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Originally Posted by Jay-qu
the focus is dependant on the shape of your setup, unless you are using non-flat mirrors.
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Yes I understand that I select the focus point of my parabolic. I'm trying to match the focal point of the dish/ reflector to match the optimal focal length of the mirrors I’m using.
x^2 = 4py p is the focal point of the parabolic.
Im using flat mirrors and even flat mirrors have focus point.
you are reflecting an image of the sun. the sun is about .5 deg wide as seen from the earth.
so using 2in^2 mirror at 19 feet the suns image blurs
(2 / tan(.5)) / 12
http://www.amasci.com/amateur/mirror.html
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MIRROR SIZE AND LONG FOCAL LENGTH
The smaller your mirror chips, the smaller and hotter the focus. After all, the hotspot is approximately the size of a single mirror. An array of 1in. mirrors a foot across will make 144 beams, but if you use 2in. mirrors for your 1ft furnace, the hotspot only receives 36 beams.
However, if you use small mirror chips and adjust your solar furnace for a very long focal length, you'll find that the hotspot grows larger, fuzzier, and cooler. This occurs because the sun is not a tiny point, instead it is a disk, and the mirror-facets act as the pinholes of a "pinhole camera." Small mirror-chips form an image of the sun, rather than an image of the mirror-chip shapes. Each little square of light will develop a blurry edge, and only the center of each square image will get "full sun."
To compensate for this blurring effect, use larger mirrors. Here's the rule: choose a focal length which is lots shorter than 120 times the width of a mirror-chip. (This 120 comes from 1/tan(.5deg), the sun being about 1/2 degree in angular size.) For example, the 1in. mirrors would give a blurry hotspot if F.L. was longer than a few feet, and at 9.5ft the blurred regions swallow the hot center of the hotspot. This "blur" is an image of the sun. If you want to burn objects from 120ft away, you'll have to build a furnace using mirrors which are wider than 1ft each. The size of the sun-disk is the cause. (If our sun was tiny, but still just as bright, then this blur would be gone, and you could form its light into an intense parallel beam like a laser!)
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1 in. 9 ft.
2 in. 19 ft.
3 in. 28 ft.
4 in. 38 ft.
