Author Topic: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.  (Read 185 times)

Jerry Gilbert

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #30 on: January 23, 2018, 07:15:00 PM »
> It would essentially have to be a perfect sensor.

Hmm. Lessee, in what sense were the Kodak 103a, 103o, and 103f emulsions usd at Mt. Wilson "a perfect detector"?

> take a look at the math of the various amounts of time light could take to travel to your sensor depending on the directness of their path and variable environmental conditions.

The math is clear as water-white float-glass, and shows that path-length delays and refractive index gradient of the atmosphere are utterly insignificant at our frequency of modulation of interest, ...120 Hertz.

Jus' sayin',

--Joe

Jerry Ridl

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #31 on: January 25, 2018, 01:03:21 PM »
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Other types of lighting may not heat up metal wires rather gases, but similarly they rely on heat generated off electric current in one way or another. Still heat does not go on and off in rapid phases.
<span>Negative.  Gas discharge tubes (Hg, Na) are just that.  Their working principle is spectral emission, not incandescence.  Near the zero-crossing times of voltage, these light fixtures are essentially dark ("OFF").
BTW, even an incandescent (Tungsten filament) light bulb reduces its white light output each cycle (albeit not to zero) near the zero-crossing times.  Such modulation is the principle behind how it is possible to "talk over a light beam" (using, say, a tungsten flashlight bulb), that is, to modulate a light beam.  To experiment with this, you can connect a solar cell to the input of an audio amplifier, turn up the volume, and allow the light of an incandescent bulb to fall on the solar cell.  You will hear the typical "120 cycle hum" created by the lightbulb's output modulated by the 60 Hz AC line current.  Light output of a tungsten filament bulb is very definitely modulated by an AC current, or an AC audio signal ((voice, music, e.g.) impressed upon a DC "bias" voltage which lights the lamp to a high level of output, for good audio fidelity..
I think that the work done at Mt. Wilson (?) with this was done at an epoch when there was a mix in the cities of incandescent street lamps and high-pressure Mercury (Hg) street lamps.  The Hg street lamps go nearly entirely "OFF" around zero-crossing times.  This is why the technique worked for the Astronomers there.  Although, again, the Intermittency Effect ("Flicker Effect") limited its usefulness with the photographic emulsion based detectors ("plates"), which were the state of the art (in imaging) of the time.
rgds,
--Joe

Not saying there is no change in light intensity in a direct cycle, so you can measure the cycle by reading the light.  But there is no effective on and off that you were looking for.

bermordliro

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #32 on: January 25, 2018, 02:25:01 PM »
"The other question of if it can be detected is a more interesting thought experiment and for that take a look at the math of the various amounts of time light could take to travel to your sensor depending on the directness of their path and variable environmental conditions."

Path is probably the least of the reasons that there is no perceivable flicker in light pollution. That fact that most light sources don't have a perceivable flicker is probably the major reason. Incandescent lights do not cool hardly at all between peaks and most florescent lighting is at kilohertz frequencies, not AC frequencies. So the ripples are small and at different frequencies so as to nullify each other before we even talk about distance from the light source, or from the power plant, or any other phase shifts or delays in the circuit.

Corey Howell

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #33 on: January 31, 2018, 03:49:52 AM »
"Not saying there is no change in light intensity in a direct cycle, so you can measure the cycle by reading the light. But there is no effective on and off that you were looking for."

FirstC8, You fell for a slight of hand.

Instead of saying that incandescent lights have virtually no perceivable flicker (because they do not cool fast enough), the OP said that they dim, but not to zero.

The point ultimately is does the sky perceivably flicker? From the evidence with high speed imaging, the answer seems to be no. And if this alleged experiment was done in the past, it seems that it failed then as well, seeing that there seems to be no trace of it or its results left.

boacamcentrumb

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #34 on: January 31, 2018, 04:02:20 AM »
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The other question of if it can be detected is a more interesting thought experiment and for that take a look at the math of the various amounts of time light could take to travel to your sensor depending on the directness of their path and variable environmental conditions.
Scattering of light is well-understood. So is the speed of light in air. I'm not sure where the 1000x factor in distance due to scattering came from, but that is completely unrealistic. Photons we see in light pollution is scattered no more than afew times and most photons that we see will be scattered only once, and the scattered path would be in the same order of magnitude as the direct path. There cannot possibly be three orders of magnitude difference. Atmospheric scattering is primarily Rayleigh scattering and as a comparative example, of the longer wavelengths of visible light from the sun, less than 5% get scattered when passing through the entire thickness of the atmosphere. Among the shorter wavelengths, around 20% get scattered. The light we see from light pollution is primarily the longer wavelength visible light that gets scattered <strong class="bbc">once[/b] on its path to space as itgets directed to our cameras by a single scattering event.

AC current has two nodes per cycle spaced 1/120 second apart. If the frequency of the scattered light is shifted by approximately 1.4E-03 seconds, then the minimum intensity will have been moved to about50% intensity compared to unshifted light. So, a scattering length that results in a 1.4E-03 delay will smear out the flickering by 50% for the scattered light. The speed of light is 186,282 miles per second. So the 1.4E-03 delay converts to 257 miles. That means for a photon to smear out the flickering of an unscattered photon by 50%, it would need to scatter 257 miles father than the path of the unscattered photon. That is simply not a realistic scattering length for atmospheric scattering. Additionally, to smear out the entire overall flickering by 50% at least half of the light pollution photons would need to be scattered by a distance of 257 miles or more.

So, atmospheric scattering is not a plausible cause for the smearing out of flickering.

That is assuming that all atmospheric scattering is elastic Rayleigh scattering. There will no doubt be some inelastic scattering including fluorescence and phosphorescence as well as more exotic inelastic scattering mechanisms that will conspire to smear out some of the flickering, but the overwhelming fraction of photons we see will be photons scattered only once from the source to our eyes. The very small increase in path length for the scattered photons will have little to no effect on the flickering.

Tim

Jay Cole

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #35 on: January 31, 2018, 07:44:42 AM »
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Quote

The other question of if it can be detected is a more interesting thought experiment and for that take a look at the math of the various amounts of time light could take to travel to your sensor depending on the directness of their path and variable environmental conditions.
Scattering of light is well-understood. So is the speed of light in air. I'm not sure where the 1000x factor in distance due to scattering came from, but that is completely unrealistic. Photons we see in light pollution is scattered no more than afew times and most photons that we see will be scattered only once, and the scattered path would be in the same order of magnitude as the direct path. There cannot possibly be three orders of magnitude difference. Atmospheric scattering is primarily Rayleigh scattering and as a comparative example, of the longer wavelengths of visible light from the sun, less than 5% get scattered when passing through the entire thickness of the atmosphere. Among the shorter wavelengths, around 20% get scattered. The light we see from light pollution is primarily the longer wavelength visible light that gets scattered once on its path to space as itgets directed to our cameras by a single scattering event.

AC current has two nodes per cycle spaced 1/120 second apart. If the frequency of the scattered light is shifted by approximately 1.4E-03 seconds, then the minimum intensity will have been moved to about50% intensity compared to unshifted light. So, a scattering length that results in a 1.4E-03 delay will smear out the flickering by 50% for the scattered light. The speed of light is 186,282 miles per second. So the 1.4E-03 delay converts to 257 miles. That means for a photon to smear out the flickering of an unscattered photon by 50%, it would need to scatter 257 miles father than the path of the unscattered photon. That is simply not a realistic scattering length for atmospheric scattering. Additionally, to smear out the entire overall flickering by 50% at least half of the light pollution photons would need to be scattered by a distance of 257 miles or more.

So, atmospheric scattering is not a plausible cause for the smearing out of flickering.

That is assuming that all atmospheric scattering is elastic Rayleigh scattering. There will no doubt be some inelastic scattering including fluorescence and phosphorescence as well as more exotic inelastic scattering mechanisms that will conspire to smear out some of the flickering, but the overwhelming fraction of photons we see will be photons scattered only once from the source to our eyes. The very small increase in path length for the scattered photons will have little to no effect on the flickering.

Tim

Very nice and thorough explanation.

pamasluocon

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #36 on: January 31, 2018, 10:12:30 AM »
"So, atmospheric scattering is not a plausible cause for the smearing out of flickering."

Which leaves us with the flickering being much less than assumed and much less in unison than assumed. Which would explain the reality that the net effect is close to zero.

propdiagairil

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #37 on: February 09, 2018, 12:07:27 AM »
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"So, atmospheric scattering is not a plausible cause for the smearing out of flickering."

Which leaves us with the flickering being much less than assumed and much less in unison than assumed. Which would explain the reality that the net effect is close to zero.

I don't know how you could have possibly drawn that conclusion from what I wrote. My conclusion was exactly the opposite.

I'm also not convinced that the net effect is close to zero. The human eye cannot detect flickering faster than about 60 Hz or so. Lights that flicker with line power do so at 120 Hz. So if there is flickering, nobody can see it.

I also reject the premise that the lack of "proof" from short-exposure photography leads to the conclusion that the net effect is close to zero. Short-exposure photography is not intended to test the hypothesis that light pollution flickers with some degree of uniformity. In general, short-exposure photography is intended to capture relatively bright objects - significantly brighter than the sky background. Otherwise, it wouldn't work at all. The result being a target that is substantially brighter than the background. If the background varied modestly for a few images that happened to be timed correctly, would anyone have noticed? We see images all the time for which the background varies for reasons that are no immediately obvious. We typically dismiss them out of hand as being due to clouds, the moon, or other factors without giving them a second thought.

Don't get me wrong - the net effect may indeed be close to zero. However, any speculation about the net effect that is made in the absence of any solid empirical evidence, is simply that - speculation.

We do know that some common sources of light pollution like high-pressure sodium lamps have substantial flicker (up to 95%). Others, like mercury vapor, flicker by about 50%. Incandescent bulbs flicker much less - on the order of 10 to 15%. We also know that the light that originates from these sources will be flickering in unison. We also know that the flickering will not be substantially reduced by scattering. So, in theory, light pollution should flicker in unison with a change in intensity somewhere between 10% (for 100% incandescent light pollution) and 95% (100% high-pressure sodium light pollution). I have yet to see anyone present a plausible mechanism by which that flicker can be reduced such that the net effect is zero.

I'm not saying such a mechanism doesn't exist. I'm just saying that it hasn't emerged in this thread.

Tim

enmumenge

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #38 on: February 09, 2018, 06:15:07 AM »
This is completely talking out my rear but as AC travels through the transmission lines aren't there affects which cause the phase to shift over time?

I tried to look this up but all I found was this:

http://fnetpublic.utk.edu/index.html

Apparently it monitors the frequency and phase from utilities over the US.

There are lots of time keeping devices that used to (and may still) rely on the frequency to keep time and this article:

https://en.wikipedia...ction_.28TEC.29

They try to keep the error down to seconds per day - but maybe this affects all devices equally and is not the spread in the devices?