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Old 17-08-2020, 04:41 PM
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Tulloch (Andrew)
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Spectra of the planets

Hi there, I was wondering if anyone has (or would) take a spectral measurement of the planets? I'm really interested in determining the "true" colour of the planets (others like Christophe Pellier have already measured Uranus and Neptune), but I'm interested in the spectra of Mars also.
https://www.cloudynights.com/topic/6...or-differences

Has anyone done this (or does anyone want to do this )? I only have a colour camera (ASI224MC) which I don't think is compatible with something like the SA100 or SA200, but maybe it is.

Thanks in advance,

Andrew
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Old 19-08-2020, 01:36 PM
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Hi Andrew! Colour really only has meaning to us within the visible spectrum, outside that it's really just wavelength even though we invent descriptions like ultraviolet and infrared. So I don't think we can have any idea of the "true" colour of Uranus unless we have visual sensitivity to the whole range of the electromagnetic spectrum.

I had a bit of a play-around with the optical (visual) spectrum of Uranus once, obtained with a DSLR and an SA100 grating. I suspect that what I did was self-fulfilling in that the DSLR sensor 'assigns' a colour to Uranus, I then split that colour with a grating, and then put that back together to give a colour for Uranus! Anyway, it was a bit of fun!

Cheers -
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Old 19-08-2020, 01:42 PM
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Merlin66 (Ken)
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Andrew,
Your camera would work with the grating.
Remember without a slit, the grating resolution is only as good as the size of the target, smaller the target, better the results i.e. stellar images.
Check out our webpage for details:
http://www.astronomicalspectroscopy.com/gratings.html
and
http://www.astronomicalspectroscopy.com/objects.html (down the page for Planets)
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Old 19-08-2020, 08:37 PM
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Thanks for that, I’ll certainly look closer ...
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Old 03-10-2020, 06:12 AM
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pmrid (Peter)
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Quote:
Originally Posted by Tulloch View Post
Hi there, I was wondering if anyone has (or would) take a spectral measurement of the planets? I'm really interested in determining the "true" colour of the planets (others like Christophe Pellier have already measured Uranus and Neptune), but I'm interested in the spectra of Mars also.
https://www.cloudynights.com/topic/6...or-differences

Has anyone done this (or does anyone want to do this )? I only have a colour camera (ASI224MC) which I don't think is compatible with something like the SA100 or SA200, but maybe it is.

Thanks in advance,

Andrew
displaying my ignorance here, but I would think that the spectra would largely be sunlight being reflected back at us. And if there are any absorbtion lines that do not appear in sunlight, this might tell us a little of their atmospheres but not much else.

Peter
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Old 04-10-2020, 07:41 AM
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Peter,
Yes, you can take a spectrum of the gas gaints and then a spectrum of a G2V star (or just use the Pickles/ Miles standard) and effectively remove the solar spectrum from the planet spectrum.
What's left behind is a fingerprint of the atmospheric absorption, usually the methane bands.
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Old 04-10-2020, 09:12 AM
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Quote:
Originally Posted by pmrid View Post
displaying my ignorance here, but I would think that the spectra would largely be sunlight being reflected back at us.
Peter
Well, what is the colour of an object, other than the spectrum reflected off the object by sunlight, then interpreted by our eyes and brain?

We see lots of images of the planets, which are usually "prettied up" by NASA or others for the general population to "ooh" and "aah" at. The worst of these are from Hubble, with the images from its extremely narrow band filters designed to capture far distant nebulae, hacked together to produce something approaching human vision with the contrast blown out.

My interest in all this started when I stopped capturing images of the planets with my DSLR (with the white balance set to "Daylight") to a dedicated planetary camera, the ASI224MC, and innocently asked the community, "What white balance settings do I use on my new camera?" When the answer came back, "Well, here are some settings provided by the manufacturer, but you need to modify the colour balance afterwards to make them images look nice", I was less than impressed.

This motivated me to embark on a quest for Colour Truth, where I investigated monitor calibration equipment, Macbeth colour charts, camera calibration matricies, G2V star measurements, atmospheric propagation effects, the CIE 1931 standard on tristimulous colour coordinates, and measured spectral data of the planets. If you have a spare few hours you can read about my quest here (along with everyone else's objections to why it won't work ).
https://www.cloudynights.com/topic/7...-final-report/

Where I eventually ended up was a NASA report by Erich Karkoschka in which he publishes the actual spectroscopic data from his experiments on the planets, which can be used to convert to the colour coordinates XYZ, L*a*b* and finally RGB for (calibrated) computer monitors. This data is an "average" colour of the planets, from which I calculated red/green and blue/green ratios for each planet, then used these values as a crude tool for "white balancing" my own images. It's an order of magnitude calculation at best, however it is at least something that could be used to provide a slightly more accurate image.

Maybe

Andrew
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Old 12-10-2020, 08:43 AM
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Found this interesting page on Titan Uranus and Neptune.

https://www.lpl.arizona.edu/~rlorenz/amateur.pdf
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Old 12-10-2020, 10:16 AM
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Thanks Ken, an interesting report - I'll have a closer look at that
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