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Stepped slit tests

by warren | September 25, 2013 13:42 25 Sep 13:42 | #9373 | #9373

What I want to do

Following up on Mathew Lippincott's printed slit tests, Dave Stoft and I had hoped to test if a stepped slit would provide un-clipped data in the center (narrower) zone due to the neighboring (wider) slit zones providing a lot more light. This is hard to test without a nicely diffused light source, but Mathew's tests from yesterday with a new kind of slit printing used the stepped concept and I was able to take a close look at the data. I took Mathew's 0.09mm slit test as a sample.

Our attempt and results

Although it seems like the light source may have been a bit varied -- maybe a CFL? -- and we should repeat these tests with a very even, diffused source, it seemed fairly clear where the 3 zones were, with the WIDE-NARROW-WIDE pattern. The lead image shows a cross section of the spectrum brightness. I'm not sure why the center zone seemed to have similar brightness to the side zones, but it probably has to do with either the image brightness adjustment algorithms evening things out, or perhaps the center zone was pointed at a brighter area of the bulb. This is a big unknown we'll have to test.

Still, I was able to take two sample rows, saving them as the following two spectra, and the "bright" zone was definitely hitting 100% more than the dimmer zone. In part because the smartphone camera is pretty good, it still wasn't really clipping, but I think that a lower quality would have at those points.

Wider slit spectrum (line 1444): https://spectralworkbench.org/analyze/spectrum/13029

Screen_Shot_2013-09-25_at_9.22.41_AM.png

Narrower slit spectrum (line 1101): https://spectralworkbench.org/analyze/spectrum/13030

Screen_Shot_2013-09-25_at_9.23.04_AM.png

And a comparison in a single graph (although RGB are averaged here so you don't see the clipping effect as well):

Screen_Shot_2013-09-25_at_9.41.17_AM.png

Questions and next steps

So while it does seem that the lower-exposed areas, corresponding to at least part of the narrow slit region of the spectrum, are less likely to clip, we haven't really demonstrated that this was because of the brighter zones to each side. In addition, we need to repeat tests with very even, diffuse illumination -- perhaps also with continuous spectrum light, which is more likely to clip in general, if you look at lots of existing data at SpectralWorkbench.org.


8 Comments

I don't think diffusion gets the stepped result expected. Here is a spectra of a very overcast sky: https://spectralworkbench.org/spectra/embed/13038 All indoor light sources got the same result-brighter in the middle, including 48" fluorescent tubes. I didn't save one of those spectra but here is a CFL from 5m. same brightness in the middle: https://spectralworkbench.org/spectra/embed/13041

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Whoa - how is that sky spectrum so varied? Was it pointed through a screen or something?

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Through a somewhat dirty window. Yeah, wierd, right? its so striated.

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@warren Hi, I wanted to ask if you have encountered collimation slits that were not perfectly parallel (i.e. in a slight V shape tapering to close). I want to try this as I should then be able to pick optimal width based on the image height (I want to use a parallel diffraction grating so the lines should be straight and easy to adjust). Do you think that would be a sensible thing to try?

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Hi @CBradley - i think it's a good idea, yes! If you have a specific beginning and end size and are willing to share your work, we can probably get some made? How are you thinking of setting up your tests?

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@warren hey, thanks for the reply. I haven't really finished tinkering with the spectrometer yet but I will try to make some simple slits with tape when it's ready. If it works generally, I'd love to try some more precise ones!

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👍

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