Ashen Light and Spectroscopy

[Andy Wilson]

The latest edition of Sky & Telescope has an article on the Ashen Light by William Sheehan and Klaus Brasch. I openly admit that I am no expert on Venus, so I am not sure if the Ashen Light has now been dismissed as an observational effect, or if it is still a genuine possibility.

The gist of the article is about the possibility of global aurora causing the Ashen Light, including a brief reference to The Planet Venus by Patrick Moore. Although Venus has no magnetic field they think global aurora may be possible. Indeed observations from Apache Point Observatory showed a green line for oxygen emission after there had been large solar flares pointed towards Venus.

If this were at all possible, then it sounds like an ideal target for amateur spectroscopy. Any light from the dark side of Venus just visible to the eye should be within easy reach of low resolution spectroscopy.

Of course this may be a wild goose chase! With my interest in spectroscopy I felt I had to put this up on the forum to canvas opinion from those with more knowledge of our nearest planetary neighbour.

Thanks,

Andy

[Richard Miles]

Very good suggestion – since the light is supposed to be associated with emission then your spectrograph would be an ideal way of tackling what is supposed to be an ephemeral phenomenon that comes and goes. You’ll have to work out a good methodology of offseting to measure the sky and back again – bobbing to and fro on some time-scale to ensure changes in the Earth’s atmosphere are properly subtracted from the Venus signal. Beware of scattered light from the bright side contaminating the signal you are after. You might therefore need to also measure the bright side (with the gain turned down on your spectrograph – or by just measuring the sky north/south of the cusps to quantify this). That makes for 2 or 3 sky positions as well as the dark side of Venus herself.

Richard

[Robin Leadbeater]

With a long slit orientated appropriatelyand a suitable image scale you might be able to get all you need in one shot (Well 2, one exposed for the bright side)  You would need a  flat field illumination source which is nice and even in the slit direction so  the subtractions work accurately. 

Funnily I was just reading Paul Abel’s beginners article on Earthshine and musing about  measuring that spectroscopically, perhaps mounting the spectrograph behind a camera  lens to get a sensible image scale to match the slit. Much easier than Ashen Light though !

Robin

[Andy Wilson]

Robin & Richard,

Thank you for your replies. It is good to hear it is not a crazy idea, though one that might only give negative results.

I had been thinking that one exposure might be sufficient with the slit appropriately aligned. Though it is a good point that 2 positions might be needed due to the contrast between day side and night side of Venus.

Robin, I have one of the newer LHIRES IIIs, do you think the flat field calibration lamp is sufficiently good?

If aurora were the cause, then I would hope there would be an “obvious” green line due to oxygen at 5577 Angstroms, or at least strong enough to be above the noise.

The other issue would be knowing when to look, as ideally this would be when CME has been ejected in the direction of Venus.

I’ll try it out with an early morning in September when Venus is better placed. See how feasible it is to get good data.

Thanks,

Andy

[Robin Leadbeater]

Hi Andy,

Hmm.. getting an even enough flat might take some arranging. I think you would need to go back to a flat light source in front of the telescope rather than rely on the internal flat lamp, to take into accout any vignetting before the spectrograph.  You could perhaps test it though by taking a daylight sky spectrum which over the small field should give  the same counts independent of position along the slit, once flat corrected.

Robin

[Andy Wilson]

Hi Robin,

Thanks, that is good to know. Taking a daylight spectrum sounds like a useful and interesting test anyway. It would be handy to see how well my flat correction works across the slit. With stellar spectroscopy I always aim to put the star in about the same position on the slit, but there will be times like this when I’ll want to use more of the slit height.

Best wishes,

Andy

[Robin Leadbeater]

Looking for auroral lines could be particularly interesting as you could use higher resolution to enhance the contrast between any continuum spectrum background. (like imaging with narrowband filters)  You would still have to watch out for auroral lines from our own atmosphere and subtract them of course.

Cheers

Robin

[Andy Wilson]

An interesting point Robin. I had thought that this would be better suited to a lower resolution setup than my own, and I should use my low resolution grating. However, since I know the spectral line I’m interested in, then high resolution should not be a problem and as you point out the increase in contrast will be a benefit.

Cheers,

Andy

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