlin66 »

Taratasy,
Your arithmetic is good....but I think you'll end up with a gap closer to 10-15micron...
This will still be OK due to the anamorphic factor.....
You may find pencil sharpener blades (flat side towards telescope) work even better.....
taratasy SHG.JPG
taratasy SHG.JPG (253.7 KiB) Viewed 10182 times


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Re: Building a basic spectroheliograph

Post by taratasy »

Thanks Merlin !

I downloaded the spreadsheet "SimSpec SHGa.xls" from the yahoo group "Astronomical Spectroscopy for Amateurs".
There is an typo error : cell D29 and D30 are referencing cell G18 (blank), I think it should be cell G17 :idea:


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Re: Building a basic spectroheliograph

Post by Merlin66 »

Whoooops,
You are correct!
I'll amend and update.
Thanks.


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Re: Building a basic spectroheliograph

Post by taratasy »

Welp, I am having problems.

My construction is definetly too poor for correct imaging.
The component can't be aligned correctly, focusing is really difficult to achieve, the camera can't fit tightly to the imaging lens, the whole setup is too heavy for my mount, it shack during acquisition, I got fringe patern on the AVI... and more and more frustration.
A total mess.

I need to reconstruct my SHG in a solid and efficient way, I got some ideas and the motivation.
However I won't be able to do it before next summer (I go back to studies), the project is now definetly in standby.


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Re: Building a basic spectroheliograph

Post by highfnum »

sorry to hear this


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Re: Building a basic spectroheliograph

Post by taratasy »

It was something I expected when I first decided to build a SHG. I lacked proper tools and materials, plus it was the first time I built something "complicated".
When I wrote my previous comment I was a little bit frustrated by the amount of time it takes for me to set everything right and take a picture, it is not so bad as I said.

I'm a still very happy to have been able to take a few shot of the sun :D

btw here the best picture I could get of the sun. It is not bad at all, it is just very difficult and time consuming to get this result with my SHG so I prefer stop and rebuild the thing later in order to get the SHG easier and less frustrating to setup and use ;)
16_12_04_001 - copie copie.jpg
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Re: Building a basic spectroheliograph

Post by highfnum »

image focus is good - not sure how you got that split image did you bump it or wind gust


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Re: Building a basic spectroheliograph

Post by taratasy »

It is caused by dropped frames.
The two parallele dark lines are caused by the slit but they normaly should be horizontal. Here they appear tilted because the optical train disalign itself during the capture...


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Re: Building a basic spectroheliograph

Post by Merlin66 »

Taratasy,
Don't be to hard on yourself...
You have managed to assemble a great "proof of concept" SHG and obtained your first spectroheliograms. Well done!
This experience will help you to understand the critical aspects of the SHG design and allow you to better appreciate the accuracy and rigidity required for ongoing success.
You've travelled a good way down the path, your next SHG I'm sure will be much better.
Onwards and Upwards


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Re: Building a basic spectroheliograph

Post by highfnum »

you are at a more advanced point then I was at same point in time (based on your post)
it took me a long time to get decent images
I still suck at post image processing
I plan to spend more time at this in my retirement
and with chapter 8 in Ken's book


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Re: Building a basic spectroheliograph

Post by taratasy »

Hi Guys, It has been a long time!
Since last time I completely stopped my SHG project because of a lack of time (end of holidays, had to go back to study) and lack of motivation, I must admit.
Now, I still don't have much time, but I want to try again and build a new SHG (the old one was too wonky, I dumped it). I still have my diffraction grating, the telescope, collimator, and I recently got the famous adjustable optic slit from SupluShed! Now I just need to put everything together in a non-wonky way :mrgreen:
You will hear from me in the coming months.


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Re: Building a basic spectroheliograph

Post by taratasy »

Hi Merlin66,
In your book, chapter 7 page 132, you write that the "Small Refractor Classical Spectrograph" would achieve "a dispersion of 0.15 Å/pixel, a resolution (at Hα −6563 Å) of 0.61 Å (R = 10800) and a wavelength coverage of 244 Å.".
With:

- Telescope—80 mm, 400 mm focal length, f5
- Collimator—50 mm, 180 mm focal length, working at f5
- Entrance Slit—15–20 μm gap, 6 mm long
- Grating—50 × 50 mm, blazed 5000 Å, 1800 L/mm reflective grating
- Total angle between collimator and imaging lens = 35°
- Imaging lens—50 mm, 150 mm focal length, working at f3
- Imaging camera—Webcam/DMK21, 5.6 μm pixel, 480×640 array, 2.8×3.7 mm chip.


However, when I use the SHG spreadsheet (July 16 revision) and enter the configuration above I get a different result:

- Dispersion (r): 0.39 Å/ pixel
- Resolving power (R) : 8460
- Spectral resolution (at 6563 Å): 0.78Å
- Wavelength range 93Å

Am I doing something wrong or is the spreadsheet working differently now?

zdzdz.jpg
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Re: Building a basic spectroheliograph

Post by Merlin66 »

Hmmmm
OK double checking, the answer you achieved seems to be correct.....I note you have selected x2 binning. Without this option the R value increases dramatically, from R=8460 to R=16921
I think I've found where the differences have crept in...During evaluation I varied the cameras and the results shown/obtained were with a DMK51 and x2 binning.
Sorry for the confusion-hope this helps.


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Re: Building a basic spectroheliograph

Post by taratasy »

Merlin66,
Thank you for your response. Mystery solved :D


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Re: Building a basic spectroheliograph

Post by taratasy »

So, I played a while with the SHG spreadsheet and I found out a good design for my new SHG. It will have a classical design using camera lens, greatly inspired by Chagnard's work.

The different parts are:

Telescope: a 50mm f/10 lens, fited in a PVC tube
Slit: the one from Surplus Shed
Collimator: 200mm f/5.6 Tele-Takumar lens
Grating: 25x25 1800 lines/mm
Camera lens: 200mm f/3.5 Vivitar lens
Camera: ZWO asi120mm-s

I should get a resolving power of 18000, a spectral resolution of 0.37A and the solar height will fit just right (102%)!
Now I need to find out how I will fit the optical elements on a plywood base and how to correctly align them.
IMG_5954.JPG
IMG_5954.JPG (424.33 KiB) Viewed 8075 times

I already found a way to fit the slit on the Tele-Takumar lens cap:
IMG_5956.JPG
IMG_5956.JPG (348.56 KiB) Viewed 8075 times
Last edited by taratasy on Wed Aug 23, 2017 11:59 pm, edited 2 times in total.


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Re: Building a basic spectroheliograph

Post by Merlin66 »

Looking promising....
The slit assembly needs to be reversed - the flat side of the slit should face the incoming beam from the telescope.
(I remove the four small screws and use the larger "adaptor" on the other side. Hope that makes sense)


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Re: Building a basic spectroheliograph

Post by taratasy »

Oh, the flat side is already facing the incoming beam from the telescope.


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Re: Building a basic spectroheliograph

Post by Merlin66 »

Ah....
I mis read your message....


"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
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"Imaging Sunlight - using a digital spectroheliograph" - Springer
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