Jupiter - First light DIY spectrometer

[jnp]

Hi Everyone,

It's been a while since I was last on here.

I have built a new modular DIY classical spectrometer (from plastic guttering parts ) which allows the potential to swap components about. Anyway here was first light on Jupiter taken 14th Nov. Most of the spectral lines are reflected sunlight, but there are also some methane bands.

Here's the uncorrected version of Jupiter

And here's the same with the continuum removed. You could barely see the Calcium lines where the UV response is so low.

Thanks for looking

John

[michael.h.f.wilkinson]

Very nice result. Would it be possible to correct using the solar spectrum to isolate the contributions from Jupiter's atmosphere?

[jnp]

Hi Michael,

You are quite right. In theory you just divide the raw result by a raw G2V class spectrum. In fact I took some lunar shots for this purpose, but something is not quite right at the moment. Probably a vignetting issue with the lunar images.

John

[Gasman]

Very interesting John. I often wonder what is happening on these objects to create the spectra that we see, for instance what could cause Methane to be absorbed rather than emitted?. I realise for this absorption to take place electrons will be escaping from the gravitational hold of the nucleus rather than be attracted to it!. I also realise in this case the solar light would be contributing greatly to the final spectra seen which maybe the reason Ch4 is seen at all?.

Cheers

Steve

[Merlin66]

The sunlight is being preferentially absorbed by the planet's atmosphere - think of the telluric bands in the solar spectrum caused by the earth's atmosphere - similar idea....

Nice spectrum by the way John..what slit gap were you using?

[jnp]

Thanks fors the kind comments guys.

Steve - Atoms emit and absorb light as electrons jump between energy levels. Molecules have more complex spectra due to more combinations of energy states. There are also electronic spins, molecular spins, rotations. It's nasty stuff and I'm no expert but I think this why they have wider absorption bands

Ken - I am using the Surplus Shed adjustable slit so can't be sure. Probably 40 microns?

I tried to 'correct' for instrument response by dividing by a G2v target. I used the moon, but there is something odd. Looking at the spectra below, the Moon appears redder than Jupiter and there is very little or detail in the blue end of the lunar spectra.
 

The same equipment was used for both spectra and I have checked the altitudes in case the moon was very low down. The  Moon was at 45 degrees at 8:45PM and Jupiter was at 44 degrees at 1AM. I now don't think vignetting is the cause, as the Moon being such as large object, will fully always illuminate the slit. This leaves focusing, but that's not obvious from the raw data. Any other causes?

thanks for looking

John

[Merlin66]

John,

I can't fault the process...the telluric lines look stronger in the moon spectrum....don't know why the moon spectrum would "fade" out in the blue......

[robin_astro]

Hi John,

There are some nasty effects duew to chromatisim in the optics or atmosphere which can lead to the slit selectively sampling the wavelengths and distorting the profile, see Christian Buil's page here for some particularly severe examples

http://www.astrosurf.com/buil/dispersion/atmo.htm

But I don't think this is likely to be the problem here as presumably  Jupiter (and the moon of course) was significantly wider than the slit. 

The moon is not a particularly good solar analogue as it is significantly redder than sunlight to a degree which also depends on the phase eg see fig 7 here 

http://iopscience.iop.org/1538-3881/129/6/2887/pdf/1538-3881_129_6_2887.pdf

but again I suspect this is not the whole answer and  would not explain  the shallower absorption lines.

I think my my money would be  on scattered light or a light leak for the lunar spectrum which would add to the background, potentially change the shape of the spectrum and wash out the absorption lines.

Cheers

Robin 

[robin_astro]

Another possibility is dewing up of the optics somewhere?

Robin

[robin_astro]

Very interesting John. I often wonder what is happening on these objects to create the spectra that we see, for instance what could cause Methane to be absorbed rather than emitted?.

Actually that is a very good question and not one to  which there is an obvious answer.  The production of absorption lines is straightforward enough. The photons of sunlight are scattered in the atmosphere of Jupiter and some of it is scattered back out in our direction (like headlights in fog) Some of these photons at particular wavelengths will be absorbed by atoms and molecules on their journey through Jupiter's atmosphere (either causing electrons to jump up to higher energy levels or molecules to vibrate in particular ways (Think atoms connected by springs)  producing the absorption lines (gaps in the scattered light spectrum)  The broad molecular bands are in fact hundreds of very closely spaced lines due to the myriad of ways molecules can vibrate.

So far so good,  but... the atoms and molecules in the atmosphere cannot keep absorbing photons or they would get hotter and hotter  so  the energy they absorbed must be re-emitted.   On the face of it you would think that this would just lead to the absorption lines being filled in again as the same energy photons are re-emitted but there are two potential mechanisms which stop this happening.

1. The light we see with the absorption lines imprinted on it is coming straight towards us of course. The photons of light, produced when the atoms/molecules lose that energy that produced the absorption line,  are sent out in all directions. Only a small fraction of them come in our direction so they do  not fill in  the absorption line. 

2. The energy taken up by the atoms/ molecules when absorbing the photons may not be re-emitted at  the same wavelength. For example the electron may jump back down  a series of smaller steps or the vibrations of the molecule may be dissipated  in various ways which result in the emitted radiation not being at the same wavelength as the absorbed photon.

I suspect both these mechanisms play a part here.  If we looked carefully across the whole spectrum (particularly I suspect in the IR) we would  find parts of the spectrum where there is an excess of energy compared with the solar spectrum due to this re-emission. 

Robin

[jnp]

Very interesting stuff Robin. Thanks for taking the time to dig it out. It's worth reproducing for ourselves what actually happens when a star deliberately drifts across a slit.

The C8N is a Newtonian so no CA on the slit (other than atmospheric, but 45 degrees is a decent altitude).

Definitely no dew. The moon was the first thing I pointed at.

There are probably issues guiding on the slit due to, firstly a very dodgy DEC axis and secondly because the EQ5 is overloaded with an 8 inch newt having an Atik314 positioned 40cm way from the focuser at the end of a transmission spectrometer, (it looks comical). I will soon use a shorter focal length collimating lens. Guiding wasn't necessary for the Moon anyway.

The mystery of the blue response remains. Will try and reproduce when the next opportunity arises!

John