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  #1  
Old 06-21-2009, 08:05 PM
Mangetout Mangetout is offline
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The practicalities of manually focusing multiple mirrors on a single target

A question provoked by the now-locked mirror protest thread (I obtained permission to start this one, BTW)...

1000 people are recruited to try to set light to a target by reflecting sunlight onto it, using individual hand-held mirrors. The question is: even if they are all positioned on a special gantry that means they can all see the target and the sun, has any individual any hope of actually reflecting sunlight so as to hit the target?

It was easy enough for me to reflect a small spot of light from my wristwatch glass onto the eye of my geography teacher when I was at school - but that's because, as the only person doing it, I was able to aim not by observing the angle of my reflector, but by seeing the reflected spot of light and moving it onto the target.

But when there are 999 other people all doing the same, I've got no hope of seeing the reflected patch of light and moving it to the target, have I?

I think this means that, without some kind of aiming mechanism, an attempt at creating a distributed, human-powered virtual parabolic reflector is doomed to being at best, rather inefficient. Am I right?

Last edited by Mangetout; 06-21-2009 at 08:05 PM..
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  #2  
Old 06-21-2009, 08:36 PM
enalzi enalzi is online now
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You could do it if you had one person aim at a time. Basically have everyone else hold their mirrors down except for one. Have that one person aim at something, then a second a person move their mirror into position. They would know which spot was theirs because it would be the only one moving. Then repeat over and over with each person.
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Old 06-21-2009, 08:45 PM
Princhester Princhester is offline
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But enalzi keeping the spot in the right place is going to take constant adjustment by the holder of the mirror. No one can hand-hold a mirror so steady that they can keep a reflected blob of light from it in exactly the right place for any length of time without constant bio-feedback (ie adjusting aim based on seeing where their particular blob of light is).

Once you have a few hundred people aiming, you will have an amorphous blob of bright light on or around the target, and you will be back to the same problem ie you can't tell which part of the blob is yours, in order to adjust your aim.

Even if you had each mirror on a tripod, and assuming (unrealistically) that you can adjust a mirror in five seconds, the rate at which the sun moves would be higher than the rate at which you could adjust 1000 mirrors.

The only way you could do it would be with 1000 mirrors on stands, each with tracking servos that kept it focussed on the same spot while you set up the other mirrors.
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Old 06-21-2009, 09:20 PM
Sleel Sleel is offline
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1. With those kind of numbers, you don't need to be all that exact, and most people are actually pretty good at estimating angles for aiming.

2. An aiming mechanism can be as simple as a small hole in the middle of the mirror to sight through. See any picture of a signaling mirror for examples.

3. A real-life test to see if Archimedes' "burning glass" legend was possible or practical showed that while it's not exactly easy to get people who haven't practiced to aim at the same spot, it is possible.
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Old 06-21-2009, 10:07 PM
Machine Elf Machine Elf is offline
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If your mirrors are at any significant distance from the target, you'll need to spend some cabbage in order to buy good quality, seriously flat mirrors that are also thick enough to resist bending/distortion when held at various angles relative to gravity. If you just use a bunch of 12"x12" mirrors made of 3/16" glass from the dollar store, you won't be able to project anything resembling a square of light onto the distant target.
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Old 06-22-2009, 03:21 PM
Mangetout Mangetout is offline
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Quote:
Originally Posted by Sleel View Post
1. With those kind of numbers, you don't need to be all that exact, and most people are actually pretty good at estimating angles for aiming.

2. An aiming mechanism can be as simple as a small hole in the middle of the mirror to sight through. See any picture of a signaling mirror for examples.
I always wondered how you're meant to use that - do you sight on a point apparently midway between the sun and your target? Obviously the goal can't be to place your mirror perpendicular to the direction of the target, so how does sighting through a hole help?

Quote:
3. A real-life test to see if Archimedes' "burning glass" legend was possible or practical showed that while it's not exactly easy to get people who haven't practiced to aim at the same spot, it is possible.
That's not really a comparable experiment - those mirrors are quite carefully arranged, preconfigured for adjustment and are not handheld. It does demonstrate that a virtual parabolic mirror is possible, but that's not really a surprise.
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Old 06-22-2009, 03:41 PM
Chronos Chronos is online now
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I always wondered how you're meant to use that - do you sight on a point apparently midway between the sun and your target? Obviously the goal can't be to place your mirror perpendicular to the direction of the target, so how does sighting through a hole help?
CalMeacham explained this once, so if I get it wrong, I hope he'll correct me. The back of the mirror needs to be at least slightly reflective, too. First, you hold your mirror in front of you so that you can see your target directly through the hole. Then, the mirror will be casting a shadow on the ground in front of you, with a little bright spot in the middle of the shadow where the Sun is shining through the hole. Now, you look at the ground reflected in the back side of your mirror, and adjust your mirror (while keeping the target directly visible) until the reflection of the bright spot in the shadow lines up with the hole from your point of view.
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  #8  
Old 06-22-2009, 04:09 PM
billfish678 billfish678 is offline
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As a very rough rule. Say you want the intensity of the sun to be 1000 times that of normal.

And you have a 1000 mirrors hand held mirrors.

If each mirror is say 4 inches by 4 inches, that means the light reflected from each mirror needs not move any more than that (and preferably a fraction of that ) AT the target. If you are talking any real distance to the target and you have hundreds of other reflections keeping you from telling exactly what your particular reflection is doing.....well, lets just say I find it highly improbable that you could pull it off. Optical Ninja's, maybe. A bunch of rednecks with mirrors? NO.
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  #9  
Old 06-22-2009, 04:41 PM
Projammer Projammer is online now
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With a standard mirror, not likely.

Give a thousand people a scaled up signal mirror and your target will be feeling the heat.

Trained with one in boy scouts mumble years ago and spotlighting someone on the other side of the field was as easy as looking at them.
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  #10  
Old 06-22-2009, 04:54 PM
Projammer Projammer is online now
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Wow. Totally missed sleel's post about signal mirrors, but my link does explain the process. No, you don't try to guestimate a point midway. You position a virtual spot directly on the target.

Your spot is dependent only upon your own mirror, so all the other points created by everyone else's mirror won't affect your aiming.
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  #11  
Old 06-22-2009, 05:03 PM
billfish678 billfish678 is offline
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Originally Posted by Projammer View Post
.

Your spot is dependent only upon your own mirror, so all the other points created by everyone else's mirror won't affect your aiming.
Yes and no.

If you just have your mirror, you can look at your target and SEE whether you are on it or not.

The other method doesnt have that positive feedback. It depends more on you just "doing it right".
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Old 06-22-2009, 05:04 PM
Exapno Mapcase Exapno Mapcase is offline
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Don't forget that the sun itself is moving through the sky, so if you're taking any significant amount of time, the first group will need to be able to change their aim to compensate for the sun's change.
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  #13  
Old 06-22-2009, 05:20 PM
billfish678 billfish678 is offline
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A few calcs.

IIRC a lens with a 100 inch or so focal length forms an image of the sun that is 1 inch across.

A lens with an F ratio of 4 can barely set fire to an optimum target (something like black painted very dry wood). Note I've actually played around with this because of an internet discussion years ago.

A 100 inch focal length lens or mirror with an F 4 ratio would have to have a diameter of 25 inches. That means the intensity of the sun is intensified by 25 squared or 550 times.

You really need significantly more than that for bursting into flames of anything other than toilet paper , so the 1000 mirrors are just gonna barely do, even with a good target and darn good aiming.

Somebody need to get a mirror, reflect an image of the sun onto something XYZ feet apart and see just how "stable" they can keep that image on something XYZ feet away.
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Old 06-22-2009, 05:58 PM
Mangetout Mangetout is offline
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Quote:
Originally Posted by billfish678 View Post
...A lens with an F ratio of 4 can barely set fire to an optimum target (something like black painted very dry wood). Note I've actually played around with this because of an internet discussion years ago.

A 100 inch focal length lens or mirror with an F 4 ratio would have to have a diameter of 25 inches. That means the intensity of the sun is intensified by 25 squared or 550 times....
I'm confused now. Aren't you talking about lenses with artificial apertures or something here? A lens with a diameter of a few feet can be used to melt concrete.

Last edited by Mangetout; 06-22-2009 at 06:02 PM..
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Old 06-22-2009, 06:51 PM
billfish678 billfish678 is offline
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Originally Posted by Mangetout View Post
I'm confused now. Aren't you talking about lenses with artificial apertures or something here? A lens with a diameter of a few feet can be used to melt concrete.
The peak temp you can achieve is basically ONLY a function of the F ratio. And its non linear. An F2 is four times as "hot" as an F4 and an F1 is 16 times as "hot" as an F4.


You make the mirror or lens ten times bigger (keeping the same F ratio) and the image becomes tens times bigger as well, equalling things out.

If you get the mirror or lens REALLY small then it breaks down due to other considerations, and then it becomes a "could Smurfs actually build those small fires ?" discussion.

Now, with your 1000 mirrors scenario, there is no focusing per se. But for reasonable distances, your 4 inch square FLAT mirrors will reflect (approximately) a 4 inch square patch of light.

Now, if you use a small mirror and a LONG distance, you get the mirror version of pinhole camera (which I have also played with) and can get a decent image of the sun. That really isnt relevant for this discussion, but they are fun to play with and and I can give you a few pointers, as it is fun to play with.
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Old 06-22-2009, 07:56 PM
lurcio lurcio is offline
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Maybe with a large stadium...
http://variety-sf.blogspot.com/2007/...sunstroke.html
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  #17  
Old 06-22-2009, 08:19 PM
sailor sailor is offline
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Originally Posted by Sleel View Post
See any picture of a signaling mirror for examples.
The heliograph.
http://www.smecc.org/heliograph_-_si...by_the_sun.htm
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Old 06-22-2009, 08:55 PM
Sleel Sleel is offline
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Originally Posted by Mangetout View Post
That's not really a comparable experiment - those mirrors are quite carefully arranged, preconfigured for adjustment and are not handheld. It does demonstrate that a virtual parabolic mirror is possible, but that's not really a surprise.
Their first test was handheld, with an ad-hoc untrained group of about 100 people. They got some effects, though not quite so dramatic. Their second test was the one you're talking about.
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  #19  
Old 06-23-2009, 01:19 AM
spinky spinky is offline
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Not speaking to the issue of handheld aiming, but this dude did the experiment of putting hundreds of little mirrors on a giant old-school satellite dish in order to focus the sun and burn things.

The first 15 or so pages are documentation of the trials and tribulations of building the thing, and page 15 or so is where he actually starts to burn stuff with it.

Last edited by spinky; 06-23-2009 at 01:19 AM..
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  #20  
Old 06-23-2009, 02:48 AM
Richard Pearse Richard Pearse is offline
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Originally Posted by Chronos View Post
CalMeacham explained this once, so if I get it wrong, I hope he'll correct me. The back of the mirror needs to be at least slightly reflective, too. First, you hold your mirror in front of you so that you can see your target directly through the hole. Then, the mirror will be casting a shadow on the ground in front of you, with a little bright spot in the middle of the shadow where the Sun is shining through the hole. Now, you look at the ground reflected in the back side of your mirror, and adjust your mirror (while keeping the target directly visible) until the reflection of the bright spot in the shadow lines up with the hole from your point of view.
An easier way is to hold your thumb out at arms length and looking through the hole in the mirror, place your thumb over the target. Then tilt the mirror so it shines on your thumb, it will then also be shining on the target.
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Old 06-23-2009, 11:56 AM
Rigamarole Rigamarole is offline
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Mythbusters did this very experiment. When they were doing a test with handheld mirrors, they had trouble even getting a dozen or so people to aim them all on one spot.

Later they built a huge rig with tons of mirrors aimed together (not handheld) to see if they could get a boat to light on fire. It didn't work.
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Old 06-23-2009, 01:35 PM
Sapo Sapo is offline
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Originally Posted by 1920s Style "Death Ray" View Post
An easier way is to hold your thumb out at arms length and looking through the hole in the mirror, place your thumb over the target. Then tilt the mirror so it shines on your thumb, it will then also be shining on the target.
This. A stick would also work and make it easier. Just rest your mirror on the ground place the stick so the tip of it is on the line from the center of the mirror to the target and you are in. As long as you see light on the tip of that stick, you know you are hitting your target. I was really pissed at the Mythbusters' half assed attempt at this one.
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Old 06-23-2009, 03:49 PM
billfish678 billfish678 is offline
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Originally Posted by Sapo View Post
This. A stick would also work and make it easier. Just rest your mirror on the ground place the stick so the tip of it is on the line from the center of the mirror to the target and you are in. As long as you see light on the tip of that stick, you know you are hitting your target. I was really pissed at the Mythbusters' half assed attempt at this one.
But with WHAT accuracy?

If the stick is at 10 feet and your target is at 500 ft, then a half inch error at the stick becomes a 25 inch error at the target. Unless you have a BIG and FLAT mirror, that kind of error is going to seriously cut in the the "concentration" you are trying so hard to achieve.
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Old 06-23-2009, 04:10 PM
Jerseyman Jerseyman is offline
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It has been done. According to legend, a 'philosopher' whose name I forget burnt Roman ships besieging Syracuse with parabolic mirrors. A few years ago the Greek navy decided that since the Romans wrote the account they didn't know what he'd used and had probably lined men up on the city walls with polished shields. So they got some sailors together to see if it was possible with metal mirrors to represent the old shield and trained the sun on a model boat. It caught fire.
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Old 06-23-2009, 05:00 PM
Chronos Chronos is online now
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Quote:
According to legend, a 'philosopher' whose name I forget burnt Roman ships besieging Syracuse with parabolic mirrors.
Archimedes. He's been mentioned in pretty much every link posted in this thread. The burning glass is somewhat disputed, but it's universally agreed that he did make a variety of very effective weapons for the defense of Syracuse.
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Old 06-23-2009, 07:02 PM
Rigamarole Rigamarole is offline
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Originally Posted by Jerseyman View Post
It has been done. According to legend, a 'philosopher' whose name I forget burnt Roman ships besieging Syracuse with parabolic mirrors. A few years ago the Greek navy decided that since the Romans wrote the account they didn't know what he'd used and had probably lined men up on the city walls with polished shields. So they got some sailors together to see if it was possible with metal mirrors to represent the old shield and trained the sun on a model boat. It caught fire.
Yeah, this is exactly the myth they were testing on Mythbusters, which I mentioned a couple posts ago. They decided it was impossible.
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Old 06-23-2009, 09:26 PM
Princhester Princhester is offline
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Originally Posted by Sapo View Post
This. A stick would also work and make it easier. Just rest your mirror on the ground place the stick so the tip of it is on the line from the center of the mirror to the target and you are in. As long as you see light on the tip of that stick, you know you are hitting your target. I was really pissed at the Mythbusters' half assed attempt at this one.
Why? What did they do wrong?

It seems to me that you can pretty much divide the attempts at re-creating the Archimedes legend into two camps: artificial attempts using some or all of modern materials, unrealistic distances, unrealistically still targets. These attempts sometimes work. Then there are realistic attempts which all fail due to various impracticalities.
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Old 06-23-2009, 09:42 PM
CalMeacham CalMeacham is online now
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Sorry -- I've been away all day, and haven't been able to answer.

1.) There are several methods of aligning a mirror so that you can reflect sunlight onto a distant target. This is, as noted above, the basis of the heliograph. During the second world war, the Air Services and the Navy were concerned that people on life rafts would want to be able to signal for help with mirrors. GE included such mirrors in their emergency kits. Two articles by R.S. Hunter in the Journal of the Optical Society of America (Vol. 35, pp. 805+ (1945) and Vol. 36, #2, pp. 110-115 (1946)) told how to use them (After the war was over, unfortunately).

2.)There were several methods described, but the two simplest were those given by 1920s Death Ray and by Chronos. Death Ray's (appropriate!) is the more obvious, but the method Chronos gives is more elegant, and doesn't require you to stick out something blocking part of the beam. This method is also described by Albert Claus in Applied Optics Vol. 12, #10, p. A14 (1973) and in Hal Clement's science fiction novel Cycle of Fire (1957). Arthur C. Clarke describes something similar in his short story "A Slight Case of Sunstroke" from 1958, which is in his collection Tales of Ten Worlds.

3.) I know that Mythbusters tried this and failed, but they really did a bad job of it. They didn't use people individually adjusting mirrors, but made a big mirror out of small sections (which is actually what Archimedes was said to have done. The idea of using individually controlled mirrors was suggested later by another Greek, Proclus). But his method of adjusting the focus was pretty crude -- they never checked them optically, as is evident from the huge size of the "focus". Ideally, this ought not to have been larger than the reflected spot from a mirror, but it was much larger. (I'm told there was a followup, but have never seen it).

4.) The experimenthasp been carried out successfully, despite the Mythbusters' failure. In 1973 Ionannis Sakkas of Greece got a team of 60 soldiers to direct sunlight using mirrors onto a ship 160 feet away (in the water) and set it on fire. The result was published (with pictures) in Time magazine, not to menion the London Times and in New Scientist. in 2002 a German team used 500 volunteers to ignite a sail 50 meters away. The Comte de Buffon had performed similar experiments back in tyhe 18th century. See Mlahanas' website at http://www.mlahanas.de/Greeks/Mirrors.htm

5.) Archimedes has been adopted as a sort of Patron Saint . of Adaptive Optics (even though, as I say, it was Proclus who used multiple movable mirrors). I've seen him cited by at least three works on Adaptive Optics (the engineering technique that uses "rubber mirrors" ) as the father of their discipline.

6.) I don't really think that either Archimedes or Proclus was able to actually burn ships. I do think it was within their technical capabilities, but I doubt that they ever built the battery of mirrors needed or rehearsed the soldiers in the techniques. There are easier ways to sink a boat.
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  #29  
Old 06-24-2009, 05:43 PM
billfish678 billfish678 is offline
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The 500 guys sounds about right, but again I imagine the situation was more optimum than realisitic. Particularly is you are talking about what the ancients could accomplish. No, not THOSE ancients.

The 60 guys? Some major cheating going on if I had to bet.

Then look at the distances. A measely 150 feet or so. A handful of guys with flaming arrows could do that. Longer and useful distances would be much harder alignment and aiming wise.
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Old 06-24-2009, 06:21 PM
CalMeacham CalMeacham is online now
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Then look at the distances. A measely 150 feet or so. A handful of guys with flaming arrows could do that. Longer and useful distances would be much harder alignment and aiming wise.

See my point #6 above. As I say, I dpubt if anyone ever actually did this in the ancient world, or if it would be worthwhile. But it's certainly possible.
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Old 06-24-2009, 06:55 PM
billfish678 billfish678 is offline
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See my point #6 above. As I say, I dpubt if anyone ever actually did this in the ancient world, or if it would be worthwhile. But it's certainly possible.
Oh, I generally agree with you agreeing with me agreeing with you...

The 60 guys one. Do you have any more info on that one? I suspect their target was either strike anywhere matches or they had large high precision long distance focusing mirrors.
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Old 06-24-2009, 07:36 PM
CalMeacham CalMeacham is online now
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The 60 guys one. Do you have any more info on that one? I suspect their target was either strike anywhere matches or they had large high precision long distance focusing mirrors.
I've seen the pictures -- it looked like a respectable test, and it was on a boat made to look like an ancient Greek craft -- not matches. I haven't run he numbers, but I can believe that under a hot, clear Mediterranean sky, such as they had, they could concentrate quite a bit of light on a ship.
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Old 06-24-2009, 08:10 PM
billfish678 billfish678 is offline
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I've seen the pictures -- it looked like a respectable test, and it was on a boat made to look like an ancient Greek craft -- not matches. I haven't run he numbers, but I can believe that under a hot, clear Mediterranean sky, such as they had, they could concentrate quite a bit of light on a ship.
okay, but southern sunny florida aint now slouch either, and for normal materials I need something closer to a 500 times "intensification" than only 60 to start fires.

Of course we been having a cloud free heat wave for nearly a month here, and just NOW its gotten all cloudy on me

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Old 06-25-2009, 12:49 PM
Sapo Sapo is offline
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Originally Posted by billfish678 View Post
But with WHAT accuracy?

If the stick is at 10 feet and your target is at 500 ft, then a half inch error at the stick becomes a 25 inch error at the target. Unless you have a BIG and FLAT mirror, that kind of error is going to seriously cut in the the "concentration" you are trying so hard to achieve.
Very true. Although I guess that's why 500 are better than 60. Find enough men and soon enough precision doesn't matter much. Then again, you have to make 500 big flat mirrors and not just 60. Good luck finding a good mirror store in Ancient Greece.

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Why? What did they do wrong?

It seems to me that you can pretty much divide the attempts at re-creating the Archimedes legend into two camps: artificial attempts using some or all of modern materials, unrealistic distances, unrealistically still targets. These attempts sometimes work. Then there are realistic attempts which all fail due to various impracticalities.
Although I am not on the pro-Archimedes camp, I found the MB effort to be specially half-hearted. 9 people looking desperately like they would rather be anywhere else are hardly equivalent to disciplined and trained soldiers looking to defend their wives and children from an invading army. They set out to prove it could not be done and, surprise, they succeeded. A failure only counts when you set out to prove you can do something.
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Old 06-25-2009, 06:08 PM
Chronos Chronos is online now
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Keep in mind, too, Archimedes' goal wasn't to defeat a ship, it was to defeat an entire navy. Even if you could have taken out the ship with arrows, that just gets you one ship. Soldiers are used to arrows; they know what to expect. But if you take out one ship with a hillside of soldiers wielding the fire of the Sun itself, the next shipful isn't going to want to stick around to find out just what the limits of your weapon are. From what I've heard, the Roman soldiers' fear of Archimedes' inventions was a major contributor to how Syracuse was able to hold them off for as long as they did.
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Old 06-25-2009, 08:10 PM
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Keep in mind, too, Archimedes' goal wasn't to defeat a ship, it was to defeat an entire navy. Even if you could have taken out the ship with arrows, that just gets you one ship. Soldiers are used to arrows; they know what to expect. But if you take out one ship with a hillside of soldiers wielding the fire of the Sun itself, the next shipful isn't going to want to stick around to find out just what the limits of your weapon are. From what I've heard, the Roman soldiers' fear of Archimedes' inventions was a major contributor to how Syracuse was able to hold them off for as long as they did.
Very true. Just starting any fire on the sails, supplies, etc or burning a couple of soldiers would be enough to sow all kinds of doubts on a superstititous bunch.
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Old 06-26-2009, 03:52 AM
Princhester Princhester is offline
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Originally Posted by Sapo View Post
Although I am not on the pro-Archimedes camp, I found the MB effort to be specially half-hearted. 9 people looking desperately like they would rather be anywhere else are hardly equivalent to disciplined and trained soldiers looking to defend their wives and children from an invading army. They set out to prove it could not be done and, surprise, they succeeded. A failure only counts when you set out to prove you can do something.
So apart from some vague waffle about motivation, you've got nothing.
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Old 06-26-2009, 08:38 AM
Sapo Sapo is offline
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So apart from some vague waffle about motivation, you've got nothing.
You may have missed the part about numbers, training and discipline. 9 pencil pushers on a first try are no match to 60-500 trained soldiers.
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Old 06-26-2009, 11:26 AM
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I need something closer to a 500 times "intensification" than only 60 to start fires.
I've been thinking about this so indulge me if you will.

The sun subtends about a one-half degree angle - about the size of a dime held at arm's length. A flat mirror projects an image of the sun of the same angle so, regardless of the distance to the target, you would need to aim the mirror within one-quarter of a degree to hit a point.

The number of mirrors you need depends on the distance and the size of the mirrors. At 500m the sun image will be 4.4m in diameter with an area of 15 square meters. If each soldier's mirror is 1 square meter then 15 soldiers will deliver one sun-power. A sun magnification of 500 times would require 7500 soldiers.

If the soldiers aim perfectly then the 500 sun intensity would be felt over the whole 4.4m diameter circle. In reality there would be a bell curve with the center point receiving the full intensity with diminishing intensity further away.
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Old 06-26-2009, 09:40 PM
Princhester Princhester is offline
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You may have missed the part about numbers, training and discipline. 9 pencil pushers on a first try are no match to 60-500 trained soldiers.
Numbers means something, perhaps, but just insulting people and waffling about training and discipline is meaningless unless you say what this training and discipline is actually going to result in, in terms of actual activity.
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Old 06-27-2009, 12:58 AM
ENugent ENugent is offline
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The Kansas City Space Pirates used basically the Archimedes Death Ray setup to power a robot climber in the 2006 Space Elevator Games. You can see the aiming difficulties if you look at the size of the spot on the climber.
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Old 06-27-2009, 09:22 AM
Sapo Sapo is offline
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Numbers means something, perhaps, but just insulting people and waffling about training and discipline is meaningless unless you say what this training and discipline is actually going to result in, in terms of actual activity.
I think that the advantages of having someone used to physical activity and following orders doing something for the nth time over an office dweller doing something completely out of his usual range of activities for the first time and wanting to prove it cannot be done are self-evident.
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but I digress
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Old 06-27-2009, 09:44 AM
Squink Squink is offline
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The problem with maintaining the mirror's position once it's aimed correctly might be minimized by standing to the side of the mirror, so that you can simultaneously see the position of the sun's reflection and a stationary background object. People would still have to adjust the position of their mirrors a few at a time, but once everyone has figured out which background object shows in their mirror when it's in the correct position, they'll be able to re-focus the sun on the target spot quickly.
Of course the sun moves a degree through the sky every four minutes, so everyone'll have to be quick about it.

Last edited by Squink; 06-27-2009 at 09:45 AM..
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Old 06-27-2009, 07:55 PM
CalMeacham CalMeacham is online now
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The problem with maintaining the mirror's position once it's aimed correctly might be minimized by standing to the side of the mirror, so that you can simultaneously see the position of the sun's reflection and a stationary background object.
See my note above about aiming, which Chronos has described earlier. There are indeed ways to aim the mirror that are better than just looking at where the spot hits the target -- especially if 500 other guys are doing exactly the same thing and it's hard to tell which of the spots that's moving around is yours. The solution in which you have a two -sided mirror with a hole in it and adjust the reflection of the sun spot on your cheek so that it appears to go out through the hole is the most elegant solution.
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Old 06-28-2009, 10:24 PM
Princhester Princhester is offline
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I think that the advantages of having someone used to physical activity and following orders doing something for the nth time over an office dweller doing something completely out of his usual range of activities for the first time and wanting to prove it cannot be done are self-evident.
I'm probably just stupid because it's not self evident to me. I don't recall there being any problem whatsoever with the Mythbuster's team getting tired, or being unco-ordinated or undisciplined. Furthermore, someone who refers to the Mythbusters as "office dwellers" begins to sound very much like someone who hasn't ever really watched the show.

For "self evident" I'm going to have to assume you mean "I've got nothing in particular I can actually spell out, so I'm going to waffle about my assumptions concerning motivation, and some strawmen and irrelevancies, then call the problems self evident because I can't actually think of anything evident to spell out".
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Old 06-29-2009, 12:31 PM
Sapo Sapo is offline
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I'm probably just stupid because it's not self evident to me.
I was assuming unwillingness to see, but I will take your answer for truth and leave it at that.
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Old 06-29-2009, 12:37 PM
billfish678 billfish678 is offline
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See my note above about aiming, which Chronos has described earlier. There are indeed ways to aim the mirror that are better than just looking at where the spot hits the target -- especially if 500 other guys are doing exactly the same thing and it's hard to tell which of the spots that's moving around is yours. The solution in which you have a two -sided mirror with a hole in it and adjust the reflection of the sun spot on your cheek so that it appears to go out through the hole is the most elegant solution.

There is still the issue (even assuming an optically perfect and elegant aim technique) of just HOW steady you can hold the mirror you are aiming.

Even slight wobbles becomes big physical distance wobbles at decent distances IMO.
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Old 06-29-2009, 04:14 PM
EdwardLost EdwardLost is offline
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Even slight wobbles becomes big physical distance wobbles at decent distances IMO.
As I mentioned in a post above, the target's distance does not make a difference because the sun's image becomes larger with distance. Your angular wiggle room is one quarter of a degree, regardless of the target's position. (That is, unless you are talking about a target close w.r.t the size of the mirror - say a few feet away.)
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Old 06-29-2009, 07:25 PM
Chronos Chronos is online now
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On the other hand, at a greater distance, you will need more mirrors. Assuming everything's aimed perfectly, an observer at the target point would observe the entire hillside to be as bright as the Sun, and how hot the target point gets will depend on how much of the field of view is taken up by something sun-bright. Double the distance, and you'll need four times as much area to take up the same amount of field of view, and so on.
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Old 06-29-2009, 10:17 PM
CalMeacham CalMeacham is online now
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Here are some accounts of an MIT team that tried the experiment of using mirrors aimed at a boat on the Mythbusters set. They were able to set fire to the boat I missed the episode. Apparently they used 127 mirrors and 100 feet, and were successful

Quote:
In October 2005 a group of students from the Massachusetts Institute of Technology carried out an experiment with 127 one-foot (30 cm) square mirror tiles, focused on a mock-up wooden ship at a range of around 100 feet (30 m). Flames broke out on a patch of the ship, but only after the sky had been cloudless and the ship had remained stationary for around ten minutes. It was concluded that the device was a feasible weapon under these conditions. The MIT group repeated the experiment for the television show MythBusters, using a wooden fishing boat in San Francisco as the target. Again some charring occurred, along with a small amount of flame. In order to catch fire, wood needs to reach its flash point, which is around 300 degrees Celsius (570 °F).[28]

When MythBusters broadcast the result of the San Francisco experiment in January 2006, the claim was placed in the category of "busted" (or failed) because of the length of time and the ideal weather conditions required for combustion to occur. It was also pointed out that since Syracuse faces the sea towards the east, the Roman fleet would have had to attack during the morning for optimal gathering of light by the mirrors. MythBusters also pointed out that conventional weaponry
http://en.wikipedia.org/wiki/Archimedes_of_Syracuse


http://web.mit.edu/2.009/www/experim...thbusters.html

http://www.boingboing.net/2005/10/06..._death_ra.html

http://www.boingboing.net/2005/10/08...s_archime.html



I note that their mirrors were fixed, no person-controlled.

As for how stable a person's holding the mirror can be -- i don't know, but it ought to be easy enough to check. I'll do so as soon as I get a chance. But I note that it's supposed to rain all week.
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