Author Topic: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.  (Read 183 times)

Ligon Payton

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #15 on: January 12, 2018, 03:37:19 AM »
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I suspect that due to all of the reflections and scattering involved, all phase information is lost and the glow is uniform and without flicker.


That's unphysical (impossible, I mean).

If the lights flicker in-phase, so does the sky-glow from those lights. It can be worth one's testing, I'd say.

This is false thinking... The light pollution that causes us so many issues is not due to direct light introduced into our sensors, but reflections and scattering in the atmosphere. That fact on its own says that the amount of time it takes for light to go from the source to its reflecting counterpart and then continued scattering as it makes its way back down to your scope varies and it means that you lose that information. This would assume all lights were in phase.

If this hypothesis were true, then you would get substantially darker frames when doing lucky imaging. That doesn't happen. There have likely been millions upon millions of tests. Not to be hard on the idea, but case closed.

Marvin Alexander

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #16 on: January 12, 2018, 04:27:06 AM »
sys.,

It's unphysical bec. the speed of light is very great. If all the lamps causing the skyglow are in-phase, so must the skyglow be.

The flicker is only 120 Hz (not GHz, or THz).

To test the skyglow itself for this physical flicker takes a dedicated experiment, I'd suggest. Again, since the effect was exploited at Mt. Wilson Obs., there may be some juice in there for amateurs of today using solid-state detectors, and not old-timey silver emulsions.

--Joe

Tommy Schmidt

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #17 on: January 12, 2018, 05:01:18 AM »
It seems like a plausible approach to me, and there is apparently some history of it being effective. Itdoes not surprise me that the phenomenon has not been more widely noticed since it would require very well-timed exposures of just the right duration coupled with an awareness of what one is looking for.

It also seems like it would be relatively simple to test. A simple triggering circuit using a Schmitt trigger sensing line current could trigger a short exposure of a light dome as the AC signal crosses zero. The wave propagation velocity of transmissions lines is a large fraction of "c", so pretty much any area with visible light pollution will also have a ready triggering signal that is nearly perfectly in synch in the local AC service.

An alternative simpler approach would be to use a rotating two-opening shutter attached to an AC synchronous motor. There would be no way obvious to me to synchronize the shutter with the zero point in the AC current other than trial and error, though.

Tim

nonbuysalcho

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #18 on: January 12, 2018, 11:37:11 AM »
But josh, you just don't have an "open" mind.

dsepinumer

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #19 on: January 12, 2018, 10:12:50 PM »
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Not to be hard on the idea, but case closed.


Josh, you must be joshing. Not to be hard on you!

Sleep on it... .

--Joe

Jeremy Butler

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #20 on: January 13, 2018, 07:00:35 AM »
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sys.,

It's unphysical bec. the speed of light is very great. If all the lamps causing the skyglow are in-phase, so must the skyglow be.

The flicker is only 120 Hz (not GHz, or THz).

To test the skyglow itself for this physical flicker takes a dedicated experiment, I'd suggest. Again, since the effect was exploited at Mt. Wilson Obs., there may be some juice in there for amateurs of today using solid-state detectors, and not old-timey silver emulsions.

--Joe


I can't think of more dedication than thousands of amateur astronomers doing high speed lucky imaging and not one of them reporting that the sky is flickering. Can you produce the example of it actually being tried in the past? Or is this an urban legend?

Cesar Lawhorn

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #21 on: January 14, 2018, 01:23:33 AM »
"It also seems like it would be relatively simple to test."

It was very simple to test. And the sky isn't flickering.

Tim Jauregui

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #22 on: January 15, 2018, 05:15:05 PM »
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Quote

Quote

I suspect that due to all of the reflections and scattering involved, all phase information is lost and the glow is uniform and without flicker.


That's unphysical (impossible, I mean).

If the lights flicker in-phase, so does the sky-glow from those lights. It can be worth one's testing, I'd say.

This is false thinking... The light pollution that causes us so many issues is not due to direct light introduced into our sensors, but reflections and scattering in the atmosphere. That fact on its own says that the amount of time it takes for light to go from the source to its reflecting counterpart and then continued scattering as it makes its way back down to your scope varies and it means that you lose that information. This would assume all lights were in phase.

If this hypothesis were true, then you would get substantially darker frames when doing lucky imaging. That doesn't happen. There have likely been millions upon millions of tests. Not to be hard on the idea, but case closed.
With all due respect, I don't think anything you offered is sufficient evidence to close the case. Reflections and scattering are irrelevant since they all take place at "c", so any phase delay introduced by them would be insignificant. As I mentioned earlier, lucky imaging would only reveal the dimming (if it actually happens) ifthe exposures wereperfectly timed and you were actually looking for the dimming. How many of us have seen some frames lighter or darker than others for no obvious reason?

I'm not arguing that light pollution can be eliminated with this approach. I am arguing that the idea has merit and that at least on paper there is no reason why we should not be able to detect an AC synchronous dimming of light pollution if we were to try to look for it.

However, even if it is possible to detect it, it may be of no practical value. By limiting exposures to only those times when the AC waveform is near zero, one would be dramatically reducing integration time. The resulting darker sky may not offset the large los in integration time.

Tim

David Pee

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #23 on: January 15, 2018, 09:51:30 PM »
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Not to be hard on the idea, but case closed.


Josh, you must be joshing. Not to be hard on you!

Sleep on it... .

--Joe
The data already exists for many of us... If shooting with lucky imaging with exposures faster than 120 Hz, then you have data that already exists to test this hypothesis. Most planetary imagers have this data accessible. Ask them to look for it. I'm looking at and have looked at it before, its not there...

Additionally, speed of light is constant in a vacuum, we are not shooting in a vacuum. Also, the light does not take a direct path to us, some photons take a direct path while others may bounce around like a pinball on a journey 1000x the distance from the source to your scope before entering your sensor. This combined with differing atmospheric conditions can't be ignored.

As a very interesting thought experiment, even though our data denies this hypothesis, how would you explain away the very different distances traveled as well differing mediums of the light paths?

Todd Treser

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #24 on: January 16, 2018, 01:38:15 AM »
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"It also seems like it would be relatively simple to test."

It was very simple to test. And the sky isn't flickering.

Just because you can't see it doesn't mean that it isn't. It most certainly is - at least to some extent. The vast majority of lighting is AC lighting and the light output varies with the AC current. All lights flicker, and at least those lights that are on the same electrical grid flicker in unison.

The question is whether the flicker can be detected with imaging cameras and of so, whether the flicker is useful for reducing light pollution. I don't think anyone has answered those questions.

Tim

rotenoter

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #25 on: January 17, 2018, 11:39:27 PM »
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It seems like a plausible approach to me, and there is apparently some history of it being effective. Itdoes not surprise me that the phenomenon has not been more widely noticed since it would require very well-timed exposures of just the right duration coupled with an awareness of what one is looking for.

It also seems like it would be relatively simple to test. A simple triggering circuit using a Schmitt trigger sensing line current could trigger a short exposure of a light dome as the AC signal crosses zero. The wave propagation velocity of transmissions lines is a large fraction of "c", so pretty much any area with visible light pollution will also have a ready triggering signal that is nearly perfectly in synch in the local AC service.

An alternative simpler approach would be to use a rotating two-opening shutter attached to an AC synchronous motor. There would be no way obvious to me to synchronize the shutter with the zero point in the AC current other than trial and error, though.


Tim, thanks, I see some well-considered pondering and projecting, there! Tnx.

I'm following the synchronous motor approach, here, myself at this point. Of course, later can come some sensible "electronicization". Much more than that, I will not immediately say. This may have commercial potential.

But the cat is out of the bag, and has been out of the bag since the early '60s ( ...the Mt. Wilson experiments, where The Intermittency-Effect, and Reciprocity-Failure, in emulsions called an early end to the practical usefulness of this exploit).

I find that to synch the slot in the synchronously-rotating shutter disk over the array-detector with the sky-flickering is easy. We can talk later, maybe off-line. And maybe here in the clear too.

Must run -- Birthday, today. Friend is taking me to breakfast (hmm, she may even pay).

g'day,

--Joe

wellbanstubars

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #26 on: January 18, 2018, 03:49:04 AM »
<p class="citation">QuoteAs a very interesting thought experiment, even though our data denies this hypothesis, how would you explain away the very different distances traveled as well differing mediums of the light paths?[/quote]

Thank you for asking.

The path-length delays and the different indices you mention are simply insignificant at 120 Hz, in 'light' of the fact of the speed of light, and in view of the fact that the index of air is nearly that of the index of vacuum (about 1.000).

BTW, in vacuum, there is no molecular light scattering at all.

--Joe

Jasper Banks

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #27 on: January 18, 2018, 06:30:37 AM »
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"It also seems like it would be relatively simple to test."

It was very simple to test. And the sky isn't flickering.

Just because you can't see it doesn't mean that it isn't. It most certainly is - at least to some extent. The vast majority of lighting is AC lighting and the light output varies with the AC current. All lights flicker, and at least those lights that are on the same electrical grid flicker in unison.

The question is whether the flicker can be detected with imaging cameras and of so, whether the flicker is useful for reducing light pollution. I don't think anyone has answered those questions.

Tim

Tim, I think the caveats of if it can be detected and whether it is useful for lp reduction is a healthy view point. That one is far easier to answer and it is a resounding no unless you are imaging with a very low read noise sensor. It would essentially have to be a perfect sensor.

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.

curnarenche

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #28 on: January 23, 2018, 10:26:00 AM »
Tim,

Thanks again for another fine-business (oops, amateur-radio-speak) consideration.

Quote
I don't think anyone has answered those questions.

Well, the old guys at Mt. Wilson did. Hats off to 'em. We'll catch up.

best,

--Joe

tinlengmmuner

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #29 on: January 23, 2018, 12:22:27 PM »
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"It also seems like it would be relatively simple to test."

It was very simple to test. And the sky isn't flickering.

Just because you can't see it doesn't mean that it isn't. It most certainly is - at least to some extent. The vast majority of lighting is AC lighting and the light output varies with the AC current. All lights flicker, and at least those lights that are on the same electrical grid flicker in unison.

The question is whether the flicker can be detected with imaging cameras and of so, whether the flicker is useful for reducing light pollution. I don't think anyone has answered those questions.

Tim

No, I tested it with imaging cameras at high frame rates and there was no perceivable flicker. And 1000s of other have basically done the same. You shouldn't base the plausibility of an idea on how interesting it sounds. The real science here would be to explain the reality of there not being any flicker in light pollution, not to just keep suggesting that there should be a flicker, even though there isn't. Given that lights flicker, why then doesn't light pollution flicker. That is the question.

And I would like to see this urban myth documented somewhere. It sounds like something someone conjectured but obviously didn't pan out and was lost to time.