Curvature to mirror and focus light?

[DaveC426913]

Re: satellite dishes: there are two types: one where the focal point (pickup location) is on the axis if symmetry (out where you think it should be) and the second type has a non-symmetric shape. These very common types have the pickup out from the antenna but closer to the bottom of it than the centre.

I think even the non-symmetrical one would suit his purpose. However, I don't recall seeing any large enough to hold 500+ mirrors. Do they come that big?

 

[Thundagere]

Hmm...forgetting for a moment about curving, 500 mirrors is 500 inches squared. With this in mind, and assuming the area of the dish is ABOUT πr^2 (it should be roughly equal), then 500=πr^2
159=r^2
r=12.6 inches, or to be safe, I'd aim for 14 to 15.
I'm looking online, not sure if I can find one. I think I might have to mount it on a flat board.
Does anyone have any ideas on how to distill the water at high noon? I mean, in the morning an afternoon you can reflect it then place the water on a stool or something in front of it...I need to come up with a design so that it can be adjusted well to the correct position.

So, now that the math is out of the way, my challenges are
a) How to attach it.
b) How to maintain positioning of the water
c) How to obtain a good mount.

 

[JeffKoch]

If you have access to a very bright illuminator light, for example https://www.amazon.com/dp/B005EILLZQ/?tag=pfamazon01-20, you might check your numbers with a small amount of water insulated as you propose and see if you can get it to at least visibly bubble a bit. You'd have to dig around to find the actual light output in W/cm², but it's much more than the solar constant and at least in the direction of your mirror focuser. You would hate to put a lot of effort into this and then fail, and in fact part of the point is to put enough of the right kind of thoughtful effort into the project to ensure that it does not fail, because it's not a guess or luck.

BTW I do not believe you are in middle school. Fess up - you are the parent of a middle-school kid looking for a science fair project?

 

[johnbbahm]

Most home satellite dishes come with a pole mount, and some sort of alignment mechanism. They are not designed to track, so you may need some work there.
As to attaching the mirrors, a tube a silicon caulk should do fine.
Your heat collector will go where the microwave feedhorn was mounted, as that is the focal point (in your case the focal area.) Do be careful, as I think this is going to get hotter than you think.

 

[DaveC426913]

Hmm...forgetting for a moment about curving, 500 mirrors is 500 inches squared. With this in mind, and assuming the area of the dish is ABOUT πr^2 (it should be roughly equal), then 500=πr^2
159=r^2
r=12.6 inches, or to be safe, I'd aim for 14 to 15.

That's awfully optimistic packing for square mirrors tilted on two axes fitting inside a circle. You figure no mirror tile will overlap any other and yet no mirror tile will have more than about 10% wastage?

I'd double the needed surface area to 1000^2 inches.

 

[Thundagere]

If you have access to a very bright illuminator light, for example https://www.amazon.com/dp/B005EILLZQ/?tag=pfamazon01-20, you might check your numbers with a small amount of water insulated as you propose and see if you can get it to at least visibly bubble a bit. You'd have to dig around to find the actual light output in W/cm², but it's much more than the solar constant and at least in the direction of your mirror focuser. You would hate to put a lot of effort into this and then fail, and in fact part of the point is to put enough of the right kind of thoughtful effort into the project to ensure that it does not fail, because it's not a guess or luck.

BTW I do not believe you are in middle school. Fess up - you are the parent of a middle-school kid looking for a science fair project?

I doubt I can, because I simply lack the tools. I do know of a professor nearby though, I think I'll see if he has something I can use.
FYI, I'm 14. I'm not a parent just yet. I just really like physics.

Most home satellite dishes come with a pole mount, and some sort of alignment mechanism. They are not designed to track, so you may need some work there.
As to attaching the mirrors, a tube a silicon caulk should do fine.
Your heat collector will go where the microwave feedhorn was mounted, as that is the focal point (in your case the focal area.) Do be careful, as I think this is going to get hotter than you think.

Good point. I think I can simply hang a glass jar at the focal point, then let it boil. I'll definitely be careful, though it getting hot is a good thing! :)

That's awfully optimistic packing for square mirrors tilted on two axes fitting inside a circle. You figure no mirror tile will overlap any other and yet no mirror tile will have more than about 10% wastage?

I'd double the needed surface area to 1000^2 inches.

I hope you don't mean 1000^2, and more of 1000 inches^2. : o. You're right, of course, there's going to be overlapping and there's going to be wastage. HOwever, 1000 inches squared seems a tad overkill.
If I may refer you guys https://www.physicsforums.com/showthread.php?t=541057", I posted earlier. So far, I've been getting conflicting replies of whether 500 mirrors will work or not. My calcs say it SHOULD work, but I will definitely be losing a lot of heat, even with a design to conserve heat.
However, I don't think I should double it. Doubling it would result in lots of open space, which might throw off my focusing. Maybe add a hundred inches squared?
At any rate, I doubt I'll find a cheap satellite that's big enough...I'll probably have to plasticine it onto a wooden board.

 

[DaveC426913]

I hope you don't mean 1000^2, and more of 1000 inches^2.

One thousand square inches.

However, I don't think I should double it. Doubling it would result in lots of open space,

I was concerned about the tilting of the mirrors, which means each will require more than 1inch^2 of space. But that was when you were using a flat board. With a tilted dish, you wouldn't need to also tilt all the tiles.

At any rate, I doubt I'll find a cheap satellite that's big enough...I'll probably have to plasticine it onto a wooden board.

Consider a glue gun. It'll last longer and require less readjustment. Do the alignment and placement in the same step.

 

[Thundagere]

One thousand square inches.

I was concerned about the tilting of the mirrors, which means each will require more than 1inch^2 of space. But that was when you were using a flat board. With a tilted dish, you wouldn't need to also tilt all the tiles.Consider a glue gun. It'll last longer and require less readjustment. Do the alignment and placement in the same step.

Oh, phew. :)
I think, if I get a big enough, but cheap enough satellite dish, I'll plasticine it, adjust it to focusing, then put down the glue gun/silicone caulk. It'd be awfully uncomfortable if I found out it wasn't aligned after I glued it down.
Or...actually, I could just put the glue gun on the back, align it to the focal point, then do all of them in this manner. I guess that's what you meant? Either way, thanks!
Now the issue is finding a cheap satellite dish that's big enough. If not, I'll have to use a very big board. Maybe, if it comes to this, I'll buy a big one, put the mirrors down starting from the center, test it, then jigsaw the excess off.

 

[Thundagere]

You know, it occurs to me that most bowls have a nice curvature on the side, it's just the bottom that's flat. Do you guys think that it would be a good method to take one, glue gun down mirrors on the sides, then put mirrors on the bottom and adjust as I do so?

 

[DaveC426913]

You know, it occurs to me that most bowls have a nice curvature on the side, it's just the bottom that's flat. Do you guys think that it would be a good method to take one, glue gun down mirrors on the sides, then put mirrors on the bottom and adjust as I do so?

You could.

It occurs to me the you don't need to worry too much about focussing. Your target's size is non-zero. As long as the mirrors hit it all, it'll heat the water.

 

[Thundagere]

Hm, in all likelihood, I'll do that. I can't get ahold of a big enough dish online, so I'll have to do this or the wood.

 

[johnbbahm]

I had another thought, you might look at some tile stores. They might have mirrors on a tile matte, this would make it very easy to lay onto the dish. Even with individual small mirrors, this might go quick. Spread a thin layer of adhesive, say 4" X 4" and stick in the mirrors edge to edge. it will look like an inverse disco ball.

 

[Thundagere]

One last question for you guys,
which way do you think is best?

Find some sort of satellite dish (difficult to find and expensive, don't like this)
Use a big bowl and paste down the mirrors
Use a flat board and adjust as needed.