building a parabolic with mirrors.

[EStein]

Originally Posted by arkain101

hehehhe,,,get'er'done.. !

Think about this. The surface of a rotating cylinder of fluid assumes a parabolic shape. This can be shown by calculus taking into account these forces:
1)The constant downward force due to gravity.
2)The outward force that is proportional to the rotational velocity times the square of the radius.
Picture a fruitcake tin filled halfway with water, centered on an old record player turntable that is spinning at 33 1/3 rpm. Would anyone care to compute the surface focal length?
What would the surface look like if the tin was off centered?

[SolarFreak]

Originally Posted by EStein

Think about this. The surface of a rotating cylinder of fluid assumes a parabolic shape. This can be shown by calculus taking into account these forces:
1)The constant downward force due to gravity.
2)The outward force that is proportional to the rotational velocity times the square of the radius.
Picture a fruitcake tin filled halfway with water, centered on an old record player turntable that is spinning at 33 1/3 rpm. Would anyone care to compute the surface focal length?
What would the surface look like if the tin was off centered?

I dont think water would be a good choice, but liquid mercury on the hand would work fine.

I think 33 1/3 might be ok
where; L = focal length g = gravity w = angular speed ( in radians/sec )
2 L = g / 2w

The problem that I think of 1st is with the liquid spinning it would make tracking the sun or your traget if it was used in a telescope. the liquid would have gyroscopic effect so if you move it to track an object it would make waves or ripples.


but I have heard of people building a parabolic using using epoxies in spining dish. after it sets up you have your parabolic

here is a good link

http://sci-toys.com/scitoys/scitoys/...20Project.html

[EStein]

Originally Posted by SolarFreak

I dont think water would be a good choice, but liquid mercury on the hand would work fine.

I think 33 1/3 might be ok
where; L = focal length g = gravity w = angular speed ( in radians/sec )
2 L = g / 2w

The problem that I think of 1st is with the liquid spinning it would make tracking the sun or your traget if it was used in a telescope. the liquid would have gyroscopic effect so if you move it to track an object it would make waves or ripples.


but I have heard of people building a parabolic using using epoxies in spining dish. after it sets up you have your parabolic

here is a good link

http://sci-toys.com/scitoys/scitoys/...20Project.html

What a great response, SolarFreak. Thanks also for that wonderful link. Yes, water is not the best choice. When I first heard of the spinning liquid problem, it was in a calculus class and we were to prove the surface was paraboloid. I wanted to see what it would look like so I put a pan with water on a record player. I remember having a candle in my hand and turning out the light. When I put it over the spinning pan, an image of the candle was floating over the focal point. I later used some kind of polymer and hardener to make a permanent "mirror". Later when some college students from MIT did it, they had a whole article in Scientific American(February 1994). There have been many projects done using the principle and several "spinning telescopes" using mercury have been made.
As far as the off centered tin of water, it makes an off centered focus. This is perfect when building a telescope that needs no secondary mirror. I actually built a very crude telescope using this technique using a magnifing glass for an eyepiece. Without the off centered focus, one's head keep getting in the way.

[Turtle]

Originally Posted by Corona

I was going to do a type of trough design but while in the shower this morning I had an idea I want to try that is more like what you did.

I believe the material needs to be headed up to around 450 F to become charcoalized. Do you know for how long a period we need to keep it at that temp?

The problem with making charcoal in a dish oven is that it allows only a limited volume of material in the receiver, wheras the parabolic trough oven is extendable to accomodate a large volume receiver.

The period of time required to charcolaize a given volume of material is going to vary widely depending on the dryness of the material, the type of material, how compact the material is in the receiver, as well as the amount of Sun light available & the ambient air temperature surrounding the receiver.

[Sajuuk]

I can finally upload a picture of the solar death ray I made with a friend in summer '05 (see attached file). As I said, there are about 500 3"x3" mirrors on a 6'x6' wooden plate and the focal length was about 5'.

Yes, this is shameless bragging.

[Turtle]

Originally Posted by Sajuuk

I can finally upload a picture of the solar death ray I made with a friend in summer '05 (see attached file). As I said, there are about 500 3"x3" mirrors on a 6'x6' wooden plate and the focal length was about 5'.

Yes, this is shameless bragging.

"Shameless bragging" is an oxymoron. Nonetheless, you built a nice looking setup. Did you use it for anything productive? Is it weatherproof? Do you still have it?

[Sajuuk]

Originally Posted by Turtle

Did you use it for anything productive? Is it weatherproof? Do you still have it?

We didn't try to make anything productive with it since it would have required some kind of device to track the sun (the whole solar death ray weights more than 200 lbs, so moving it is not an easy task). We sheltered it under a plastic sheet and it spent the winter in a warehouse. We still have it but I haven't tested it much this summer (not at all in fact) because I was out of town. Some of the mirrors are now misaligned and I don't know what we are going to do with it for the coming winter.

I attached some pics of a burning cellphone.

[SolarFreak]

Wow!!!! Very nice! I’m starting to have a case of “mirror envy”
What kind of temperatures could to achieve with 500 3" x 3" mirrors?


It looks like you mounted the mirrors on woodblocks cut at the tangent angle of the parabolic.

are the mirrors adjustable?

[Sajuuk]

Originally Posted by SolarFreak

What kind of temperatures could to achieve with 500 3" x 3" mirrors?


It looks like you mounted the mirrors on woodblocks cut at the tangent angle of the parabolic.

are the mirrors adjustable?

Based on the diameter of our focal point (about 6"), the diameter of the parabolae (6') and the sun power (1000W/m^2), we calculated that a blackbody should have reach 1400C at the focal point. Yet we don't know if we ever reached that, since the temperature reached by an object depends strongly on its emissivity (I attached the pic of a burned beer can, it didn't melt much since it is quite reflective).

Indeed, we fixed the mirrors with cement glue on woodblocks cut at the tangent angle of the parabolic. Then we screwed them directly on the wooden plate, so they are not adjustable. We thought that building an adjustable parabolic mirror would take way too much time, so we didn't do it.

[Michaelangelica]

http://hypography.com/forums/science...l-oven-29.html