Anyone actively working spectroscopy?

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Pete
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Anyone actively working spectroscopy?

Unread post by Pete » Wed Jun 24, 2020 5:26 pm

Perhaps it's a case of having too much down time, but curiosity got me sucked into exploring the feasibility of doing spectroscopy, possibly identifying the composition of the gas giants and the distance to a quasar.

This kind of analysis involves lots of different corrections and it may prove more complex than desired. But with inexpensive diffraction gratings and free analysis software it's tempting.

Might even be something that we could team up on.....
Pete P.
Bruce D
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Re: Anyone actively working spectroscopy?

Unread post by Bruce D » Wed Jun 24, 2020 8:19 pm

Gee in the past year i saw an article detailing construction of 2 different units- darned if i can recall where
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mark.m
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Re: Anyone actively working spectroscopy?

Unread post by mark.m » Thu Jun 25, 2020 2:25 pm

Pete:
Here are some references I found. The first is an S&T article:
https://skyandtelescope.org/get-involve ... ctroscopy/
The second is the AAVSO Spectroscopy Handbook. It's more focused on how to extract science from the spectrum, but I've attached it.

The S&T article makes it sound like you might be able to record a spectrum just by fitting a diffraction grating into a filter holder as if it was an LRGB (or BVRI) filter. Hmmmm. That doesn't sound too hard.

- Mark
Attachments
AAVSO Guide to Getting Started in Spectroscopy December 2019.pdf
(935.17 KiB) Downloaded 2 times
Mark M
Portsmouth, RI
10" Meade SCT
SBIG ST-9E
GM2000 (10Micron)
American Association of Variable Star Observers (AAVSO) observer code: MMU
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mark.m
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Re: Anyone actively working spectroscopy?

Unread post by mark.m » Thu Jun 25, 2020 4:07 pm

Okay, just for fun, let's try this.
My junk box has a bunch of cheap mylar diffraction gratings from Amazon that I use with high school classes:
DSC_0007.jpg
DSC_0007.jpg (147.66 KiB) Viewed 63 times
I removed the grating from the cardboard holder and cut out a 1" disk from the center of the mylar. I put that "grating disk" into an unused 1.25" filter cell:
DSC_0005.jpg
DSC_0005.jpg (304.27 KiB) Viewed 63 times
And then put that into the last remaining unused cell of my filter wheel (the slot at the top):
DSC_0006.jpg
DSC_0006.jpg (251.79 KiB) Viewed 63 times
... and then wait for sunset.

I have no idea whether this will actually work at all.

- Mark M
Mark M
Portsmouth, RI
10" Meade SCT
SBIG ST-9E
GM2000 (10Micron)
American Association of Variable Star Observers (AAVSO) observer code: MMU
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Pete
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Re: Anyone actively working spectroscopy?

Unread post by Pete » Thu Jun 25, 2020 8:40 pm

Good show Mark. It's not that often that we can find yet another facet to our stargazing. And what will take me 5 months seems to have taken you 5 minutes!
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Re: Anyone actively working spectroscopy?

Unread post by mark.m » Fri Jun 26, 2020 11:45 am

Oh, I can easily see how this could become an all-consuming specialty for someone...

My poor-man's spectrograph was set up like this:
diffraction_grating-1.jpg
diffraction_grating-1.jpg (34.21 KiB) Viewed 59 times
Light going through the diffraction grating is split. Most of it continues straight, but some is bent off at an angle (I labeled it theta -- in my case it seems to be about 20-degrees) and spread into a spectrum. The angle is big enough that it pushes the spectrum off the edge of my CCD.

But by offsetting the star to the edge of the diffraction grating (about 15 arcminutes away from the optical centerline), I can get a portion of the spectrum to fall onto the CCD:
diffraction_grating-2.jpg
diffraction_grating-2.jpg (25.35 KiB) Viewed 59 times
My first light image is of Vega:
image034.jpg
image034.jpg (31.42 KiB) Viewed 59 times
The streak is the spectrum. Focus is a mess. If you look in the first drawing, the path length for red light is longer than the path length for blue light. When I focus, I get the focus correct for one single color; everything else is out of focus, which smears the spectral lines. I only looked at mag 1 stars, and needed exposure times of about 15-20 seconds.

So, extracting the spectrum from the image for Vega yields:
vega-image.jpg
vega-image.jpg (3.57 KiB) Viewed 59 times
And I can graph the intensity of the light along the spectrum (blue is to the left, red is to the right):
vega.png
vega.png (3.33 KiB) Viewed 59 times
Some absorption line features are immediately visible. Google tells me that the Hydrogen Balmer series are prominent in Vega, so if I try to map them to the spectral features I get this:
vega_annotated.jpg
vega_annotated.jpg (22.45 KiB) Viewed 59 times
(I'm not at all convinced that I have that mapping correct!) What's cool is looking at some other stars. Antares first (very red star, notice how the spectral peak is way off to the right):
antares.png
antares.png (3.28 KiB) Viewed 59 times
Arcturus is not as red; the peak shifts to the left. Google says that Hydrogen lines in Arcturus are very weak, with sodium and calcium being strong. I didn't spend any time trying to align those lines with the features here:
arcturus.png
arcturus.png (3.36 KiB) Viewed 59 times
So, it is certainly possible to capture spectra and extract features. With this kind of a setup, details in the spectra are pretty impossible to capture, because of focus issues. And you could certainly go down several magnitudes and get usable data with longer exposures.

- Mark
Mark M
Portsmouth, RI
10" Meade SCT
SBIG ST-9E
GM2000 (10Micron)
American Association of Variable Star Observers (AAVSO) observer code: MMU
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Pete
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Re: Anyone actively working spectroscopy?

Unread post by Pete » Fri Jun 26, 2020 12:13 pm

Oh wow! Now your one night of playing around would take me a month of experimentation. Now that there's a baseline it's possible to make some preliminary observations -

* Adjusting the distance between the defraction grating and the imaging chip would impact the length of the spectrum.

* Since the little I've read cautions against using a focal reducer this is an application where a very large imaging chip is helpful. The minimal prepackaged kit https://optcorp.com/products/field-test ... py-package uses a tiny 4.8 X 3.7 mm chip.

* By scattering photons all over the chip, to achieve workable spectroscopy images you're either going to have to take very long exposures or favor large aperture.

And Mark, you obviously already have some appropriate software. Extracting the spectrum and running a graphic analysis isn't trivial. Wonderful experiment.

As a practical matter, imaging even a relatively bright 6th magnitude Uranus looks pretty difficult, and getting redshift on a 13th magnitude quasar mucho difficult.
Pete P.
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