Detecting Methane in Neptune's Atmosphere

[julianh72]

The attached image is a stacked composite of 8 x 60-second frames captured on the evening of 25 August 2016 from my light-polluted backyard in suburban Brisbane. (Captured using a 200 mm Meade LX-90 telescope with 0.63 Focal Reducer, QHY5 mono camera, and a Star Analyser SA100 grating.) The exposure is reasonable, but for an improved quality spectrum for analysis, a longer capture session with a greater number of stacked frames would have helped.

Neptune had an apparent visual magnitude of 7.8 at the time; Neptune is at centre-left, with its spectrum spread across the centre of frame, and there are three other unidentified “field stars” visible in the frame. (Could the dim star just below and left of Neptune be Triton?)

Methane is commonly known for its absorption in the infrared (hence its significance as a Greenhouse Gas in the Earth’s atmosphere), but it also has a number of absorption / emission lines in the visible spectrum, at wavelengths of 4860, 5430, 5760, 5960, 6190 & 6680 Å. When we overlay these lines on the spectrum, we see an excellent correlation. Bingo! Neptune’s atmosphere is confirmed to contain a significant amount of Methane!

This may not come as any news to experienced planetary scientists (I don't think NASA needs to launch a mission to confirm my “findings”!), but I find it quite astonishing what can be accomplished by a very inexpert amateur with very modest equipment in their backyard!

(There's a longer write-up on my blog: http://julianh72.blogspot.com.au/2016/09/methane-in-neptunes-atmosphere-from-my.html )

[Tinderboxsky]

Hi Julian,

Excellent. Very interesting. Thanks for sharing.

Cheer

Steve

[Merlin66]

Julian,
Well done mate!
Just a comment - focus on the spectrum (not the zero order) to maximise resolution.
There are some comparison planetary spectra (Titan, Uranus and Neptune) taken with a similar set-up by Torsten Hansen in "Astronomical Spectroscopy for Amateurs", p128
Onwards and Upwards

[julianh72]

Thanks, Ken.

This capture was taken a couple of weeks ago. I've been trying to focus on the spectrum rather than the zero-order, but I've also being working on optimal exposure settings, etc. Part of the issues with this capture are probably sub-optimal focus, but I think it's also affected by pretty average seeing (I was shooting over the roof of my house, and into the sky-glow of Brisbane City, which is only 6 km from my home), and not enough frames to get a really clean stacked image.

Every night that I get outdoors now, I am getting better and better data, but I was pretty excited by the fact that even this clumsy beginner's effort was able to find methane in Neptune's atmosphere.

I've got a copy of "Astronomical Spectroscopy for Amateurs" (a great reference, by the way - highly recommended to any other beginners!) - I'd forgotten about the section on planetary spectra. My extracted spectrum is pretty rough, but it's still quite a good fit with Torsten Hansen's analysis - if I was of a suspicious mind, I'd think that my data had been "fudged to fit"! :question:

[Merlin66]

Julian,
If you were lucky enough to download a copy of Walker's spectral atlas before it was removed....
In Table 91, p119 he gives an annotated spectrum of Saturn which also includes the Methane bands....

Have you tried John's BASS Project software?
I also find AstroArt (I use for image processing guiding and control) also does an excellent job of stacking my subs.

[julianh72]

Yes, I have a PDF copy of Richard Walker's excellent "Spectroscopic Atlas", downloaded before he arranged for professional publishing. It's one of my three "Go To" spectroscopy references. (I hope you won't too embarrassed when I say that the other two are both yours!) I'll probably look at buying a copy when it launches officially, as I am a great believer in rewarding the efforts of those whose work is helpful to me. (Yes, I'm one of those strange people who donate to Shareware / Freeware authors, if I'm still using their software after a month or so of trial use.)

In addition to RSpec, I've also looked at BASS and VSpec, and I'm still experimenting with several stacking packages. (Sometimes, I wonder whether my progress is hampered by the fact that I like to experiment with all sorts of software packages, instead of just picking one, and getting on with the task at hand!) One feature I really like about RSpec is the ability to calibrate and view the spectrum from a live video feed in real-time, which is really cool when one of my family come out into the backyard to see what I'm up to!

[Merlin66]

Julian,
Thanks....
Tom has done a good job with RSpec (he's working on getting it FITS ready...) it's an ideal introduction software for the gratings.
IMHO something like BASS or VSpec or ISIR is the way to go forward. They are much more comprehensive and can give results acceptable to many of the ProAm campaigns. They allow you to "grow" in spectroscopy without having to relearn.....
The other books you "should have" on the shelf:
Robinson's "Spectroscopy -The key to the stars" - an easy introduction to the astrophysics
and
Kaler's "Stars and their spectra" - a great overview with lots of background and detail.