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stanislas-jean
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Re: Ashen Light - Redux new [Re: GlennLeDrew]
      #5783147 - 04/07/13 05:55 AM

If you place the crescent limit just on the edge of the occulting side, you have noticeable light diffraction accessible to eyes even in daylight.
With few seconds of arc shift of the crescent limit beyond the occulting edge, this is disappearing.
I did the exercise. Perform this when you donot notice the AL presence or what it seems, so when the planet phase is around 0.5 for having the best conditions of test.
Occulting side use has needs.
Stanislas-Jean


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GlennLeDrew
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Re: Ashen Light - Redux new [Re: GlennLeDrew]
      #5783150 - 04/07/13 06:00 AM

Stanislas-Jean,
For the purpose of *detection*, calibration is not required. One is merely looking for any brightness level above that of the sky. If the sky glow is recorded well enough above the instrumental noise, and all sources of glare and scatter are taken care of to the extent possible, then one has a sufficiently good image with which to say 'yes' or 'no' for the detection.

That same image, if not processed, can then be measured for the determination of the brightness ratio, or contrast. And if one knows the sky brightness at the time of the image, the intrinsic surface brightness on the planet (sky subtracted) follows directly.

If the visual observer is using a sensor--the eye--which is not calibrated, why then must a camera be calibrated beforehand? Just as the eye detects a brightness ratio, so does the camera. What's important is that the exposure put the sky glow sufficiently above the camera noise, and not more than about 2/3 of the way to saturation. That's a pretty good margin which allows several f/stops' range on exposure.


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stanislas-jean
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Re: Ashen Light - Redux new [Re: GlennLeDrew]
      #5783153 - 04/07/13 06:06 AM

How do you consider the fact that the amount of light created by earthshine reflexion on venus dark side being above of the background sky light level not revealed by the chipset?
Stanislas-Jean


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brianb11213
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Re: Ashen Light - Redux new [Re: GlennLeDrew]
      #5783168 - 04/07/13 06:17 AM

Quote:

detectors (and film) where glare is concerned, as they have less dynamic range than the eye. But a suitable occulting device near the focus will obviate this.



But there is still scatter in the optics positioned before the occulting bar, and in the atmosphere.

I suspect we are going to need observations made with a coronograph from either a high altitude desert location or from a high altitude balloon or satellite to solve the question of Venusian ashen light with any degree of conclusiveness.


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GlennLeDrew
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Re: Ashen Light - Redux new [Re: brianb11213]
      #5783390 - 04/07/13 09:24 AM

Stanislas-Jean,
If, after taking due care to minimize extraneous light, Venus' night side is not detectably brighter than the sky, then we can assign a *maximum* intrinsic surface brightness. For example, suppose the sky has SB = 19, and Venus' night side is identically bright (no detectable ashen light at all.) furthermore, suppose that, based on the camera's bit depth and its noise level in the image we deduce that we should discriminate a brightness difference of 2%. We know then that the night side cannot be any brighter than 4.3 MPSAS below the sky brightness, or 23.3 MPSAS.


Brian,
Indeed, as I have pointed out in earlier posts, simply occulting the source at the focus does not quell atmospheric and instrumental scatter.

But the eye has to deal with these as well. To a camera, this will be *very* much less injurious than the blazing, bloomed, far over-saturated blob of light which would be the sunlit portion. But with an occulter in place, the residual scatter most assuredly will be no more of an impediment than it is to the eye.


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Rick Woods
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Re: Ashen Light - Redux new [Re: GlennLeDrew]
      #5783534 - 04/07/13 10:52 AM

To elaborate on one point: The daytime observation of Cruikshank and Hartmann revealed not a brighter unlit side of Venus, but rather a slightly coppery colored one against the blue background.
The effect was one of color, not brightness.

(PS: See S&T, March 2012, pg 53.)

Edited by Rick Woods (04/07/13 11:13 AM)


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stanislas-jean
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Re: Ashen Light - Redux new [Re: Rick Woods]
      #5783771 - 04/07/13 01:06 PM

Not only Rick because the AL coppered color you recall was effectively reported with a moltenned appearance.
However this moltenned appearance was also noted on the sky around.
This comment is strange and make an objection.
Personnally for reporting, this AL color was seen also moltened coppered color (rather grainy than moltened) but not on the sky around and concerning the whole dark side part or only partially without the filter use and AL seen also under color filters R, G, B.

Glenn, I understand what you mean about the ccd procedure used. My evaluation of this situation for the moment is that the earthshine reflection conducts to a light level being above the back ground skylight level used as a reference in the ccd procedure (light level being few above the noise of the ccd chipset, you recalled). This is qualitative settlement but
this point is still disturbing me and I am wandering who did this acurate evaluation by calculation and which results he get.
I would exercise these verifications.
Stanislas-Jean


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azure1961p
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Re: Ashen Light - Redux new [Re: stanislas-jean]
      #5784071 - 04/07/13 03:26 PM

The coppery or warm tone would seem to he easily explained away as a contrast effect against so much blue sky, day or evening. Surely you've seen the effect if this in doublestars where the blue companion can make the primary (or vice versa) take on a decidedly warm hue. In this case because the ashen light is seen or seemed to possess a certain brightness however faint then it would make all the sense in the world this take in the hue it does.

How though this topic keeps thriving in the light of some pretty damning evidence is beyond me. I thought Glenn said it all rather succintly .


Pete
PS: oh, I know where you going to go with this rebuttal Stan: the doublestar analogy is invalid because its point sources versus extended object. That's irrelevant as any kind of counter claim. What differences arise are more an exercise in semantics than anything that would matter one way or the other.


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Rick Woods
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Re: Ashen Light - Redux new [Re: azure1961p]
      #5784722 - 04/07/13 08:30 PM

Pete,

I sure haven't seen any damning evidence yet. And the double star analogy is *really* thin. And the thread continues because it's an interesting topic, and is, still, an unsolved mystery.


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buddyjesus
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Re: Ashen Light - Redux new [Re: GlennLeDrew]
      #5785072 - 04/07/13 11:49 PM

the venus watch yahoo group has an image available for making venus occultion masks with 3 different arcs on it(for different magnifications.) I didn't see anyone use it last season though.

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stanislas-jean
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Re: Ashen Light - Redux new [Re: azure1961p]
      #5785202 - 04/08/13 02:02 AM

This doesnot explain the fact when the occulting side is used the AL is still visible on the dark side and sometimes with this coppered color and grainy appearance.
Take in consideration the procedure used with the occulting side and with the second observer who locates the planet aleatory to be found by the 1st obaserver who doesn't know where the dark side is actually.
I suggest you perform this, this is very instructive for ourselves.
Stanislas-Jean.


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stanislas-jean
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Re: Ashen Light - Redux new [Re: stanislas-jean]
      #5785212 - 04/08/13 02:16 AM

Sirius double star was well separated in a 3" achromat well designed on the 80ies (not interested now with doubles).
This needs a high doublet where the main star is still a luminous point without spikes, the sky around remaining black deeply. All the scopes on use are on this level (absence of light diffusion around a strongly lighted surface or point): this is a need for this kind of exercise on venus.
The observation of doubles is interresting on that view but a 1st step.
Stanislas-Jean.


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azure1961p
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Re: Ashen Light - Redux new [Re: stanislas-jean]
      #5785250 - 04/08/13 03:25 AM

Persistence of vision.

Pete


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stanislas-jean
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Re: Ashen Light - Redux new [Re: azure1961p]
      #5785330 - 04/08/13 06:55 AM

That persists.
Stanislas-Jean


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GlennLeDrew
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Re: Ashen Light - Redux new [Re: stanislas-jean]
      #5786116 - 04/08/13 02:10 PM

Indeed,
If the sunlit crescent could be well occulted so that it's fully blocked, an observation of a night side illumination would be more secure.

Note, however, that there can remain one or more sources which can introduce uncertainty.

1) If the occulter is made so as to have a 'concavity' which permits the seeing of the maximum area of the night side, and if the occulter is readily seen as a black silhouette, that very concavity could well provide a 'frame' for the visual system to provide a spurious 'filling in' of the circle.

2) There will likely be some atmospheric and instrumental scatter, appearing as a brightening of the sky toward the occulted crescent. Such a gradient could have some bearing on the perception of a spurious glow.

An image would definitely be preferred in any case, as then it could be objectively measured, with the capability to find the limb and ascertain if it's both well defined and smoothly circular, of the expected size.


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stanislas-jean
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Re: Ashen Light - Redux new [Re: GlennLeDrew]
      #5786413 - 04/08/13 05:02 PM

The question of the occulting is important to define.
Frankly, when spurious light is occuring this does not create a circular appearance (a dark side lighted) but rather a lighted band with a decreasing light intensity. The width of the this band is the diameter of venus.
For annealing this it is just necessary to shift the crescent few seconds of arc behind the occulter edge in order to be fully covered by the occulter.
This is the condition to achieve for not having spurious (diffraction edge).
When some AL appears on the sky the dark side of venus not occulted appears lighted without the vision of the crescent.
For the occulter I am using piece of dense argentic films (the TP 2415 from kodak is excellent, the opacity is easy to get, it is unsensitive to moisture, it remains at home some coils still not used. To check the density we have access to a densitometer used for industrial radioscopy). Just to glue the oculter on the FOV ring of the eyepiece, so just at the focus plan, less the film thickness.
My results were got with this. The drawing reports were done under pure sky.
At final after 2 days tests as described, I didn't use any occulter, evaluating that its useless role.
Long distance tests on cibles convinced me also to abandon the use of the occulter bringing in fact more trouble than solving some hypothetic situation.
You know the separation of double star as Sirius, as a 1st approach and long distance test on cibles that create sensitive surfaces lighted with a known light ratio.
Stanislas-Jean


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GlennLeDrew
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Re: Ashen Light - Redux new [Re: stanislas-jean]
      #5786769 - 04/08/13 08:19 PM

S-J,
I gather you're stating that the occulter is not necessary? From the standpoint of diffraction, certainly this is the case. But sensor (eye or camera) saturation without it introduces a very real potential for masking of the dim phenomenon we're trying to detect. Due to the excessive brightness ratio between day and night sides, and most crucially their proximity, every effort to block that bright source can only pay dividends.

For one thing, if the visual detection of the AL is seen to depend, even slightly, on the *presence* of the sunlit crescent, this must be seriously considered as evidence for the 'filling-in-the-circle' illusion.


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Rick Woods
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Re: Ashen Light - Redux new [Re: GlennLeDrew]
      #5786859 - 04/08/13 09:06 PM

I'd like to slip in here and thank everyone for your opinions and viewpoints so far. I'm really enjoying this thread - lots of good stuff here!


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stanislas-jean
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Re: Ashen Light - Redux new [Re: Rick Woods]
      #5787282 - 04/09/13 02:20 AM

Thanks Rick for your presence.
Glenn, yes this is my conclusion after performance of 2 kind of tests.
Intellectually this is disturbing for sure but this is not a fact but a result.
I know that when making ccdd images (I did) the crescent is invaded by glaring. But I think this is light levels concern and probably depending on the observer also.
We take an example: driving during night with people on the opposite way with lights full and you following a driver with its little red lights more or less powerfull, under rain.
This is an analogy but similar.
Now all these assessments doesnot constitute prooves.
And for the moment the ccd procedure is the way for prooving.
This is what I noted through the present CN forum.
And the fact that an AL light was seen under conditions we know that merits deeper analysis, because the light is not illusory, but can the result of physical processus created by the observation method or the kind of light observed (an edge create diffracted light with an amount of polarised light).
Consider the fact that, for ccd too, that light are the result of cause that can be splitted according to calc.
The eye has the possibility to merge a certain calc (that correspond to a certain light and a cause) and avoid the others (the drivers analogy is correct for illustration of this, it is almost impossible to discerm the red lights if you consider only the overall scene).
Stanislas-Jean


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GlennLeDrew
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Re: Ashen Light - Redux new [Re: stanislas-jean]
      #5787421 - 04/09/13 05:50 AM

S-J,
There are some confusing parts in your previous post.

I'm still not certain whether you say *to use* an occulter or *not to use* an occulter.

You often mention diffraction. I feel that this by itself poses no *significant* problem to the detection of AL. With any reasonable size telescope, the diffraction intrudes into the night side only some several arcseconds, compared to the disk diameter of the better part of an arcminute. Moreover, diffraction is always present, irrespective of the presence or lack of an occulter.

You have often mentioned polarization. The combination of eye and telescope introduces polarization only (?) when light is reflected at an angle somewhere in the system, as when a diagonal is used. I should think a straight-through arrangement will not result in instrument-induced polarization.

Is polarization of real concern? The object, by virtue of the scattering property of the cloud droplets, should not be expected to polarize light itself. And if the source light is not polarized, instrumental polarization cannot alter the contrast of the view.

Of course, our atmosphere does partially polarize that light it scatters. The maximum polarization occurs at some angle (around 60 degrees?) from the source. Given that Venus is rather closer to the Sun than this, about 25-30 degrees and less, the polarization of scattered sunlight is weaker. And so a polarization filter is less efficacious.

Does the atmosphere polarize the light of objects seen through it? I don't think so; not to any notable extent anyway. If this is the case, then atmospheric polarization cannot alter the contrast of the components making up the scene beyond. All we can do is exploit atmospheric polarization, by cross-polarizing at the appropriate angle in order to dim the sky glow relative to the scene beyond. (Everything will be dimmed somewhat, but the sky more so.)

I do wonder about the degree of polarization of light from the object which is scattered and seen in the immediate vicinity of the object itself (i.e., in direct forward scattering.) I believe such light is not really polarized, and if it is it would be effectively, and weakly, circularly polarized. In such case, there may be no effective way to suppress the glow of scattered light emanating from Venus' day side.


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