SHS/ SHG Holographic Grating choices

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solarGain
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SHS/ SHG Holographic Grating choices

Post by solarGain »

Well I am again embarking on my second SHS/ SHG. This time its a Fluvio Mete Littrow imaging train layout. A ultra lightweight setup. And its linear.
I have the rail, the two telescopes, FS-60 taking the incoming light and a ST80 taking the retroreflective grating that returns the light to a 3-port box/ wedge etc. To be discussed later.
So if been looking at a Holographic grating with 2400 g/mm and there is only one company here in the UK, Edmundsoptics.com, that provides off the shelf holographic gratings. What I find difficult to understand, is why are the Diffraction Efficiency numbers so low - 45 to 65%. So thats one problem to figure out.
The second is whether to go for a Rowland curved grating that would allow for little or no tuning/ turning to find a selected wavelength, with more than >85% reflection on zerodur glass.

What do you think?
John


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Spectral Joe
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Re: SHS/ SHG Holographic Grating choices

Post by Spectral Joe »

The difference in efficiency is due to groove profile. Holographic gratings have a sinusoidal groove profile, because of the way they are made. Ruled gratings have grooves with flat sides, the angle of which is chosen to enhance efficiency in the spectral range of interest, called "blazing". Holographic gratings have lower ghosting and scattered light, in applications where this is important it offsets the lower overall efficiency. As for a concave grating, the mount geometry gets complicated, and astigmatism keeps the usable field very small.


Observing the Sun with complex optical systems since 1966, and still haven't burned, melted or damaged anything.
Not blind yet, either!
Light pollution? I only observe the Sun, magnitude -26.74. Pollute that!
solarGain
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Re: SHS/ SHG Holographic Grating choices

Post by solarGain »

Joe much appreciated. Thank you for clearing up the lower efficiency.
As regard the Rowland, ultimately want we want is a 'small' portion of the spectrum that revels Ha, Ca-K etc. Or is it too fiddly to reach these lines? Or is it something else.
And secondly is astigmatism on a zerodur base still prevalent in today's machining tolerances practice. This unit is also sold by edmundoptics.com
All the best
John


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Re: SHS/ SHG Holographic Grating choices

Post by Spectral Joe »

The rules for resolution and dispersion are the same for concave gratings, Rowland circle mountings aren't used much outside of the vacuum ultraviolet, Eagle or Wadsworth mountings are the usual choice. The astigmatism problem with concave gratings isn't due to the figure of the substrate, it's from the grating being tilted to get to the wavelength of interest and the optical axis now being far off of the axis of the sphere the grating is ruled on. This angle can be large, far greater than the off axis angles in plane grating mounts. Edison Pettit published a partial solution in 1931: http://adsabs.harvard.edu/full/1931PASP...43...75P and in recent times gratings ruled on toroidal substrates have been used to correct this.

Joe


Observing the Sun with complex optical systems since 1966, and still haven't burned, melted or damaged anything.
Not blind yet, either!
Light pollution? I only observe the Sun, magnitude -26.74. Pollute that!
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Re: SHS/ SHG Holographic Grating choices

Post by solarGain »

Well, thank you for clearing up this last question up, and the additional short paper supports this.
John


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Re: SHS/ SHG Holographic Grating choices

Post by solarGain »

Oh, what precautions to do you use to keep dust away from the face of your gratings? Or do you just dismantle and put back into box, face up.
I believe that Edmunds' allow bulb blowers to be used.
Any thoughts.
John


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Re: SHS/ SHG Holographic Grating choices

Post by Spectral Joe »

The whole thing is enclosed and well sealed, it's only opened for occasional adjustments. All the optics stay clean. A safe way to remove dust is to use a regular duster can with a long extension made from thin plastic tubing, the can is left stationary on a flat surface so there's no chance of liquid coming out and the tube can reach places the can never could.

Joe


Observing the Sun with complex optical systems since 1966, and still haven't burned, melted or damaged anything.
Not blind yet, either!
Light pollution? I only observe the Sun, magnitude -26.74. Pollute that!
solarGain
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Re: SHS/ SHG Holographic Grating choices

Post by solarGain »

It would be extremely useful to have a photo that shows this (enclosed and well sealed) housing. Over the years I have done many searches on the internet that bring up 'nothing'. A professional such as yourself has tried and tested this particular safeguard and I think this would be quite instructive to this audience.
Many thanks for your insight.
John


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Re: SHS/ SHG Holographic Grating choices

Post by Spectral Joe »

The box is 1/4" plywood, with a double layer of 3/4" plywood for the bottom. It's internally baffled and divided into five sections, the objective section, the turning and flat field optics, the grating and Littrow lens, the slit, scanner and detector and an intermediate center area that the light traverses. It's all painted black inside, with four access panels that are removable. These panels are 1/4" plywood with 1/2" square wood strips that fit into the openings in the top of the enclosure, light and dust would have to make two turns to get in, and the covers fit closely. This enclosure has been in use for 7 years in a dusty garage, no dust on any internal optics in that time. Only the coelostat mirrors and the first surface of the objective are exposed to the outside, and they have dust covers.

One of the lids
GEDC1788.JPG
GEDC1788.JPG (994.64 KiB) Viewed 1826 times


Observing the Sun with complex optical systems since 1966, and still haven't burned, melted or damaged anything.
Not blind yet, either!
Light pollution? I only observe the Sun, magnitude -26.74. Pollute that!
solarGain
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Posts: 97
Joined: Fri May 08, 2020 1:38 am
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Re: SHS/ SHG Holographic Grating choices

Post by solarGain »

More mysteries being exposed and more questions to ask...
Thank you very much Joe.
John


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