Giant Magnefying Lens in the Sky

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This certainly expands my options. I was planning on always staying indoors during daylight hours at places that resemble the Guggenheim Bilbao:

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Oh yeah! Is there a matching jacket?

Aside from the problem of clouds, a magnifying lens, like the kind you'd use to fry an object like an ant, needs to have a very small focal ratio, like f/4 or less. (That's the ratio of the focal length to the lens diameter.) For a lens 100 miles up in space, the lens would have to be at least 25 miles across, or more, really. (Plus there is the problem of steering the lens as it orbits around the Earth, with it's focal point constantly changing position.)

From geosynchronous orbit, the lens would have to be 5500 miles across. And there is still a need to steer the lens as the Earth orbit the Sun and that angle changes.

The ability to direct and focus the light would certainly be critical, but I doubt insurmountable.

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I am curious about you assertion that burning things would require a very small f-number. While it make sense to me that for a given lens/collection area, a small f-number would be better for burning things, I'm not sure it holds true when you have a collection area large enough to provide sufficient energy to heat more than a pin point.

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Let's just considers some numbers. Assume you have a circular lens/collector in space about 17.85 meters in diameter.... for ease of consideration,since that makes it 1000 m² area. Solar radiation up there that might reasonably be concentrated is roughly 1300 watts per m² , but for ease of calculation, lets assume that since we are using light weight reconfigurable focusing material, that right off the bat, we can only assume 1000 watts per m². Okay so that means we are dealing with 1 MW. Sunlight normally loses about 25% of its energy traveling through the atmosphere, so let's assume it is higher for concentrated light, say 30% and lets throw another 10% on for clouds, dust, and naysayer wind. Actually lets make it easy and know we are being conservative and just whack the number in half. That brings us down to 500,000 watts.

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Lets assume that because of atmospheric distortion...

That the very best that can reliably be accomplished is focusing the light into a 1 m² effective area. Okay, fine, 2 m² then. It seems like adding 250,000 watts/m² to the 2 m² immediately surrounding someone would be highly lethal? I would expect to hear reports of spontaneous human combustion.

"It seems like adding 250,000 watts/m² to the 2 m²immediately surrounding someone would be highly lethal? I would expect to hear reports of spontaneous human combustion."
Seems that everyone here is overlooking one major problem with using this thing this way.
Time.
It will take at least a moment to get the lens system lined up between the sun and on its target plus get focused. By then the guy who you were trying to melt would have ran away.

If you had to get the lens lined up between the target and the sun, it wouldn't be feasible. However, the beam could be steered and corrected either with reflection or refraction or some combination, then aiming a beam would be much faster than sending a drone.

Using 'simple lens' algebra, the image size of an object is given by: S = f*tan(α) where α is the angular size of the object and f is the focal length.

The Sun's angular image size is a half of a degree, f would be the focal length of the lens, in this case 22,000 miles. So the image size of the Sun, at the focal point on Earth would be (22,000 miles) * (0.00873) = 192 miles in diameter - not exactly a pinpoint.

If the lens was 192 miles in diameter, you'd only get a 1:1 concentration sunlight, i.e., no effective increase in intensity. To get, say, a 50:1 concentration of the Sun's energy, the lens would have to be 50*192 miles in diameter, or 9600 miles across.

"....the image size of an object is given by: S = f*tan(α) where α is the angular size of the object and f is the focal length..."

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To me, this doesn't make sense. You are using the angular size of the object (the sun) that we see unaided. This would suggest that things like the diameter of the lens have no part in the concentration of sunlight. Using the angular size that we see without a lens, and only allowing the focal length in as a variable would lead to all lenses of the same focal length concentrating the same amount.

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I'm not well versed in optics, but that didn't sit right with me. I decided to become more familiar with the ins and outs of optics, since my intuition seemed to be leading me astray. The formulas presented relating image size and focal length I found (and then confirmed at other pages) differ in important ways from what you used.

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α = 2 ⁢ arctan ⁡ B 2 ⁢ f using: α Angle of view B Image size f Focal length If you derive a formula for f you will get:

f = B 2 ⁢ tan ⁡ α 2

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It is important to note that the 0.5° angular diameter is not the angular size being referenced. Instead the angle of view IS dependent on the size and distance of the lens, since that affects the angle of view.

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Another important thing to note is that the focal length being used is apparently considerably less than the full distance to the image. It seems pretty straight forward that the size of the image is not dictated by the focal length, but instead by how close ' b (+) ' is to ' f' (-) '.

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And that makes sense. The size of the spot formed with a hand held magnifying glass was never static, even though the size of the sun and the focal length of the magnifying glass were static. The size of the spot, and therefor the concentration available always depended on how close you could get the focal point to the surface of whatever you were trying to burn.

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Ultimately it would be better to use non-imaging optics, since the ability to concentrate can be theoretically much higher than with imaging optics. It would also make a lot more sense to have several of these in LEO than so far away geosynchronous.

The image size is proportional to the focal length, so the image area is proportional to the square of the focal length. The amount of energy is proportional to the area of the lens or to the square of the lens diameter. So the amount of energy per unit area is proportional to the square of (lens diameter / focal length) or square of the f number. Therefore, a f/8 lens would produce 4 times the energy density as a f/16 lens of the same diameter, only in 1/4 the area.

That makes sense to me, and seems to indicate that (putting atmospheric concerns away for the moment) in a vacuum, there would be no insurmountable obstacle to creating a 30 meter diameter lens that would concentrate light onto a very small area a few thousand kilometers away.

Because the image size (of the sun, in this case) increases in size with focal length, it would be a circle with a diameter of about 8 3/4 km for each 1000 km focal length, given that the sun diameter subtends 1/2 degree. That would spread your 30 meter circle of sunshine pretty thin. Not a very powerful death ray.

Okay, I think are getting onto something that is not critical since there is no need to produce a correct image (non image producing optics can provide much greater concentration), but I'm learning, so lets continue.

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Is your assertion is that if I try to focus light from the sun onto a small point on the earth's surface using a 30 meter lens in low earth orbit that the BEST I can do is over 8 km diameter per 1000 km focal length, before we even consider atmosphere, lens imperfections, and dust?

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It sounds like just imagining a circle would be more effective at focusing light. What effect is preventing a lens filling that circle from directing the rays close to its edge ever so slightly inward?

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I don't think this would make a very effective weapon, even if they could get the beam focused...unless they just wanted to start fires.

Any human would be able to detect the light and heat, and simply run out of the beam.

Remember how uncooperative those ants were, when we were kids, and trying to burn them as they came out of the mound?

It didn't work out very well, because they wouldn't stay still.

Yeah, I realize now that what I must have meant all along was "hey doesn't that really look like a concentrator for a massive solar pumped laser that could be used to take outdoor terrorist types?"

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Yeah... all along.

Seems like a lot of effort, when the drone strikes are working so nicely.

The rays from the sun are not exactly parallel, but arrive from an angle of 0.5 degrees. A 0.5 degree cone from the lens 1000 km away makes a circle 8 3/4 km in diameter.

Hey, I might be coming around. Reading about 'circle of confusion' and 'depth of field' is bringing me around with an intuitive explanation that if the angular diameter of the lens is not larger than the angular diameter of the sun, then there is no way for imaging optics to concentrate additional light. Does that seem right?

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Thank you Rixter and USBPort for being persistent and thick skinned.

Thanks for putting it so succinctly. #11 had me brainswoggled for a few minutes!

Sorry about that.

No problem

I have my moments, when it's all as clear as mud!

'...all lenses of the same focal length concentrating the same amount.'

All lenses of the same focal length produce the same size image of a given object; the intensity (brightness) and detail within the image will vary with the diameter of the lens.

The formula I gave is standard among astronomers; I've used it for years. It's the equation used for calculating the size of the image of a galaxy or nebula (or the Sun, or the Moon) on a CCD sensor.

You might not like the equation, but it is what it is.

I just marked that comment 'off topic'. It was written in a rude way. I shouldn't have posted like that. Sorry.

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My opinion hasn't changed, but I have a lot to learn in optics and I appreciate your willingness to engage.

Here is another way to look at it: Each point on the sun's surface is imaged to a point at the lens's focal plane and all these points subtend an angle of 0.5 degrees across as seen from the lens. That's at the focal point and it's as small as it's going to get.

It would be great for "Peeping Tom" types. They could look over the fences of nudist camps without being seen!

Good Luck, Old Salt

I hope it's just a telescope!!!!

If-if-if-if they tried to use it as a weapon, it would heat the atmosphere, and that would melt the glaciers, and-and -and -and.....it would get hotter and increase global warming. And the earth would boil over.

DARPA is part of the government, and the government can only make things better for us. They would never do anything to increase global warming...so this can only be a good thing, that will help keep us safe.

The extra feeling of security, along with a further elimination of privacy, is well worth having a giant eye in the sky...maybe lots of them.

OK, suppose there is in space a huge solar collector, lens, mirror, or whatever, and the collected energy is beamed to earth, concentrated on a small spot. It seems to me that would *decrease* global warming. First,t he big collector would essentially cast a shadow, deprive the earth of some insolation, thus cooling it. The hot spot of concentrated energy would, because it is so hot, radiate heat to space more efficiently than the usual temperature surface, thus further cooling the earth. This scheme might be a good candidate for a government "stimulus" project, better than feeding money to failing "green" industries.

Good thinking.

Maybe we could use it to zap the green energy companies, before the president has time to sink billions into them.

...or maybe we could use them to melt down the "clunkers" from the brilliantly conceived "cash for clunker" program. It would be free energy!!!!!

Aha! A great way to make glass factories without nuclear weapons...I like it!

We could just throw the clunkers and anything else we didn't want into "The Pit".

Speaking of a pit, I just read Roger Pink's thread on the Grand Canyon and learned there is a bigger on one under all that ice in Antarctica. Once we've filled up the Grand Canyon with clunkers we can use the beam to melt the ice and start filling up that pit too.

I'm thinking no. http://www.gizmag.com/darpa-folding-telescope/30039/

That article doesn't really do anything to convince me that it isn't a real possibility. It is nothing more than the official story. The artist conception is nice though.

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That reddish glow isn't the artists rendition of a solar pumped laser, is it? What purpose does the weird film connecting the trusses serve other than to deflect attention from the possibility that sunlight will be striking those optics pretty often.

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Okay, it is too easy to play 'what if' here. I find it interesting that everyone has come out fairly unanimously in favor of the idea it is not possible or at least not feasible.

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It doesn't seem like that big of a stretch to me. I seems like it could be cheap enough to have hundreds up there, certainly decreasing the demands on steering the beams at great angles.

The entire thing is nothing but conception, regardless of if it's a weapon, or a way to spy.

Even if it were to work in either scenario, who would we hire to deliver it (them) to space?

Russia?

China?

India?

I doubt any of them would take the job.

We didn't loose all capability for putting stuff in orbit when we retired the shuttle. We still put satellites up regularly. The design they are talking about doesn't even look like a very big satellite.

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I don't getting the satellites into orbit as a challenge at all for the US.

Sure enough. I'm not sure what a big telescope would be able to do better than this.

http://www.cnn.com/2013/08/28/us/spy-satellite-launch/index.html

It also looks like something that would be very susceptible to flying space junk.

We have deliveries leaving for space all the time.....

http://en.wikipedia.org/wiki/SpaceX

Looks like the time is right, for them to move into manned flight.

http://www.scienceworldreport.com/articles/6560/20130501/nasa-pays-70-million-seat-blast-astronauts-space-russian-spacecraft.htm

You really think our government can do it for less than $70 million?

And 44 years ago we were walking on the moon. We've come a long way backwards since then.

Moonwalking is backwards...

Ah, that explains it!

Each one of these burning glasses would increase the solar energy density in a 8.75 km diameter circle by (30/8750)^2 = 0.0012 percent. It might not even be possible to detect.

Now if I were going to do it, I would have a big solar cell array, store the energy, and fire off a big laser. It would be easier to aim and more highly collimated than sunlight. (I hope this doesn't exist, I don't want the men in black looking for me!)

...should reduce collateral damage by just frying one ant at a time

Right after 09/11 terror attacks on the U.S. I emailed the DOD my idea: Launch a satellite with a reasonably large array of solar panels, then upgrade the power (that is to be stored on board using a supercapacitor ?) to microwave (maser) that (1) transmits through water vapor easily and (2) could be sufficiently high powered (say 50MW) to allow for raster scanning of an entire mountain pass, say in Afghanistan.

Unfortunately, this method would be somewhat non-selective on the target area.