Custom-made fused quartz slits for SHG -- 9 micron x 12 mm

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thesmiths
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Custom-made fused quartz slits for SHG -- 9 micron x 12 mm

Post by thesmiths »

For some time, the limiting factor in our spectroheliograph (SHG) experiments has been the slit. Just as a review of the existing landscape, the three main types of slits are: (1) adjustable metal jaws; (2) fixed-width laser-etched stainless steel foil; (3) lithographically etched metal on glass. Each of these has some unsatisfactory features:

(1) The adjustable metal slits are long but typically limited to how narrow the slits can get because of issues with edge roughness and parallelism.

(2) Laser-etched stainless steel slits are quite good but are usually quite short (around 3mm), which limits the imaging size. They also have the problem of being very prone to dust landing on the air gap; the slits are basically impossible to clean (apart from what can be removed by blowing air). In my experience, slit widths below 20 micron do not provide very good uniformity. The Thorlabs version costs about 90 pounds https://www.thorlabs.com/newgrouppage9. ... up_id=1464. These slits were originally designed for laser experiments, where an air gap is desirable and length is not that critical (laser beam diameters are typically quite small). They are not reflective (in fact, they are usually blackened) and therefore tend to suffer under intensive solar radiation after long periods of exposure.

(3) The current state of the art is probably lithographically patterned metal on glass. There is no real limit to either the slit length or width and if they get dirty, they can be easily cleaned. A widely used example is the one sold by Shelyak https://www.shelyak.com/produit/se0223a ... t/?lang=en. These were originally designed for night-time spectroscopy and so have a few undesirable features. First of all, they are also fairly short (4.5mm) and the substrate (which I believe is soda-lime glass) is prone to thermal shocks -- i.e. cracking under intense solar radiation. The Shelyak example also is mounted in a way that requires "shielding" so that solar radiation does not "cook" the glue and potentially damage the black anodised aluminium holder. The cost is approximately 150 euros.

After trying to convince a few existing suppliers to make a design more tailored to SHG users, I decided to take a fresh look at the problem.

Length of slit: the maximum telescope focal length likely to be used is around 1000mm so the solar disk will have a projected diameter of about 9.2mm. Allowing room for positional error, it was decided that a 12mm slit length should be sufficient.

Slit width: the community of SHG users seems to have settled on a 10 micron width. There are a number of experimental parameters that depend on the slit width (spectral resolution, spatial resolution, light throughput, diffraction effects) so the choice of slit width is a complex compromise. Narrower is better, up to a point; but a few expert users recommended not going below 7 or 8 microns. I decided to be conservative and use 9 micron, close to the current commonly used 10 micron width that most users seem to be happy with.

Substrate material: the "chrome on glass" lithographic slits from Shelyak and others is a good starting point but this can be improved significantly. After some thought and investigation, it was clear that a much better material would be chrome on fused quartz. This material is what is used to make the semiconductor masks that are used in lithographic steppers. These steppers use very high intensity UV light and fused quartz is a much superior choice than both soda lime and borosilicate glass.

Why fused quartz? Due to the very low coefficient of thermal expansion (1/7 vs borosilicate and 1/18 vs soda lime glass), fused quartz is far more shock resistant. It also has a much wider transparent window (over 90% between 220 and 2000 nm). It has a much higher melting temperature and therefore much less likely to thermally deform. It is also harder, and therefore more scratch resistant, than glass.

glass versus quartz physical parameters
glass versus quartz physical parameters
glass vs quartz.JPG (99.65 KiB) Viewed 2714 times

Fused quartz is more expensive than glass but the main manufacturing cost is the photolithography, with fused quartz only adding around 10% to the final cost. The substrate was a standard 5 inch square plate (the typical size of a semiconductor mask), 2.25mm thick with a chrome coating of optical density of 5 (i.e. 1/100,000 attenuation). The 5 inch plate can be cut into 15mm x 15mm squares, similar the Shelyak design, in a 7x7 array. So I ended up with a lot of slits!

After a few months of planning, the actually manufacturing only took about a week and I received the finished quartz plate a few days ago. Here is a photo of the plate held up to sun-lit window so that the slits are visible. The 5 inch square plate is attached to a sticky plastic sheet and each cut 15mm square chip can be peeled off individually.

5 inch quartz plate with chrome photolithography
5 inch quartz plate with chrome photolithography
1368.jpg (237.59 KiB) Viewed 2714 times

Below is a photo taken with a 40X metallurgical microscope objective using transmitted light with a 25A red filter. The slit looks very straight and essentially "perfect". I was told by the manufacturer that it would require a scanning electron microscope at 1000X magnification to see any irregularities.

9 micron wide slit, fused quartz, at 40X magnification
9 micron wide slit, fused quartz, at 40X magnification
0857.jpg (27.27 KiB) Viewed 2714 times

For comparison, here is a 20 micron by 3mm Thorlabs slit taken with the same microscope. The defects seen are minor but they do have some effect on spectroscopic imaging. I believe the defects seen here are not dust or dirt but rather irregularities from the manufacturing process. I have also examined 10 micron and 5 micron Thorlabs slits -- they look much worse under the microscope since the defects stay about the same size but the width is narrower so the percentage variation is correspondingly larger.

20 micron wide slit, laser etched stainless steel, at 40X magnification
20 micron wide slit, laser etched stainless steel, at 40X magnification
0848.jpg (30.41 KiB) Viewed 2714 times

I have done some preliminary tests in a spectroheliograph just using standard light sources such as sodium and hydrogen. The results are nice clean spectra, as expected. I am currently doing some tests to evaluate durability when exposed to focused solar radiation. So far, the chrome on quartz chips have survived but I would like to do more extensive tests as there could be potential safety issues.


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Re: Custom-made fused quartz slits for SHG -- 9 micron x 12 mm

Post by ChrisHalpha2017 »

indeed the 9 um fused quartz is very neat and clean edge!! But i'wondering if a too thin slit wouldnt be counterproductive :

I mean at the focal plane of the main telescope you image the sun's image on the entrance slit. That means that the size of the smallest image point will have a certain physical size depending on the diameter, seeing and focal lengh. You can calculate the size of this "detail" and I think that the slit width should match the size of it or we loose light and maybe spatial resolution. This is what we do in stellar spectroscopy.

I dont do spectroheliography yet but im working on it. I use the wide slit method to image the prominences so i dont need to scan the sun but i noticed something important : when the anamorphosis is present I was gaining spectral resolution at the expense of spatial resolution. But still, shouldnt the size of the "airy disc" on the slit be taken into account ?


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Re: Custom-made fused quartz slits for SHG -- 9 micron x 12 mm

Post by Montana »

Wow!! I look forward to more testing and the ultimate test on the Sun :hamster:

Alexandra


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Re: Custom-made fused quartz slits for SHG -- 9 micron x 12 mm

Post by fulvio.mete »

Hi , The Smiths,
I almost totally agree about your analysis of slit type for solar spectrographs.
Anyway, for imaging (Spectroheliograms) I use on my four Hires spectrographs: VHIRSS, POSS2, UPS, HIRSS2, the SurpluShed adjustable air slits, (about € 20 cost )whose blades have been modified and rectified by me with a very simple method. The only problem is a light disuniformity in parallelism of the blades when i shut them to about 15 micron.Obviously the raw images suffers of some transversalium, that can be deleted in post processing.
For detection of Zeeman effect in sunspots with SolarScan (my Hires fully automated spectrograph) I use a 5 micron fixed air slit I bought at Edmund Optics some years ago, that works very well.I must say: however, another similar slit I bought last year was not worked so well and its perfomances are low (too much transversalium).
Your options of Quarz slits seems interesting, but, I'm afraid the cost of your slit plate could be high.
Fulvio


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Re: Custom-made fused quartz slits for SHG -- 9 micron x 12 mm

Post by thesmiths »

ChrisHalpha2017 wrote: Thu Jun 09, 2022 4:54 am I'm wondering if a too thin slit wouldn't be counterproductive: I mean at the focal plane of the main telescope you image the Sun's image on the entrance slit. That means that the size of the smallest image point will have a certain physical size depending on the diameter, seeing and focal length. You can calculate the size of this "detail" and I think that the slit width should match the size of it or we loose light and maybe spatial resolution. This is what we do in stellar spectroscopy. I don't do spectroheliography yet but I'm working on it. I use the wide slit method to image the prominences so I don't need to scan the Sun but I noticed something important: when the anamorphosis is present I was gaining spectral resolution at the expense of spatial resolution.
Yes, it's a bit like choosing the pixel size of a camera sensor depending on focal length of the telescope and the imaging scale. So clearly this will change depending on what telescope you chose. In SHG, the goal is primarily full disk images. My goal here was to target a focal length such that the Sun's projected diameter would be 5mm to 10mm.

As I understand it, the anamorphorphic effect "squashes" the image of the slit at the camera sensor. So this gives narrower spectral lines. But I can see if you are doing "wide slit" prominence imaging then by narrowing the apparent width of the slit at the camera sensor, you make it harder to achieve good pixel sampling of spatial features in the direction perpendicular to the slit length. I would imagine there is even an actual geometric distortion, with different scale factors along vs across the slit?


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Re: Custom-made fused quartz slits for SHG -- 9 micron x 12 mm

Post by thesmiths »

fulvio.mete wrote: Thu Jun 09, 2022 10:16 am I almost totally agree about your analysis of slit type for solar spectrographs.

Your option of quartz slits seems interesting, but I'm afraid the cost of your slit plate could be high.
Like with all things related to semiconductor processing, the fixed cost is high but the unit cost can be quite low. The cost of the making the 5 inch mask was 1400 pounds so the unit cost was 1/49 of this. I could have made the chips even smaller to get to 1/64 but I didn't think I could ever use, give away or sell that many so I made them larger to make handling easier.

The two most demanding optical elements are the diffraction grating and slit. The cost of the Edmund Optics 2400 l/mm 30mmx30mm visible holographic grating is around 200 pounds so a high quality slit with a unit cost below 50 pounds seemed reasonable.


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Re: Custom-made fused quartz slits for SHG -- 9 micron x 12 mm

Post by p_zetner »

This looks very promising!
Thanks for pursuing this.
Cheers.
Peter


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Re: Custom-made fused quartz slits for SHG -- 9 micron x 12 mm

Post by Massimo »

Good morning everyone. I recently bought the optics for the realization of Sol'ex (the rest I printed with carbon fiber filament, stronger and more heat resistant). I watched the video chat with Doug (really very interesting and informative, thank you) . I wanted to know if it is possible to buy one of those custom made quartz fused slits that I read about in this chat. Thanks again for all the very useful information you make available.


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Re: Custom-made fused quartz slits for SHG -- 9 micron x 12 mm

Post by thesmiths »

Massimo wrote: Fri Aug 18, 2023 11:18 pm I wanted to know if it is possible to buy one of those custom made quartz fused slits that I read about in this chat. Thanks again for all the very useful information you make available.
I still have a few quartz slits left from the original 49. You can email me or message me here about details. The most recent technical document on the slits can be found here: https://github.com/thelondonsmiths/Sole ... -SHG-slits


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Re: Custom-made fused quartz slits for SHG -- 9 micron x 12 mm

Post by pBandi »

thesmiths wrote: Sat Aug 19, 2023 3:15 pm I still have a few quartz slits left from the original 49. You can email me or message me here about details. The most recent technical document on the slits can be found here: https://github.com/thelondonsmiths/Sole ... -SHG-slits
Good Morning Doug,
Until now I followed only read only, the experiments you did for the whole community. Currently I use the optical design of solex, where the focal length is limited, but my aim is to go further in terms on fl. Unfortunately I did not found any direct contact to you, may I ask you to send me also the details how can one buy this 12mm long slit you managed to produce?

Best regards
Andras


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Re: Custom-made fused quartz slits for SHG -- 9 micron x 12 mm

Post by Rodler »

Hello,
I would like to buy such a slit from you. Would it be possible


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Re: Custom-made fused quartz slits for SHG -- 9 micron x 12 mm

Post by thesmiths »

Rodler wrote: Sun Nov 19, 2023 7:05 am I would like to buy such a slit from you. Would it be possible?
All the 9 micron wide slits are now gone. But I will shortly be making a second batch of quartz slits, which will likely be 8 micron wide. You can send me a private message regarding the new batch.


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Re: Custom-made fused quartz slits for SHG -- 9 micron x 12 mm

Post by WilcoS »

Hi, I am also interested in a slit from the new batch. But I have only recently joined this forum and I am not allowed to send you a message (yet). Can you send me a private message instead? Thank you very much!


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Re: Custom-made fused quartz slits for SHG -- 9 micron x 12 mm

Post by solarchat »

WilcoS , you should be good now. you have to await moderator approval on the first message so we can make sure you are not a scammer. :)


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Re: Custom-made fused quartz slits for SHG -- 9 micron x 12 mm

Post by WilcoS »

Thank you Stephen! I have sent Douglas a private message.

Kind regards,

Wilco


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Re: Custom-made fused quartz slits for SHG -- 9 micron x 12 mm

Post by piscesdiver55 »

Hello Mr. Smith. I too am new to the forum and not yet allowed to send a personal message. I would like to purchase one of your new batch slits if available. I'm able to acquire all of the components necessary to build a SHG except for the preferred slit.
Thank you,
John, Pisgah Forest, NC US


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