Author Topic: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.  (Read 184 times)

soecolerfe

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Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« on: December 24, 2017, 05:39:41 AM »
Years ago (40-50) the topic circulated about astrophotography performed at a mountain-top observatory (at California?  Mt. Wilson?)  Where the thought was to minimize the impact of this then-existing light pollution from towns, which was beginning to pollute the site badly.

A method was developed by which a rectangular round vane using a slot cut into it ("shutter") would pass light in the telescope to the camera ONLY at a short space of time close to the zero-crossings of this line-current.  The shutter rotated in synch using the AC line current, and has been open TWICE each cycle.

The success of this process is dependent on the line-current of a distributed (large-extent) town ALL (or largely) being in-phase (a pretty good bet).  To put it differently, the town is flashing at 120 Hz (with two zero-crossings and two peaks in each cycle).

Now, in the time when photographic-emulsion on photographic plates was king, the "Intermittency Effect" played havoc with the success of this method.  It worked, but a latent picture reached a point of diminishing returns pretty quickly, at the emulsions used.

With a solid-state detector, I am unaware that the Intermittency Effect has any significance at all... .

So, I ask, if anyone is using this, from bright websites.

Gas-discharge road lamps utilized popularly are almost totally "OFF", under a certain voltage in each cycle (still some slight shine, from heating).  A town skies should be quite <strong>dark </strong>during these times when the sine wave is close to the zero-line.

A simple hand-rotated strobe-disk may be utilized to explore this -- which is, if a town (yours... ) is flashing all in-phase (or not).  Or, a slotted disk spun by an AC synchronous motor, state, at 3600 RPM, could be employed.

CCD integrations may be composed of many short "gulps" of starlight when the streetlights are only "OFF", and the town's light-dome is reasonably dark.

(Even incandescent bulbs experience a substantial dimming during dips in the line present around zero in each cycle of the sine wave, BTW).

Is anyone here harnessing this effect?

First is to check the reality of it.   I.e., <strong>do most of the town's egregious lights blink in unison?  </strong>

best,

--Joe



coepupinsynch

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #1 on: December 28, 2017, 04:28:44 AM »
I haven't seen such an effect in any of the video I have taken. Not just through a telescope (lucky imaging), but ever, with any video, or photograph. If this was realistic, then we would notice strobing in video or fast exposures.

Tim Massey

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #2 on: December 28, 2017, 11:38:45 AM »
Thanks.

But, consider: line current has to be in-phase, everywhere, if the power distrib.-system is to work. Maybe even across state boundaries (imagine the consequences if it is not  ).

And, it worked at Mt. Wilson, I think (1950s? 60s?). A note appeared in S&amp;T some time after. Should be searchable if you have S&amp;T indices on-disk (I misplaced mine, between computers, had it on 3.5 inch floppy).

--Joe

Tarence Allen

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #3 on: December 28, 2017, 08:55:57 PM »
sys.,

Quote

If this was realistic, then we would notice strobing in video or fast exposures.


Take a DSLR, set for 1 sec. exp. some night, while you scan the f.o.v. over a set of street lights nearby or distant (do a pirouette). You'll see interrupted trails in the image, no?

There you go.

Just a proof-of-concept. Of course it still leaves plenty of good work to be done.

--Joe

Marlin Riewer

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #4 on: December 29, 2017, 02:25:06 AM »
A single light maybe, but there is no overall effect in any images or videos I have seen, so therefore the combined effect of all of the lighting must average out, correct? If this idea (that all of the lighting strobes in unison) were true, it would be apparent in so many everyday phenomena.

calbeyrefrows

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #5 on: December 29, 2017, 05:40:23 AM »
sys,

Quote

A single light maybe, but there is no overall effect in any images or videos I have seen, so therefore the combined effect of all of the lighting must average out, correct? If this idea (that all of the lighting strobes in unison) were true, it would be apparent in so many everyday phenomena.


I certainly believe you may not have noticed it, in work not directed toward detecting or exploiting this fact.

Again, just consider the effects if it were not true that line-current over huge geographical regions were not in phase.

We can take this as a very handy technological "given", and just go ahead and exploit it. But it's also fun to test it six ways from Sunday, first, if a person likes. I suppose it's possible that details ("ballasts"?) of some lighting-fixture circuits might pervert the phase (lag or advance it) of the line current, but maybe not. And if, say, 80 percent of them do not, then that's 80 percent darker for us, during part of a cycle.

Now, there are mechanical shutters which can be phase-locked to the line-current, but I think there are also optical means. The "Pockels Cell" may be one, if volatile memory allows me to recall correctly.  A simple mechanical means is a safe way to start, though, and for me, that prob. means a fast synchronous AC motor (say, 3600 or 7200 RPM), with a disk attached to the shaft.

One can determine an effective and not "leaky" angular width of a cutout (sector) in a rotating disk which would pass light from the optics to the array-detector only when most of the lighting-types are effectively "OFF". For the various gas streetlight types in use (Na, Hg, high-pressure, low-pressure, etc.) I don't know what those excitation voltages are, and hence the corresponding fraction of a cycle. Better to determine this observationally in one's own town. Maybe ten percent of a cycle will be dark? A six minute exposure in a dark place might then take one hour in the city.

best,

--Joe

wallnewsspheryz

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #6 on: December 30, 2017, 03:56:12 AM »
I agree with syscore, if there were such phased effect we would have known it long ago.
More specifically you are referring to a theory started over 70 years ago.  Back then most lights were incandescents I believe.  I don't think incandescent bulb emit light in phases.  It generates light by heating metal wires to high temperature.  Heat does not just flick on and off rapidly, so there should be no flickering light either.
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.
Probably why we never heard that theory again in over half a century.

tradunjuwa

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #7 on: December 31, 2017, 03:46:27 PM »
This is kooky. All my imaging stuff is either convertedDC or battery, no phase, or polyphase, anywhere, DC... sure it's converted AC and digitally switched DC, but that is then sent to rectifiers and orvoltage regulating diode like thinga majingy stuff...

Rick Nocturnum

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #8 on: December 31, 2017, 04:17:28 PM »
Quote
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.


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

guizietropid

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #9 on: January 01, 2018, 05:58:52 AM »
C.,

Quote

This is kooky. All my imaging stuff is either convertedDC or battery, no phase, or polyphase, anywhere, DC... sure it's converted AC and digitally switched DC, but that is then sent to rectifiers and orvoltage regulating diode like thinga majingy stuff...


Yes, indeed, that's your equipment (and mine).

But streetlamps are excited directly by AC line current, not rectified current. And the streetlamps go OFF for a part of each cycle.

To exploit the availability of darkness so kindly created during this brief time each cycle, astrophotographers can devise a novel synchronous shutter that is open in synchronism with this part of the cycle, the claim being that your house-current AC is in phase with the street-light AC power. Amateurs may test this fact, if interested.

best!,

--Joe

Richard Reed

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #10 on: January 01, 2018, 07:28:26 AM »
LED street lights would not flicker (they run on DC), so this would have no effect against them.

chlorleifilwhirl

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #11 on: January 08, 2018, 01:43:04 AM »
p.,

Quote

LED street lights would not flicker (they run on DC), so this would have no effect against them.


That's the most simplistic view.

Is the duty-cycle 100 percent? Have you been able yet to test this?

A full-wave rectifier would give pretty smooth (nearly ripple-free) DC, but I don't know yet just how LED streetlights are implemented. It's worth a check if you've got some exemplars in your nabe. We don't have them here (yet... ).

best,

--Joe

Alejandro Taylor

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #12 on: January 08, 2018, 08:58:12 AM »
I can't find any reference that indicates skyglow caused by artificial lighting is flickering. 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.

fibcapoti

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #13 on: January 10, 2018, 03:11:47 AM »
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.

--Joe

p.s. BTW, none of my interest and advocacy of this is to denigrate, dismiss, nor discourage the good work of the International Dark Sky Committee (based here in my town). Our birthright is dark night skies, and I'd sorely (and surely) like them back.

Javier Gutierrez

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Re: Bright-Site Astrophotography -- Line-Current "Shutter" Synch.
« Reply #14 on: January 11, 2018, 03:56:45 PM »
"If the lights flicker in-phase, so does the sky-glow from those lights. It can be worth one's testing, I'd say. ;)"

I've already tested it. There is no discernible flicker to light pollution from frame rates of 0 to 200 hz.

And why do you say it is impossible for the phase information to be loss? It was there with the individual lights, but when we test the skyglow, it is gone. Isn't that enough to say that it was "loss"? Light pollution is incoherent. Is that better?