Ashen Light - Redux
Posted 09 April 2013 - 06:49 AM
I am speaking about the diffraction edge created by the edge of the occulter: this effect can be seen visually at the eyepiece.
The light is spreading perpendicullary to the edge. You have to shift the lighted part of the planet disk fully behind the occulter for avoiding this.
After an exhaustive campain of tests on 2 days, with and without an occulter my conclusion was that the use of the oculter brought actually nothing consistent.
This was reinforced with the practice of tests on long distanced cibles consisting in a part highly lighted and a part low lighted with a known ratio of intensities.
Also same conclusion, the occulter bring nothing more and through was determined some possibilities: a ratio of 100 000 was still possible, the lighted part being 5000cd/m2.
Hope this is clearer.
All the numbers mentionned in my old posts were coming from these tests.
Posted 10 April 2013 - 12:48 AM
Posted 10 April 2013 - 01:24 AM
"Gotcha" is American slang for "I've got you", which in this case means "I understand now".
Posted 10 April 2013 - 01:39 AM
Posted 10 April 2013 - 08:58 PM
Posted 14 April 2013 - 04:11 PM
These NASA handbooks are outstanding! The Mars one is available free as a downloadable PDF several places on the web. The Venus one may be also, but I didn't look (printing costs would have been greater than the cost of buying it.)
Posted 16 April 2013 - 01:34 AM
"Weinberg and Newkirk were unable to identify any emission lines on 20 spectrograms taken of the unilluminated portion of Venus in 1959. But their failure in 1959 and Newkirk's success in 1958 are consistant with independent visual searches during these two periods. However, Weinberg and Newkirk are not convinced of the reality of the ashen light phenomenon and, at present, it must be concluded that there is neither a satisfactory explanation nor definite proof of its existance."
There's a bit of explanatory test before this; but they really give it short shrift, and end up with the same inclusive conclusions we still have now. I was hoping for a little more in-depth treatment of the subject, similar to the (superior) Mars version of this book.
Ah well - I laughed, I cried, I kissed six bucks goodbye. There's a lot more stuff in this booklet though, so it was still a good buy.
Posted 22 April 2013 - 01:20 PM
Another article cites a German astrophotographer who imaged the AL in ultraviolet at the same time an observer was reporting it cisually through a blue filter.
Still another suggests lightning, as considerable evidence of lightning has been seen by spacecraft, and displays a preference for the evening side of Venus as opposed to the morning; the visual record mirrors this preference, but may be due to people preferring to observe in the evening instead of the morning.
My vote right now is for refraction; glowing surface runs second.
Posted 23 April 2013 - 01:06 PM
Such a degree of scattering can only occur with a significant degree of attenuation--after all, the scattered light is being 'removed' from continued direct propagation. In such case, any ashen light so resulting should be seen only as a glow confined to the terminator region, decreasing in surface brightness toward the anti-solar point, probably dimming to invisibility (against the other illumination from external sources) after perhaps some 5-10 degrees (several hundred to *maybe* 1,000 km) into the night side.
Posted 23 April 2013 - 02:31 PM
Ok back to scheduled programming.
Posted 23 April 2013 - 05:00 PM
How high are the cloud tops? If at sufficient altitude, the clear atmosphere above will not have a particularly high density.
The easiest answer I could find without lots of digging was from Patrick Moore ("Venus", 2002, p.151):
"There are various cloud layers. The upper clouds lie at around 43 miles above ground level; the cloud deck ends at 18 miles above the surface, and below this the visibility is unexpectedly good..."
Does this tell you anything?
Posted 24 April 2013 - 11:58 AM
ive seen it a few times myself over the years,but sometimes i think the brain causes some of it ,but patrick moore has also stated he an other good astronomers saw that as well.in daylight when the crescent is real big i saw it a few times over the years and the surface area was definitely darker on venus opposite the crescent.
Well, very very near inferior conjunction with Venus as narrow a crescent as it can get, there is light scattered by the atmosphere of Venus that will sort of "complete the circle" of the entire edge of the disk of the planet (and may be visible and recordable in daylight observations), but this is not the Ashen light. The Ashen light is quite faint and is reportedly seen on the un-illuminated disk of Venus under darker sky conditions with a planetary phase that is not as strongly crescent as the inferior conjunction is and is also seen away from the limb of the planet. I have never seen it, but it may be a variable phenomena. To my knowledge, it has never been imaged and there isn't much hard evidence for its existence. Clear skies to you.
Posted 24 April 2013 - 06:06 PM
At an altitude of 50 km or so, Venus's atmospheric pressure is much like that at sea level on earth. At the cloud tops of 43 km, the air pressure would be a big greater than at sea level on Earth. Our atmosphere refracts light at sea level by just over 1/2 degree. At cloud top level on Venus, then, we might assume a refraction of about a degree.
A check on this is afforded by the observation of the completion of the atmospheric ring of light around the full disk when very near inferior conjunction. The elongation of the planet from the Sun st that time tells us that this is angle light is refracted into the night side.
Posted 25 April 2013 - 12:52 PM
That was 43 miles, not kilometers.
Also, you'd have to consider the density, constitution, and conditions of the atmosphere. The AL isn't always there; there must be conditions that occur periodically that make it visible.
Posted 30 April 2013 - 12:52 PM
So far, no response has been forthcoming; so much as I'd love to see such an image, I think we're back to the "never been imaged" point.