I need an efficient way to capture the light into a closed container, so i can't just leave the aperture open to the environment, it must be closed.
yeah but the photons from the edge of the parabolic mirror will bounce off the clear window
Only some of them. If you're really worried about it then you can use a window that curves outward on both sides, that way each light ray is entering and leaving the window perpendicular to the surface.
right, so like a convex lens? or a convex lens that is more semi-spherical? Fresnel lenses do a good job with transmitting light as well too tho
A convex corrector lens in front of the primary is called a maksutov telescope. This configuration yields images that rival a refractor, but, at a cost. The lens is heavy, relatively expensive, and not seen in apertures exceeding 7" in the commercial market. A Fresnel lens would suffer a great deal of light loss, rendering it largely impractical. A Schmidt corrector lens is a thin, lightly figured transparent 'window' that is extremely popular in the commercial market. It is somewhat unclear what you are asking about. There are also lenses used within the optical train that are generally classified as correctors [e.g., barlows and field flatteners]. It sounds, however, like you want the optical tube enclosed given your comment about not wishing to leave the aperture exposed to the environment. I assume by aperture you mean the primary light gathering element [objective].
So you are not talking about optical quality and you are not even worried too much about efficiency (as long as it's in the high 90%s). Overall efficiency will also be affected by heat loss and other factors so you need not worry too much about the optics. The window could be a mylar (or cheap plastic) sheet.
If you want the best out of your parabolic area then an offset reflector could be worth considering. (Here we go, redesigning your pet project; that's PF for you.) How 'good' will the image be from your paraboloid?
Are you starting with a reflector that you already have or would you be starting from scratch? As long as the image of the sun, where the light falls through the window is smaller than the window, you don't need any other optics. Your tracking should take care of any problem with a small window.
Yeah but if the window is a flat plastic sheet then I'm going to get a lot of reflection of the photons that are entering from the outer edges of the parabolic mirror, and they will not enter the container. This is because the angle of incidence will be close to 90 degrees.
What is the fraction of light coming in at the edge of the window? Just make it bigger. I think it's time for a diagram with some indication of dimensions here. What efficiency are you aiming for? This is a very relevant question because just making the thing a bit bigger would achieve whatever you want. Is there a size limit?
What is the fraction of light coming in at the edge of the window? Just make it bigger. I think it's time for a diagram with some indication of dimensions here. What efficiency are you aiming for? This is a very relevant question because just making the thing a bit bigger would achieve whatever you want. Is there a size limit?
agree with Sophiecentaur, Definitely time for a diagram !
karnifax
do you realize that anything put near the focal point of the mirror is going to get extremely hot !
you won't be able to use a plastic, or glass window or lens. Its going to either melt (plastic/Perspex) or crack (glass). I doubt that you would find anything easily available that could withstand the temperatures involved
It seems you are going an odd way about making a solar water heater. You don't need the light focussed into a container. You should have the water IN the container and the container being placed at the focal point of the mirror so that the OUTSIDE of the container is heated
cheers
Dave
I am talking about the angle of incidence at that angle would be so great that the light would simply bounce off the plane of glass. Think about it, the light approaching the focal point from the outer edges of the parabolic reflector will be close to parallel with the flat plane of plastic(which will be located at the focal point) thus not having hitting it directly enough for the photons to enter, they will bounce off. the focal point is about 1 inch in diamter, and i will be using a 24" diameter parabolic reflector
I did "think about it" and my mental picture, in the absence of a diagram from you, had a longer focal length for the paraboloid. Your problem of reflection would be much less with a longer focus. Btw, have you actually calculated the loss that you are worried about? Is it really significant? This is not an optical instrument where blooming is a requirement.. As with all Engineering, the actual numbers are important and you should tyres to estimate the actual efficiency.