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B For OmCheeto (astrospectroscopy)

  1. May 16, 2017 #1

    Andy Resnick

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    Tried to do some astrospectroscopy last night (fail), but figured I'd share the results anyway.

    I taped a diffraction grating onto the lens flange (lens side, not camera side) to image some spectra- I was hoping to compare Jupiter, Spica, and Arcturus. For whatever reason, my old Sony worked much better for this- I think that Sony puts their sensor much closer to the lens flange than Nikon so the spectra still fall onto the chip. I ended up having to use a wide-angle lens to capture the spectrum, the trade-off being much less throughput. Images from my backyard- one of a streetlight (with Jupiter and Spica visible), and the other a neighbor's back porch light. These are 6s ISO 1000 exposures:



    Unfortunately, the planet/starlight was just too faint for me to capture. The lights, however, have measurably different spectra (first one is the streetlight, most likely a Sodium lamp), the second an LED. Note also, the horizontal axis goes from red to blue and you can clearly see the Sodium lamp spectral peaks:



    (for comparison: https://image.slidesharecdn.com/201...-all-you-need-to-know-7-638.jpg?cb=1358421058)
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  3. May 16, 2017 #2
  4. May 16, 2017 #3


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    A guy at my local Astro Society gave a talk about Spectroscopy and he had some great pictures and graphs of spectra he'd obtained with a diffraction grating, using a section of the digital colour i mage. You could distinctly see the overall curve which corresponded to the temperature of the chosen star and also some absorption lines. All from a humble back yard!
    We didn't discuss the optical arrangement but I sort of assumed he was using prime focus and just put the grating 'somewhere' up the tube and it all came out right. (I should have known better. Nothing just comes out right with Astrophotography.
  5. May 16, 2017 #4


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    I carved my diffraction grating to fit my lens.


    I'm also a big fan of "night lights" spectroscopy:


    Was this thread inspired by the Pamela Gay interview from yesterday?
    By the end of it, I had half a dozen new tabs on my browser open.
  6. May 16, 2017 #5

    Andy Resnick

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    Nope- I was photographing the Markarian Chain and clouds started to move in, so I grabbed the opportunity to goof around.
  7. May 18, 2017 #6


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    The last thing she mentioned was the upcoming eclipse.
    That prompted me to check out the society she mentioned that she belongs to; "...I’m the Director of Technology and Citizen Science at the Astronomical Society of the Pacific."
    There I found that they had the RSpec Explorer Spectrometer for sale. Only $440.
    I thought that would be cool to own, but the price was a bit too much, so I'll probably stick with my $1.25 diffraction grating.
    The RSpec Spectrometer also doesn't appear to have the range I'm looking for:
    Spectral range is approximately 390 to 700 nanometers. Accuracy is generally 1% or better. Spectroscopic features as narrow as 3 nanometers can be measured.
    That looks suspiciously like the visual range. If I'm going to spend more than $100 on something, it had better also measure IR & UV.
    Perhaps 200 to 1500 nanometers.
    I'm not really sure. Some day I'll research that.
    (google google google)

    Ok. After I win the lottery.
    Low Cost GREEN-Wave Spectrometer
    350-1150nm wavelength range
  8. May 18, 2017 #7

    Andy Resnick

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    Yeah, I'm with you- we hand out those cheap gratings to students (we buy in bulk, I think they end up about $0.50 each) and let them look at spectra themselves- why involve a computer?

    Just decided to get a big sheet/roll of grating (like https://www.onlinesciencemall.com/p...=27348598342&gclid=CLOsw_nn-dMCFY-BaQod4x0CuA) to put over the front element of my telephoto. Not sure it will work, but I can afford the $6.50 to try. Science!
  9. May 23, 2017 #8

    Andy Resnick

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    As per the image in post #4, I put a grating sheet in front of the lens and was easily able to get spectra of Jupiter, Spica, and Arcturus. The spectra are about 10 degrees off normal, so it took me a bit to realize that I can't look at the object itself in order to capture the spectra. By happenstance, the spectra of Jupiter and Spica appear together- Jupiter is off the frame to the upper right, Spica is off to the left to the lower left:

    Here's Arcturus:

    Now, using this to make quantitative measurements is tricky: Bayer filter, etc. Here are RGB lineplots through the spectra of Jupiter, Spica, and Arcturus:




    You can see where Jupiter's spectrum overlapped Spica's (the narrow peak). It's not immediately obvious how these correspond to the different colors, but if I plot the spectral intensity ('brightness') rather than the actual colors in the same order, the difference is more clear:




    Now, Spica's curve shows how predominantly blue it is, while Arcturus emphasizes the red end. Not sure why there is a 'notch' in the yellow band....
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