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azure1961p
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Re: Ashen Light - Redux [Re: EJN]
      #5748863 - 03/22/13 12:25 AM

Quote:

This is my sketch of the crescent Venus. It should be considered definitive.




I think it just may be.

Pete


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David Gray
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Re: Ashen Light - Redux [Re: Rick Woods]
      #5749091 - 03/22/13 06:00 AM Attachment (19 downloads)

Quote:

Good post!

IIRC, in the other thread David Gray said he had unusually good eyesight in the near IR. (I could be remembering wrong.)

Whether or no, I'd never accept the results of a machine as a gauge of what is possible for a human. (At least, not a machine created by humans.)





Move over piranha I’m jumping in!

Hi Rick: yes I have said I am very (“freakishly?”!) red-sensitive. Never liked a red observing light but whatever colour the level needs to be right. I have an ordinary dimmable ‘white’ light on my 1968 vintage homemade drawing board as I do a lot of colour work, and is fine for any object: planets, DSOs etc at the right level.

I have reported the AL in the past and am attaching two impressions from 2007. The first I have to admit is pretty weird. Actually this was shown on BBC TV’s Sky at Night and in spite of my insistence/pains that the brightness/contrast levels should be maintained apparently the people involved seemed to want to use ‘artistic’ licence and it appeared on the screen far more garish than I have ever seen earthlight on the moon. Total embarrassment, they, and certain others, get no more from me! At the same time I can’t anticipate how others have their monitors calibrated; but some types (mine!) will need to be viewed square-on for these.

Due to our house obscuring much of the s’west sky I do not often catch Venus for AL at evening elongations unless it is north of declination +20 deg. This was the case last May when I caught the planet on a number of nights in near perfect conditions with the 415mm D-K. Apart from a possible vague (brownish?) shading twixt the cusps on one night saw nothing of the AL even when hiding the crescent and with/without filters etc – you name it. These views were mostly in a very dark sky.

It has been said that it is more commonly seen at evening elongations than mornings; but I think anyone looking at this statistically would have to take account of, among other things, lay-a-beds and stove-huggers!

Not sure if the attachments will come out ok so please bear with me if I have to edit!


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David Gray
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Re: Ashen Light - Redux [Re: David Gray]
      #5749093 - 03/22/13 06:11 AM Attachment (18 downloads)

That's about right - here's the other!

With the very greatest of respect before anyone takes issue please read with care my message and notes on attachment. Having more than once been misread and even taken out of context(in the recent Uranus threads) and used as an apparent pretext to give me a lecture on honesty and observing procedure etc.......please......!


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azure1961p
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Re: Ashen Light - Redux [Re: David Gray]
      #5749158 - 03/22/13 07:48 AM

Exquisite work David. Your technique is impeccable and I appreciate your observing skills. I've seen the AL - I just don't attribute it to a real phenomena on the planet itself. Great work.
Pete
Lay in beds and stove huggers


Edited by azure1961p (03/22/13 07:50 AM)


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David Gray
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Re: Ashen Light - Redux [Re: azure1961p]
      #5749333 - 03/22/13 09:55 AM Attachment (18 downloads)

Quote:

Exquisite work David. Your technique is impeccable and I appreciate your observing skills. I've seen the AL - I just don't attribute it to a real phenomena on the planet itself. Great work.
Pete
Lay in beds and stove huggers





OK Pete – I’ll assume that you’re not being just a tiny bit patronising here – thanks!

It has been said/proven(?) that spider vanes can produce a false AL effect so I will rule it out in my case with the attached – secondary affixed to optical window.

Now: after I reported the AL to one-time BAA Section Director Richard Baum he subsequently passed back to me queries from John Westphal re. the state/quality of the filters and optical window. As reported back:-

Filters: were (are) in a pristine condition and often used on the moon, and shadow-filled large craters make a fair approximation to crescent Venus and would soon have been concerned if they were showing such internal illumination/light-spill. Rotating filters/eyepieces is another check of course.

Optical window: kept clean – and dew-checked often during a session. There is a slight ghost but this is offset a few arc minutes from the planet/s. Presumably it is slightly not squared on but I like to keep it that way so as not to interfere with seeking faint satellites etc.

Incidentally after getting the rather superb WO Amici and Meade 5000 Plossls, when Saturn was at a greater altitude with rings pretty wide open I started seeing Mimas quite regularly without hiding Saturn. Clean optics and good seeing essential here as well as transparency otherwise you get a situation as in M42 and some PNs where faint stars are faded/lost in poor definition giving illusion of variability (yes I know many are in M42!)

Here is a message to Richard which also details the recently posted AL obs.

From: David Gray
Sent: 27 September 2007 16:35
To: Richard Baum
Subject: Venus AL

Hello Richard,
Ashen light again but much more elusive than the [Sept] 23rd . A lot of cloud was appearing just as Venus became high enough. I attempted to eliminate some of the possible causes of any spurious effects but due to the conditions only managed to check out the Amici and the binoviewer - see attachment. The Amici is by Williams Optics, and is far superior to the Edmund's that I used since the early 80s. I can report that with the Williams and the bino combined I was routinely spotting Mimas near elongation last apparition [2006] and without hiding Saturn - indeed it often [irritatingly!] caught my attention whilst scrutinising the planet - without being sought. Also Enceladus often seen near its conjunctions with the globe [all later checked on Sky Map Pro].
Again I have copied the drawing by 'drawing/painting' it into Corel. I must, however, chance installing the scanner software as I am not happy by abusing my eyesight spending so long staring so intently at the monitor. I feel that this can affect the vision for critical planetary observation. Years back I got hooked on playing chess for hours on the PC, and I am sure that this took the edge off my acuity, and for some days/weeks after realisation and packed it


If real it is apparently very transient - we need to keep looking (also at Uranus!!)

David.

Edited by David Gray (03/22/13 09:59 AM)


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azure1961p
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Re: Ashen Light - Redux [Re: David Gray]
      #5749516 - 03/22/13 11:46 AM

Oh not at all, I was patronizing in spades lol. I'm a fan of your work - particularly your Galiean moon drawings. I like that you weighed in on this despite my own experiences.

Mimas I know is visible to amateurs with large scopes. I'd like to make it happen in my medium reflector. In mentioning it I was not implying it was off limits but that the CCD despite the wash of glare from Saturn brings it out with great clarity in a way the eye cannot perceive it. The thread Saturns Moons in this forum has a pic imaged through a ten inch aperture of Mimas that shows it quite clearly. I was using it as an example where dynamic range if lesser can be off set by contrast manipulation.

Best,

Pete


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photonovore
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Re: Ashen Light - Redux [Re: azure1961p]
      #5751951 - 03/23/13 01:50 PM

Just a stray thought...There is another possible mechanism for Venusian ashen light, that being the earthshine during periods of high combined albedo of the earth/moon system. Lest one declare this idea (not mine BTW) unrealistic, consider that Venusian light (at crescent phase) is strong enough to cast shadows detectable by the human eye. (Even Jupiter has been documented as having the power to cast detectable shadows!) Oppositely, the earth at the same time is illuminated from a Venusian perspective at a gibbous phase and has ~10% more area overall in addition. Earth's albedo is generally considered to be half that of Venus, but greater illuminated area (gibbeous vs. crescent) would make the Earth at least as bright as seen from Venus than Venus as seen from Earth.

re; CCD, mimas saturn is an invalid analogy for the ashen light case as the former deals with a point source of high per pixel intrinsic contrast where the latter is dispersed over a large area giving a significantly lower sum contrast ratio--and thus a significantly *higher* contrast ratio with the illuminated source. Comparison of illumination per unit area between the two primary target areas is the controlling factor and when one is intrinsically low and the other extremely high, this makes for a perfect storm of high dynamic contrast, where the ccd remains utterly inferior to the human eye. HDR methods have issues due to blooming vis a vis target size. I wonder what the magnitude record is for capturing a star occulted by the Venusian darkside? In the observing record i find that the limit visually was supposed to be between 8 and 9th magnitude.

Still, lack of imaging evidence remains the best argument for ashen light being an illusory effect. However, the observational record argues against illusion because of the intermittent nature of the observations. An illusion is typically reproducible and consistent across large populations, given conditional restraints (ex. moon/horizon illusion). However, ashen light observations are anything but..being very inconsistent in both occurrence and across populations- and this would argue against it being a garden varity illusory effect.

As earth's albedo varies a good deal with the current weather on the sunlit side plus seasonal variability, and also considering concurrence of fuller phases of the Moon (from a Venusian perspective) in conjunction with anomalously high earth albedo periods, the moon/earthshine hypothesis seems sort of appealing to me as a possible explanation at least. there also remains the possibility of a combination of effects; intermittent Venusian atmospheric phenomenon plus extant earthshine.


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stanislas-jean
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Re: Ashen Light - Redux [Re: photonovore]
      #5752019 - 03/23/13 02:37 PM

Did you exercise some calculations regarding this assessment?
My expectations conduct to get a light level ratio of largely more than 10 000x between the crescent and the dark side levels.

Agree with you concerning the meaning of the dark side light accessibility being a surfacic light level to be captured and to be on comparison. Nothing to see with a pin point star/satelite with light levels, perceptibility access is not on the same level. The time of observation is also a disturbing parameter in regards because the light levels difference becomes narrower, crescent, dark side (not black hole appearance) and backgroud sky of blue, deep blue, dark blue, never black.
The results, I reported for AL captured, during day, dawn and just after sunset would conduct to get an intensity not confidential or negligeable. This needs confirmation indeed.
Stanislas-Jean


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GlennLeDrew
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Re: Ashen Light - Redux [Re: photonovore]
      #5752206 - 03/23/13 04:13 PM

Excellent comments, Mardi. And Stanislas-Jean.

Regarding Earth/Moon shine. One could calculate the maximal integrated magnitude of our 'dual planet' as seen from Venus. Then the illumination level on Venus' cloud tops could be compared to that of Venus on Earthly snow; both surfaces have reasonably similar albedo.

For my own observations, -4m Venus shining in a dark sky on snow casts a not terribly obvious shadow, the latter being filled in to some extent by the -8m light of the night sky (air glow, primarily, augmented to some extent by stars, Milky Way and zodiacal light.) If not for these other bright sources, Venus' shadow would be rather apparent due to significantly higher contrast. (But the overall illumination would of course be less.)

For Venus we will have the following contributors to its nighttime illumination:
- Stars; -4m
- Milky Way; up to -5m
- Zodiacal light; -5m
- Earth (and Moon, which contributes very little due to 1/16 the projected area and 1/3 the albedo); -4m (?)
- Venus' own air glow; ?? (If present and of similar intensity to that of Earth, its surface brightness against the orb of the planet might be something like 22 mag/arcsec^2.)

(Regarding my provisional guess for Earth's maximum apparent magnitude as seen from Venus. While it's a bit larger the Venus and is always at least at gibbous phase, its albedo is 1/2 or a bit less, and it's farther from the Sun, receiving less than 1/2 the light.)

If we integrate all these sources (neglecting for the moment any Venusian air glow), we get around -6 to -6.5m. If we assume an air glow layer like ours, the total irradiance would be much like that here on Earth, it being around -8m.

The question then becomes this. Can a nighttime illumination like that of snow on a moonless night out in the country be seen when:
- Some fraction of the sunlit planet is immediately adjacent?
- When the contrast is reduced by intervening terrestrial air glow (or, worse still, twilight glow if the observation must be made with the planet well enough above the horizon)?

Out in the country, a snowscape had a surface brightness very nearly equal to the integrated light of the night sky, which is 21-22 mag/arcsec^2, and as noted be can take Venus' night side to have a surface brightness of this order. The sky against which we see Venus is always brighter than this, it being hardly ever darker than 20 MPSAS. If Venus' night side is, say, 2 magnitudes darker than the sky, its light and that of the sky combined makes that surface appear 16% brighter than the surrounding g sky.

A not terribly low contrast, but not a very easy detection, either. *Especially* with that blazing -4m crescent--with a surface brightness higher than sunlit snow--up against it! And considering that the exit pupil tends to be on the small side in order to achieve sufficient magnification, this dims the view and by itself makes that low contrast view of the night side against the sky more difficult still.

Why there is doubt regarding the utility of electronic detectors to image here what the eye supposedly sees astonishes me. If the ashen light is real, an imaging system of no great sophistication will reveal it, when a suitable exposure is employed. If the light scatter from the sunlit crescent is no impediment visually, neither will it be for a camera.

Finally, I do wonder if something along the lines of the Martian canal phenomenon, in 'reverse', if you will, might not be in operation??

The foregoing provides a line of reasoning to build upon for the determination of a likely Venusian night side surface brightness required for terrestrial detection. And from these considerations I am currently quite doubtful of the reality of the ashen light.


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David Gray
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Re: Ashen Light - Redux [Re: photonovore]
      #5752222 - 03/23/13 04:20 PM

I recall Patrick Moore giving serious consideration of Earthshine as being a possible cause for AL in the BAA Journal but I believe he decided against it in later years. I understand that a near full Earth would be as bright as mag. at least -6 and the moon similar to how we see Jupiter. Jupiter from Venus would not be a lot fainter than we see it – should this be factored in? Perhaps so, but only when it is suitably positioned in relation to Venus and does this provide the ‘tipping’ point to render AL visible to us? I think not really: Jupiter + moon would only add fractionally to Earthshine.

With earthshine we perhaps expect AL to be bluish or simply grey, but it is most often described as reddish/brownish (even purple!); and this more accords with my filter experiences (W#22: Orange). But using W#23A (light red) I feel it is disproportionately more elusive and very fleeting at best in the denser W#25 (red). This colour impression I feel somewhat works against the illusion explanation. I agree if it was pure illusion it should be more consistently seen than reported; and more likely smooth than mottled.

Has it been (can it be) ascertained, I wonder, if the reflectivity – the very nature even – of Venus’ atmosphere increases(?)/changes on the night-side as another line of thought?


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David Gray
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Re: Ashen Light - Redux [Re: David Gray]
      #5752265 - 03/23/13 04:41 PM

Discussions such as this can be Googled:
http://cosmoquest.org/forum/archive/index.php/t-63462.html

Edited by David Gray (03/23/13 04:45 PM)


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GlennLeDrew
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Re: Ashen Light - Redux [Re: David Gray]
      #5752290 - 03/23/13 04:52 PM

If Earth light were dominant, the color would be only a *pale* bluish, due to the higher reflectivity of the clouds vs the ocean. Close enough to white to make blue filters likely detrimental due to their attenuation of that non-blue light contributing greatly.

The appearance of a reddish hue (or any color, for that matter) is almost certainly illusory if the surface brightness is below the color detection threshold of about 19 magnitudes per square arcsecond (as *perceived*, taking into account image diminution from a smaller exit pupil). Contributing to the reddish hue *might* be the well known phenomenon whereby dimmer parts of a scene take on a warmer hue.

I should think the characteristics of the cloud cover are of the normal, expected variety, with no peculiar reflectance at night vs day. Nor is the scattering markedly skewed over the visual spectrum, as evidenced by the pretty neutral white under sunlight.

Anomalous reflectance (or emission) outside the visual spectrum can hardly be expected to elicit a visual response. This is certainly where the expanded spectral sensitivity of electronic detectors affords a decided advantage over the eye.


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David Gray
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Re: Ashen Light - Redux [Re: GlennLeDrew]
      #5752329 - 03/23/13 05:18 PM

Agreed in principle, but personally I have never seen it in other than the filters reported in the above observation attachments; so can't attest to any hue! That is never have I detected it in intergrated light; and presumably it has, as alluded to in my above reports, been rendered visible by: "This [filter] combination rendering the sky almost black." (Sept 23) and in addition conceivably dimming the bright crescent relatively more than the presumed AL??

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GlennLeDrew
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Re: Ashen Light - Redux [Re: GlennLeDrew]
      #5752334 - 03/23/13 05:24 PM

David,
I checked out the discussion you linked to. It was interesting to see a number of congruencies with my own line of reasoning conducted in isolation. It seems my 'gut feeling' of -4m for the Earth as seen from Venus is rather less than the -7 bandied about in that discussion. I should calculate it myself, if for no other reason than to refute my incorrect mental image.

That notwithstanding, a -7m Earth (augmented by the other sources I enumerated earlier) will still result in a cloud deck on Venus' night side having a surface brightness hardly brighter than 20 mag/arcsec^2.


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David Gray
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Re: Ashen Light - Redux new [Re: GlennLeDrew]
      #5752388 - 03/23/13 06:01 PM

Quote:

David,
I checked out the discussion you linked to. It was interesting to see a number of congruencies with my own line of reasoning conducted in isolation. It seems my 'gut feeling' of -4m for the Earth as seen from Venus is rather less than the -7 bandied about in that discussion. I should calculate it myself, if for no other reason than to refute my incorrect mental image.

That notwithstanding, a -7m Earth (augmented by the other sources I enumerated earlier) will still result in a cloud deck on Venus' night side having a surface brightness hardly brighter than 20 mag/arcsec^2.




Glenn,
Fully agree: as indicated in my earlier post, that Earthshine is a non-starter.

I will, of course, keep checking the planet out but realistically my observatory is not that well placed for the already difficult pursuit of systematic AL study/detection. Too many seasoned observers (some initially sceptical) have reported this to walk away crying illusion, illusion!

Cheers,
David.


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stanislas-jean
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Re: Ashen Light - Redux new [Re: GlennLeDrew]
      #5752439 - 03/23/13 06:25 PM

We could conclude for a combination of "effects" that produce this possible AL.
Personnally at present I am sure of the presence of no effect.
The light reflection from the planets producing a light level on the dark side which is reflected also by the dark side atmosphere to us is hot to hold. The overall status will be poor and quite depending on the localisation of the planets respectively. This cannot occur on a long period may be few days.
The problem is that this reported over months before after conjunction.
Personally for the last conjunction this was collected from phase 0.35 until almost 0 with strong fluctuations of light intensity.
This is not observed in a colour more than an other colour channel, the blus channel seems to reveal also AL.
The intensity seems not to be depending on time moments from day until dawn and after sunset (but not black night).
The peak intensity was noted 30 days before conjunction and 15days after. Around the peaks of intensity fluctuations were observed, dark features were noted that seemed to be in relation with those collected by others with ccd.
I found this interresting but not enough by the number of data collected.
Systematically it should be used filters from the blue until the red and no filter, should be on observation at different moment in order to assess the background intensity of the sky for comparing with the observationnal data.
Daily basis observations should be done.
This will help to know:
- if this is transcient, I donot believe this,
- if fluctuations will occur again,
- when peak intensities will survey again,
_ the influence of sun activity.
It is easy to undertake such.
But before the observation method should be clean.
Occulting bar may involve diffraction edge with a light intensity "grainy", at final with the test practiced with a second observer i decided to donot continue the use of occulting bar. But i decided to use a second aperture in order to modify the respective light levels, a R50mm and a 200mm, 100 and 150mm used usually.
It's not simple to state but the method must be clean and absolutly sure.
Because we can be under an observationnal artifact , not under the pseudo illusion of eye limitations set by some. This aspect has to be well verified but how?
Stanislas-Jean


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buddyjesus
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Re: Ashen Light - Redux new [Re: stanislas-jean]
      #5752500 - 03/23/13 07:10 PM

I think the earthshine idea is a no go based on your approximations Glenn, but also must based on the inverse square law.

http://en.wikipedia.org/wiki/Inverse-square_law

What I think is most likely is the contrast effect that the area closest to the lit segment appears extra dark like the areas near the martian polar caps. Then the rest of the dark disk doesn't look as dark in comparison. Just a thought I am throwing out there to be bantered.

I think suggestion of it being there also tinges our ability to be blinded appropriately to trust our eyes.


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buddyjesus
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Re: Ashen Light - Redux new [Re: buddyjesus]
      #5752513 - 03/23/13 07:15 PM

here is a link to the article about suggestion and observation. http://www.skyatnightmagazine.com/feature/column-excerpts/seeing-believing-%E...

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Rick Woods
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Re: Ashen Light - Redux new [Re: buddyjesus]
      #5752729 - 03/23/13 09:12 PM

Quote:

What I think is most likely is the contrast effect that the area closest to the lit segment appears extra dark like the areas near the martian polar caps. Then the rest of the dark disk doesn't look as dark in comparison.




Yet, the dark Martian polar cap collar is a physical reality.


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azure1961p
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Re: Ashen Light - Redux new [Re: Rick Woods]
      #5752807 - 03/23/13 09:48 PM

The earthshine is a stretch (of millions of miles actually) but its a compelling notion. It would be interesting to model this out and I don't see why a msgnitude surface bright was couldn't be worked out for such a known reflective surface.

I'm still deep into the idea its not a true physical phenomenon of this world but earthshine is the first thing that actually holds some plausible notions.
The red color if its true would seem to be a contrast from a grey area playing opposite the contrast of a blue of blue/indigo sky. The light needed to illuminate it though from our planet... It'd be more plausible if our planet were Jupiter.

Mardi you are disagreeing with me for the sake of form and Im not addressing it. It's enough at this point.

Pete


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