A peek inside a spectroheliograph

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thesmiths
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A peek inside a spectroheliograph

Post by thesmiths »

I took this interesting photo today while doing some tests. It shows the Sun projected onto one of my chrome on quartz SHG slits, as viewed from where the spectrometer would normally be placed. You can actually see some clouds in front of the Sun in the photo.

15mm x 15mm quartz chip with 9 micron x 12mm long slit. Mounted on a copper disk with Torr Seal epoxy.
15mm x 15mm quartz chip with 9 micron x 12mm long slit. Mounted on a copper disk with Torr Seal epoxy.
5892.jpg (80.15 KiB) Viewed 1078 times

By moving the mount, the slit will scan across the projected solar image and generate a time varying spectral line when passed through a dispersive optical element (e.g. a diffraction grating). For this test, I used a 300mm f5.6 camera lens, which results in a projected solar diameter of approximately 2.8mm. Normally, a longer focal length lens or telescope would be used, resulting in a larger projected image and therefore higher spatial resolution.

The average optical density of the chrome / chrome oxide film is about the same as a typical visual solar filter, although it attenuates more strongly in the UV than the red (as seen in the photo). The slit looks very wide in this photo but that's due to saturation of the camera sensor. A better representation of the slit is the photo below, which shows the other side of the quartz chip, mounted in a 1.25-inch filter cell.

Chrome on quartz slit mounted on 27.0mm copper disk and held in a 1.25-inch filter cell.
Chrome on quartz slit mounted on 27.0mm copper disk and held in a 1.25-inch filter cell.
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I should mention that I did not use any attenuation (ND filter, Hershel wedge, UV/IR filter) for this test. Generally, however, I would recommend using a UV/IR filter since the chrome / chrome oxide will eventually be degraded by strong UV light. Note that the chrome oxide layer is non-reflective so there are relatively few reflections.

A full description of the slits can be found in the document posted here (recently updated): https://github.com/thelondonsmiths/Sole ... -SHG-slits


NM_mike
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Re: A peek inside a spectroheliograph

Post by NM_mike »

Doug,

Thanks for the peek inside and especially the slit details. Have you tried to measure the temperature rise of the copper heat sink? Just curious about heat transfer.

-Mike


thesmiths
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Re: A peek inside a spectroheliograph

Post by thesmiths »

NM_mike wrote: Sun May 21, 2023 12:05 pm Have you tried to measure the temperature rise of the copper heat sink? Just curious about heat transfer.
I have not directly measured the temperature of the copper disk. I often will touch the M42 tube into which the disk is mounted and I haven't noticed any external temperature rise. I deliberately chose copper since it is a good thermal conductor so would transfer heat away from the quartz chip. I'm guessing the copper stays very close to temperature of the thing it's mounted to (there is quite good thermal contact). A bigger question is how hot the middle of the quartz chip gets.


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Re: A peek inside a spectroheliograph

Post by Montana »

That's an interesting first photo, I some how imagined that light would only pass through the slit, I am presuming the red spot is the Sun :)

:bow :hamster:

Alexandra


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Re: A peek inside a spectroheliograph

Post by thesmiths »

Montana wrote: Thu May 25, 2023 7:03 am I am presuming the red spot is the Sun
Yes, it is. I didn't expect to see that. But then I realised that the attenuation is specified as OD5, which is the same as Baader solar film (the visual kind). Apparently, thin metal films become more transparent with longer wavelength so that's why it looks red.

The material I used is actually design for photolithographic masks, where they typically expose photoresist in the UV. Therefore, they are more concerned with making them opaque in the UV (the photoresist does not responds to red light, which is why semiconductor fabs are typically lit with orange light).


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