Some advise on building a SHG?

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Brett
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Some advise on building a SHG?

Post by Brett » Sat Sep 19, 2015 7:41 pm

I have been wanting to build a SHG for a long while now. I will finally have some time towards the end of the year and would like to get my resources ready in the meantime. Any guidance on where to start would be appreciated.

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Re: Some advise on building a SHG?

Post by highfnum » Sat Sep 19, 2015 9:17 pm

well you have come to the right place
I just finished mine a few month back
Merlin66 help me a lot also help from others -- Wah --etc
see my threads in this forum "higfnum"
some of the issues I had to deal with
I am still learning - there is a lot to learn - post capture image processing in particular

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Re: Some advise on building a SHG?

Post by Merlin66 » Sat Sep 19, 2015 10:09 pm

Brett,
Hopefully by then "Imaging Sunlight - using a digital SHG" (Springer) should be available.
It has examples of all the current SHG, reviews and examples of all the SHG processing software as well as the technical details you need to know.
In the meantime google on "Spectroheliograph" and look for names like Rondi, Rousselle, Defourneau and Mete.
If I can assist further, just drop me an email.
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.yahoo.com/neo/groups/ast ... scopy/info
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer

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Re: Some advise on building a SHG?

Post by Brett » Sat Sep 19, 2015 10:42 pm

Thanks you guys. I will read up a bit more and ask. The information on the web I found on spectroheliographs is quite vague.

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Re: Some advise on building a SHG?

Post by Merlin66 » Sat Sep 19, 2015 11:26 pm

The construction of an SHG is straight forward:
A telescope to image the solar disk onto the slit of a spectrograph
A spectrograph (Slit/collimator/ grating/imaging lens)
Harrison Fig 6_26.jpg
Defourneau SHG
Harrison Fig 6_26.jpg (43.15 KiB) Viewed 1849 times
Harrison Fig 4_1.jpg
Skywatcher SHG
Harrison Fig 4_1.jpg (50.98 KiB) Viewed 1849 times
and a fast frame mono camera.
There are four or five freeware software packages which can do the spectroheliogram processing.
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.yahoo.com/neo/groups/ast ... scopy/info
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer

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Re: Some advise on building a SHG?

Post by highfnum » Sun Sep 20, 2015 11:40 am

my SHG a real home made job
myhgs.jpg
myhgs.jpg (363 KiB) Viewed 1832 times

Brett
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Re: Some advise on building a SHG?

Post by Brett » Sun Sep 20, 2015 1:46 pm

Thank you for the info so far.

I have an 80mm , 1000mm fl telescope objective. From what I gather, this should be focused on a slit 1000mm away. This can be done using the focuser of the blue telescope in Highfnum's photo.

So now, this brings me to a couple of questions:

1.) Is there a guideline as to what slit I should buy?
2.) You want the beam from the main objective to go through another lense that will make the beam parallel, so that it strikes the grating as a parallel light beam. What lense do you use to get the light parallel? Just a negative lens or barlow? How does the diameter and FL of this lens correlate with the main telescope objective?
3.) Grating should be Blazed at 500nm? And about 1800 lines per mm? For 1000mm FL, Sun image is about 10mm So this grating should do fine? https://www.thorlabs.com/thorproduct.cf ... =GR25-1850

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Re: Some advise on building a SHG?

Post by Brett » Sun Sep 20, 2015 2:46 pm

In my question number 2 above, I assume that the negative lens should have a focal ratio that matches the focal ratio of the main telescope, in my case f/12.5. How far should this be placed from the slit on main telescope side and grating on the other side?

Merlin66, it seems that you made a parallel beam by just "reversing" your telescope closest to the grating? So it is just achieved by reversing a normal achromat? I have a binocular objective of 50mm diameter and 190mm focal length. Can I just place this reversed in the optical path and stop it down to 15mm to get the f/12.5 ratio?

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Re: Some advise on building a SHG?

Post by Brett » Sun Sep 20, 2015 3:36 pm

Okay I have now figured out that if using an achromatic objective for the collimator, it should be placed its focal length away from the slit. So in theory I would have a 80mm f/12.5 objective, placed from this a slit at 1000mm (at focal plane), then placed 250mm away from slit a 20mm achromatic lens at f/12.5. This would give me a collimated beam. Inside this, the distance does not matter, and I can add the grating given in a previous post link. The grating is 25x25mm, and the D of the collimated beam is ((250 x 80)/1000=20mm). This falls inside the grating.

Is this correct so far?

As for the slit, I assume the vertical distance of the slit should at least be the diameter of the Sun at the image plane? So in this case, at least 10mm? How should the width of the slit be calculated?

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Re: Some advise on building a SHG?

Post by Brett » Sun Sep 20, 2015 4:03 pm

Any opinions on this slit? Width is 150 microns. The photographic version is much cheaper, dont really know if I should rather get that one.

http://www.lenoxlaser.com/index.php?dis ... duct_id=17

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Re: Some advise on building a SHG?

Post by Merlin66 » Sun Sep 20, 2015 5:11 pm

Brett,
You're getting the idea....
Which camera and imaging system do you intend using?
First approximation the entrance slit gap should be 2-3 times the pixel size of the camera..
i.e. a 5.6 pixel camera would use a 15 micron gap.
The grating needs to be large enough to accept the height of the solar image - say 6-8mm = 20 +8 = 30mm grating.
10mm long slits on such a small SHG is problematic - 6mm seems to be closer to standard.
Unfortunately Surplus Shed are out of stock (again!) of their adjustable slit assembly which is widely used by the amateur.
My book "Astronomical Spectroscopy for Amateurs" covers all the design theory (and more).
Over in the files area of the Yahoo group there are construction details for a couple of spectrographs which may be of interest.
https://groups.yahoo.com/neo/groups/ast ... copy/files
Hope this helps.
Ken
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.yahoo.com/neo/groups/ast ... scopy/info
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer

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Re: Some advise on building a SHG?

Post by Brett » Sun Sep 20, 2015 6:49 pm

Hello Ken

Thank you for this info. I will head over check out some of the yahoo files.

Camera is a monochrome QHY5 L ii CMOS sensor at 3.7 microns. But I may upgrade to a 5.6 micron in the next two years, so I will go for an approximate 10 micron gap. As for a second lens to focus the image onto the camera chip, I was thinking of just using a normal achromatic lens...? This should work or what do you recommend?

Just another question regarding the slit: Why are we using a slit with a length much smaller than the solar disk? In other words you suggest 6mm, but the disk at 1000mm FL is 10mm?

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Re: Some advise on building a SHG?

Post by Brett » Sun Sep 20, 2015 7:03 pm

Ken, will this grating do? It is on the bottom of the page, in the 4th column:

http://www.optometrics.com/reflection-gratings

Specs are 30mm x 30mm, 1800 lines/mm, Blaze = 500nm

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Re: Some advise on building a SHG?

Post by Merlin66 » Sun Sep 20, 2015 11:28 pm

Brett,
You can have a long slit length, but it would probably mean you have to make your own slit assembly. The commercial slits I think are restricted to 6mm or so.
For the past ten years I've successfully used Optometric gratings in all my spectroscope designs. The 30mm sq. 1800 l/mm would be a good starting point.

The Classical design of the spectrograph makes use of two achromats - definitely needs a focusing mechanism to compensate for the chromatic aberrations.
With the QHY, the size of the chip is only 3.85 x 4.8mm, these means that the collimator/ imaging lenses must provide a reduction magnification (if you want to achieve full disk results), based on your 10mm solar image at the entrance, of 4.8/10 = x0.4.
This then would lead to a collimator of at least 30mm diameter, fl 375mm (f12.5 to match the scope) and a slightly larger aperture ( to catch all the dispersed beam from the grating) of say +/-35mm diameter, but with a fl of 150mm or so.
This would then produce a spectral image approx. 4mm high on the camera chip.
The angle between the collimator/grating/ imaging lens should be kept to a minimum; 30-40 degrees with both lenses as close to the grating as possible.

Hope this helps.
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.yahoo.com/neo/groups/ast ... scopy/info
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer

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Re: Some advise on building a SHG?

Post by Brett » Sun Sep 20, 2015 11:44 pm

Hi Ken

I will see how I can fit everything in the design so that I have the correct parameters. Will do a sketch in the meantime.

Why did you mention in your previous post that a slit of 10 mm is problematic?

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Re: Some advise on building a SHG?

Post by Merlin66 » Mon Sep 21, 2015 12:13 am

Brett,
Only that it's easier (!?) for the novice to start with a commercial slit - more accurate...
The use of long slits on short focal length (fast f ratio) telescopes can give vignetting and cause line distortions (smile) within the spectrograph. Fresnel/ field lens can be used behind the slit but this adds to the complexity....KISS

ADDED: You should look at some of the older SLR telelenses - Zuiko/ Pentax etc. Some of these are very good for use as imaging lenses (and you get a focus mechanism for free ;-) ) In the example a 135mm f4 lens would work.
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.yahoo.com/neo/groups/ast ... scopy/info
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer

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