SHG slits from China: chrome on borosilicate glass, 7 micron by 13mm

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thesmiths
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SHG slits from China: chrome on borosilicate glass, 7 micron by 13mm

Post by thesmiths »

I have been working with Maurice in Australia on various mounting designs for spectroheliograph slits. We began with the chrome on fused quartz design I had made some time ago. It was mentioned in a post a few months ago that chrome on borosilicate glass slits were available in China viewtopic.php?p=411342#p411342. I ordered a pair of them from Taobao, as did Maurice. The quality looked quite good under the microscope, only perhaps a little "rougher" and with a bit more debris than the ones I had made (this could have come from the transportation).

The design I have lately been using for my 15mm x 15mm quartz chips we adapted to the 10mm x 16mm glass chips. This helped to eliminate stray light that can come from the edges, particularly because the Chinese slit is quite long (13mm) and there is not much room at the ends. Also, because the chrome layer has no antireflection coating, there is likely to be some internal reflections inside the glass, which could eventually emerge from the edges.

The chrome on glass slits were attached with Torr Seal epoxy onto a 27mm diameter aluminium disk, which then fit into a 1.25-inch filter cell (as I have lately been doing with my chrome on quartz slits). This design reflects a large amount of sunlight before it ever hits the glass/quartz, reducing the energy load. Borosilicate glass is much more robust that soda lime glass and we found this design could be used with no ND filter to attenuate the sunlight. We just used a 2-inch UV/IR filter at the telescope focuser (the main purpose being to extend the life of the chrome).

The last image below was taken by Maurice from Australia on Jan 26 at the Ca-K wavelength. The quality is very good. Since the width is 7 micron (vs 9 micron for my quartz slits and 10 micron for the standard Shelyak slit), the spectral resolution seems a little higher, although the light throughput is, as expected, somewhat lower. So there are trade-offs with a reduced slit width.

The chrome layer on the Chinese slits is said to be 400nm, which is quite thick. I noticed this led to better opacity in the red and infrared, which is an advantage. But using a thicker metal layer will typically lead to rougher lines during the photolithographic lift-off process. The chrome/chrome oxide layer that I had used was around 200nm thick, which is actually optimised from opacity near the UV, but which has extremely low line edge roughness. So there are trade offs on this parameter as well.

All in all, I think the 7 micron chrome on borosilicate glass slits are a very viable alternative for SHG use and narrowband spectroscopy in general. I spent some time looking at these slits before placing my recent order for a new batch of chrome/chrome oxide on fused quartz slits (the same material I used before). These will be 8 micron wide and 13mm long (vs 9 micron by 12mm previously) and should be ready in a few weeks time.

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Re: SHG slits from China: chrome on borosilicate glass, 7 micron by 13mm

Post by marktownley »

Looks promising!


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Re: SHG slits from China: chrome on borosilicate glass, 7 micron by 13mm

Post by thesmiths »

I actually got these slits in order to try the infrared Calcium lines again. I found my slits gave too much background light at wavelengths above H-alpha since the transmission of thin metal layers increases with wavelength.


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