DIY Video Projector

[ChuckReese]

Originally posted by Xterrian
I found several posts about parabolic mirrors, but they all talk about using them to focus a light source. Has anyone thought about using one after the LCD?

I cannot easily see how you plan to keep the pixels in the middle of the LCD from getting blocked.

CR

[ChuckReese]

This is how you could use a reflecting telescope together with a large LCD panel. (The LCD would be on the far left.) It seems like the secondary mirror will block the center pixels of the LCD.

[ChuckReese]

Xterrian:

I think the parabolic mirrors are going to cost too much. I could not find a 16" parabolic mirror for less than $5000. This is far more expensive than getting an 18" fresnel lens from fresnel tech for $120.

Here are the parabolic mirrors from EO:
http://www.edmundoptics.com/IOD/Disp...Productid=1660

You could go the other route like those of us who bought OHP's, and instead buy a reflecting telescope for its optics. However, I doubt you can get a large aperture reflecting telescope very cheaply.

Good luck,

CR

[Xterrian]

I'm not sure exactly how it works, but the secondary mirror doesn't block the starfield when you look through the telescope, so I have to assume that it won't block the LCD either. I would think that you wouldn't need a primary mirror larger than the LCD you are using. There are ATM(Amature Telescope Maker) sites that show you how to make your own Parabolic mirror. After reading the instructions, I don't think it would be all that hard, just time consuming. The only real problem I see is the focal length of an f8, 8 inch mirror is something like 46 inches. That might be a bit long. I'm still looking into it and will post a drawing once I do the math and come up with something reasonable.

[piGuy]

Instead of buying a $1000 high grade parabolic mirror to reflect light from a cheap bulb, why not just make the mirror yourself? A decent quality mirror surface that would work great is aluminized mylar (I posted this before but nobody listened). And a spherical mirror can be made very easily since the center of curvature is constant. I recently made a rather large spherical mirror (about 1m diameter) for a solar hot dog cooking contest. Despite the fact that the mylar was applied VERY poorly in about half an hour, the light was still reflected rather well. (I looked at the mirror from the focal point to see how much light from the sun was reaching it. Needless to say, LOTS of light was reaching it. Everything I saw for about 10 minutes afterwards was yellow! )
In fact, you could make a good mirror really easily by taking an old satellite dish, stripping the paint off, and applying the mylar. Of course, this is all assuming you have a light source that is an ideal point. Maybe I'm missing something, but I'm not sure why everyone is focusing on parabolic mirrors when a spherical mirror can be made very easily, much easier than a parabolic. Both parabolic and spherical mirrors assume a point light source. So making a spherical mirror would be the way to go. Just find a material to shape the dish. Then use a cutting tool attached to a sweepable arm centered above the material. Sweep the tool around till the dish is formed. Then, apply mylar. That's all it takes, really. Even buying high grade mylar will be dirt cheap for the size of reflectors we're talking about. The only real issue would be the geometry of the light. I don't know enough about all the different light sources you guys are using, but whatever system is closest to a point light source would obviously work best. I've been looking into cold cathode tube lights. An array of these lights with trough style reflectors could work well. The only issue is brightness. I can't find any light-related data on easily attainable (computer case) CCFL lights.

[ChuckReese]

Xterrian:

Yeah, I just stumbled across a few amateur telescope building sites. You definately want to make the mirror yourself, but I am not sure how hard it would be. If it can be made compact and the mirror is parabolic (not spherical), then it made be an interesting way to go.

CR

[ChuckReese]

Originally posted by piGuy
Maybe I'm missing something, but I'm not sure why everyone is focusing on parabolic mirrors when a spherical mirror can be made very easily, much easier than a parabolic.

A parabolic mirror takes a collimated beam and focuses it to a point. I don't think a spherical mirror does this.

CR

[piGuy]

A spherical mirror will do this. If you put a point light at the focus of a spherical mirror, you will get parallel light.

[xblocker]

Parabolic mirrors will concentrate parallel lightrays along the optical axis into one focus point.
Spherical mirrors behave the same way, as long as they go near optical axis, the more outer rays are reflected slightly shifted to the the focal point, similar as PCX lenses do.

xblocker

[ChuckReese]

Sounds to me like a spherical mirror kind of works, but to get rid of spherical aberration, you need a parabolic mirror. Thanks for the clarification.

CR