# The practicalities of manually focusing multiple mirrors on a single target **URL:** \ **Category:** Factual Questions\ **Created:** [June 22, 2009, 1:05am UTC](https://boards.straightdope.com/t/the-practicalities-of-manually-focusing-multiple-mirrors-on-a-single-target/500535 "2009-06-22T01:05:17Z")\ **Posts on this page:** 19\ **Page:** 3
**Author:** ![ENugent](https://avatars.discourse-cdn.com/v4/letter/e/4af34b/32.png) [@ENugent](https://boards.straightdope.com/u/ENugent)\ **Post date:** [June 27, 2009, 5:58am UTC](https://boards.straightdope.com/t/the-practicalities-of-manually-focusing-multiple-mirrors-on-a-single-target/500535/41 "2009-06-27T05:58:39Z")
The [Kansas City Space Pirates](http://kcspacepirates.com/content/blogcategory/19/33/) used basically the Archimedes Death Ray setup to [power a robot climber](http://kcspacepirates.com/content/view/58/39/) in the 2006 Space Elevator Games. You can see the aiming difficulties if you look at the size of the spot on the climber. ---
**Author:** ![Sapo](https://avatars.discourse-cdn.com/v4/letter/s/ec9cab/32.png) [@Sapo](https://boards.straightdope.com/u/Sapo)\ **Post date:** [June 27, 2009, 2:22pm UTC](https://boards.straightdope.com/t/the-practicalities-of-manually-focusing-multiple-mirrors-on-a-single-target/500535/42 "2009-06-27T14:22:39Z")
> [@Princhester](#): > > 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. ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [June 27, 2009, 2:44pm UTC](https://boards.straightdope.com/t/the-practicalities-of-manually-focusing-multiple-mirrors-on-a-single-target/500535/43 "2009-06-27T14:44:08Z")
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. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [June 28, 2009, 12:55am UTC](https://boards.straightdope.com/t/the-practicalities-of-manually-focusing-multiple-mirrors-on-a-single-target/500535/44 "2009-06-28T00:55:04Z")
> [@](#): > > 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. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [June 29, 2009, 3:24am UTC](https://boards.straightdope.com/t/the-practicalities-of-manually-focusing-multiple-mirrors-on-a-single-target/500535/45 "2009-06-29T03:24:48Z")
> [@Sapo](#): > > 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”. ---
**Author:** ![Sapo](https://avatars.discourse-cdn.com/v4/letter/s/ec9cab/32.png) [@Sapo](https://boards.straightdope.com/u/Sapo)\ **Post date:** [June 29, 2009, 5:31pm UTC](https://boards.straightdope.com/t/the-practicalities-of-manually-focusing-multiple-mirrors-on-a-single-target/500535/46 "2009-06-29T17:31:07Z")
> [@Princhester](#): > > 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. ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [June 29, 2009, 5:37pm UTC](https://boards.straightdope.com/t/the-practicalities-of-manually-focusing-multiple-mirrors-on-a-single-target/500535/47 "2009-06-29T17:37:27Z")
> [@CalMeacham](#): > > 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. ---
**Author:** ![EdwardLost](https://avatars.discourse-cdn.com/v4/letter/e/bbe5ce/32.png) [@EdwardLost](https://boards.straightdope.com/u/EdwardLost)\ **Post date:** [June 29, 2009, 9:14pm UTC](https://boards.straightdope.com/t/the-practicalities-of-manually-focusing-multiple-mirrors-on-a-single-target/500535/48 "2009-06-29T21:14:27Z")
> [@billfish678](#): > > 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.) ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 30, 2009, 12:25am UTC](https://boards.straightdope.com/t/the-practicalities-of-manually-focusing-multiple-mirrors-on-a-single-target/500535/49 "2009-06-30T00:25:11Z")
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. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [June 30, 2009, 3:17am UTC](https://boards.straightdope.com/t/the-practicalities-of-manually-focusing-multiple-mirrors-on-a-single-target/500535/50 "2009-06-30T03:17:06Z")
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 > [@](#): > > 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 > **[Archimedes](https://en.wikipedia.org/wiki/Archimedes_of_Syracuse)** > > Archimedes of Syracuse\[a\] (/ˌɑːrkɪˈmiːdiːz/ AR-kih-MEE-deez; c. 287 – c. 212 BC) was an Ancient Greek mathematician, physicist, engineer, astronomer, and inventor from the city of Syracuse in Sicily. Although few details of his life are known, based on his surviving work, he is considered one of the leading scientists in classical antiquity, and one of the greatest mathematicians of all time. Archimedes anticipated modern calculus and analysis by applying the concept of the infinitesimals an... [http://web.mit.edu/2.009/www/experiments/deathray/10\_Mythbusters.html](http://web.mit.edu/2.009/www/experiments/deathray/10_Mythbusters.html) [http://www.boingboing.net/2005/10/06/archimedess\_death\_ra.html](http://www.boingboing.net/2005/10/06/archimedess_death_ra.html) [http://www.boingboing.net/2005/10/08/more\_on\_mits\_archime.html](http://www.boingboing.net/2005/10/08/more_on_mits_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. ---
**Author:** ![EdwardLost](https://avatars.discourse-cdn.com/v4/letter/e/bbe5ce/32.png) [@EdwardLost](https://boards.straightdope.com/u/EdwardLost)\ **Post date:** [June 30, 2009, 8:46pm UTC](https://boards.straightdope.com/t/the-practicalities-of-manually-focusing-multiple-mirrors-on-a-single-target/500535/51 "2009-06-30T20:46:52Z")
> [@Chronos](#): > > 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. Exactly. If you were the target, looking at a near-by hillside covered in well-aimed mirrors, you could hold a dime at arm’s length towards the hill and estimate how many dimes it would take to block out the hill. That’s how many “suns” would be focused on you. If you walked away until the area of the hillside just matched the area of your dime you would be getting one “sun” of light from the hill. Set up your lawn chair and relax. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [July 1, 2009, 10:17am UTC](https://boards.straightdope.com/t/the-practicalities-of-manually-focusing-multiple-mirrors-on-a-single-target/500535/52 "2009-07-01T10:17:34Z")
> [@Sapo](#): > > I was assuming unwillingness to see, but I will take your answer for truth and leave it at that. ie you’ve got nothing or you would have spelled it out by now. ---
**Author:** ![Sapo](https://avatars.discourse-cdn.com/v4/letter/s/ec9cab/32.png) [@Sapo](https://boards.straightdope.com/u/Sapo)\ **Post date:** [July 1, 2009, 4:43pm UTC](https://boards.straightdope.com/t/the-practicalities-of-manually-focusing-multiple-mirrors-on-a-single-target/500535/53 "2009-07-01T16:43:40Z")
> [@Princhester](#): > > ie you’ve got nothing or you would have spelled it out by now. Ok, since I seem to be unable to explain the obvious to you, let’s try the opposite approach. Explain to me how the exercise they did in Mythbusters is a fair trial of the alleged attempt of Archimedes on the Roman fleet. How do you reckon that 9 people trying to disprove something and giving up at their first attempt are a measure of what is possible of 60-500 trained soldiers defending their home? ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [July 2, 2009, 12:12am UTC](https://boards.straightdope.com/t/the-practicalities-of-manually-focusing-multiple-mirrors-on-a-single-target/500535/54 "2009-07-02T00:12:06Z")
> [@Sapo](#): > > Ok, since I seem to be unable to explain the obvious to you, let’s try the opposite approach. Not playing that game. You suggest their test had a fault. Say what it is. If you can’t explain it, it probably just doesn’t exist. You can’t even get your very basic facts straight: Mythbusters did not make one attempt. So far what you’ve got is that they used less people. Others have talked above about scaling and aiming and so on and why more may not be more. Other than that you’ve whined about motivation and called people names. Unless you can be more specific about what they did wrong, you’ve got nothing and you’re wasting electrons. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 2, 2009, 1:55am UTC](https://boards.straightdope.com/t/the-practicalities-of-manually-focusing-multiple-mirrors-on-a-single-target/500535/55 "2009-07-02T01:55:36Z")
The fact that the Mythbusters test failed where others succeeded should itself be sufficient evidence that the Mythbusters setup was flawed, should it not? ---
**Author:** ![Sapo](https://avatars.discourse-cdn.com/v4/letter/s/ec9cab/32.png) [@Sapo](https://boards.straightdope.com/u/Sapo)\ **Post date:** [July 2, 2009, 10:40am UTC](https://boards.straightdope.com/t/the-practicalities-of-manually-focusing-multiple-mirrors-on-a-single-target/500535/56 "2009-07-02T10:40:51Z")
> [@Princhester](#): > > Not playing that game. You suggest their test had a fault. Say what it is. If you can’t explain it, it probably just doesn’t exist. > > You can’t even get your very basic facts straight: Mythbusters did not make one attempt. > > So far what you’ve got is that they used less people. Others have talked above about scaling and aiming and so on and why more may not be more. > > Other than that you’ve whined about motivation and called people names. Unless you can be more specific about what they did wrong, you’ve got nothing and you’re wasting electrons. I did but you just wave it away calling it waffles. As Chronos points out, just about every time someone sets out to show it can be done, it works. The Mythbusters try to prove it cannot be done and it doesn’t work. They made no attempt at coordinating their efforts, aiming the mirrors intelligently or anything that would lead to success. That was not an experiment. That was a demonstration set to fail. Just a line up of people told to aim the mirrors all trying their own way with no coordination and giving up right away. They set out to prove it would be difficult and of course it is difficult. There is a long way from difficult to impossible though. I insist that that demonstration is no measure of what a number of better skilled and better motivated people might achieve after training for it under the direction of a genius. Proof of that is that group after group has succeeded trying the same. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [July 2, 2009, 1:01pm UTC](https://boards.straightdope.com/t/the-practicalities-of-manually-focusing-multiple-mirrors-on-a-single-target/500535/58 "2009-07-02T13:01:42Z")
> [@Sapo](#): > > I did but you just wave it away calling it waffles. No, you haven’t answered but now that it’s been a while you think you can get away with alleging that you have earlier given some clear explanation that you never have. I want a post # of your post where you have given a clear statement of exactly what they should have done that they didn’t. Insults, vague references to motivation and other crap will not be accepted. What exactly did they do, and what exactly should they have done instead. > [@](#): > > The Mythbusters try to prove it cannot be done and it doesn’t work. The Mythbusters set out to test the myth. The myth involves a number of elements: for a start, such accounts as exist _do not talk about 500 soldiers_. See [here](http://www.roger-pearse.com/weblog/?p=1420): > [@](#): > > When Marcellus withdrew them [his ships] a bow-shot, the old man [Archimedes] constructed a kind of hexagonal mirror, and at an interval proportionate to the size of the mirror he set similar small mirrors with four edges, moved by links and by a form of hinge, Another element is that it involves setting boats on fire. Actual boats. Wet, or at least damp, rocking around in tide and wave, with people on board who don’t want to get their boat burnt. The recreations you talk about all involve breaching one or more of these requirements. They involve shorter distances and or more people and mirrors and or bone dry mockups, often on dry land, and often long ignition times for which no real boat would have stood still. You talk about romantic notions of perfectly drilled soldiers, yet what about the sailors on the boats? They were all incompetent clowns, were they? What do you think they would have done when the first smoke occurred? I’ll give you a clue: it involves the words “water” and “bucket”. There’s no doubt that it is possible to set things alight at a distance under ideal conditions. Mythbusters however test the myth not whether something vaguely like the myth might be possible. ---
**Author:** ![Sapo](https://avatars.discourse-cdn.com/v4/letter/s/ec9cab/32.png) [@Sapo](https://boards.straightdope.com/u/Sapo)\ **Post date:** [July 2, 2009, 1:32pm UTC](https://boards.straightdope.com/t/the-practicalities-of-manually-focusing-multiple-mirrors-on-a-single-target/500535/59 "2009-07-02T13:32:01Z")
I am not arguing the matter of setting boats on fire. I am arguing the matter of focusing multiple mirrors on one target (as per the title). I don’t think you could have set those boats on fire, definitely not with polished shields for mirrors or whatever passed for a mirror way back when, but that is not the question. The question is whether you can aim independent mirrors on one target, not whether those aimed lights can set what on fire. My point is that the Mythbusters did not make any effort to focus those mirrors intelligently. Just stand there and all try at the same time on your own. Many have offered methods to make sure your mirror is pointing at the target without having to look for your blob of light among many. Whether the Mythbusters ignored them or chose not to use them I don’t know. But the methods are there and they did not try any of them. ---
**Author:** ![Sapo](https://avatars.discourse-cdn.com/v4/letter/s/ec9cab/32.png) [@Sapo](https://boards.straightdope.com/u/Sapo)\ **Post date:** [July 2, 2009, 4:47pm UTC](https://boards.straightdope.com/t/the-practicalities-of-manually-focusing-multiple-mirrors-on-a-single-target/500535/60 "2009-07-02T16:47:34Z")
how did that happen? disregard. [Previous page](https://boards.straightdope.com/t/the-practicalities-of-manually-focusing-multiple-mirrors-on-a-single-target/500535.md?page=2)