quick question

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highfnum
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quick question

Post by highfnum » Mon Dec 07, 2015 11:47 pm

does anyone use prisms instead of gratings for SHG?
I read they are more efficient because they only have
1 order
true?

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Merlin66
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Re: quick question

Post by Merlin66 » Tue Dec 08, 2015 3:29 am

Yes, a prism is more efficient, but needs to be pretty large to give good dispersion.
The early spectroscopes could have up to seven prisms in a train to give a high dispersion.
To day, seven quality prisms would cost far more than a 2400 l/mm grating!
"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

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Re: quick question

Post by highfnum » Tue Dec 08, 2015 9:35 pm

i understand you need odd number
but why 7 is that a bit overkill?

what types of prism are used ?

anyone in this forum use them for SHG or other spectroscopes?

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Merlin66
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Re: quick question

Post by Merlin66 » Tue Dec 08, 2015 11:31 pm

Normally heavy flint 60 deg prisms are used.
The dispersion is directly related to the length of the prism base - larger prisms (or more prisms in the train) give larger dispersion.
It takes approx. 10mm length of prism to give the same dispersion as a 100 l/mm grating....
So, for a 1200 l/mm you'd need 12 x 10mm of prism length!
Surplus Shed sell small prisms and also a Littrow prism (a 30 deg back silvered prism) - interesting to play with.
Hale's original SHS used two large prisms....
The Amici direct vision spectroscopes (five prisms - flint and crown combo) were widely used in the early years of spectroscopy, but again have limited dispersion.
"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: quick question

Post by Wah » Thu Dec 10, 2015 12:54 am

Two questions:
1. If the light source is bright enough, why should we consider high efficiency?
2. Does prism absorb some parts of the spectrum?

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Merlin66
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Re: quick question

Post by Merlin66 » Thu Dec 10, 2015 12:56 am

Wah!,
It's not the efficiency (for solar) but the lack of dispersion/ resolution......
The flint glass doesn't transmit well in the UV....
"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: quick question

Post by Wah » Thu Dec 10, 2015 5:49 am

My first question raised
"1. If the light source is bright enough, why should we consider high efficiency?"

because of the topic question:
"does anyone use prisms instead of gratings for SHG?"

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Re: quick question

Post by Merlin66 » Thu Dec 10, 2015 5:58 am

Wah!,
OK.
For astro spectroscopy prisms are still popular (generally as objective prisms) due to the efficiency and lack of a zero order image. The real problem is the limited sizes (and high costs) of good quality prisms.
As I mentioned, usually we are looking for high dispersions for the SHG which calls for multiple/ or large prisms. Much more cost effective to go with a good 1200/ 2400 reflection grating.
HTH
"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: quick question

Post by highfnum » Sun Dec 13, 2015 5:26 am

any one use transmission grating for SHG's

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Merlin66
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Re: quick question

Post by Merlin66 » Sun Dec 13, 2015 6:07 am

Yes, Douglas Smith uses a transmission grating 1200 l/mm, 25mm x 25mm (Thorlabs part number GT25-12) with a grism arrangement. This is detailed in the Book "Imaging Sunlight"
"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

highfnum
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Re: quick question

Post by highfnum » Sun Dec 13, 2015 1:38 pm

yea - but appears that 1200lpmm is limit for transmission grating while
reflective grating go much higher..

also I enjoy looking "inside" the h-alpha line visually while sun parades across it seeing proms and bright spots

has anyone -- tried by hand to developed a full sun disk image based just watching
whats happening visually and then record it down
I may try this and see how close I get to a recorded image

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