Im not sure if theres a reason ive missed, but my projector seems to work fine with my lcd straight ontop of my fresnel. Sure its there to collimate the light, but if its just gonna get bent back to a point why bother?

If you're referring the the OHP fresnel you are using in your $20 projector, then that is in fact two fresnels fused together (you may be able to spot the seam).

Is there a quality difference between two seperate fresnels & two fused together?
Obviously you can't build a split design if your fresnels are one.
Unless you can seperate them.

but that sounds like a bad idea to me.

You need two fresnels because of how the fresnels are shaped. Consider the following illustration:



I've used the colors red and blue to indicate separate beams of light. This has nothing to do with chromatic aberrations. Look at the way the light goes from the point source to a column of light in part A. If we move the light further away, we certainly could focus the light to a point as in part B. But then another problem arises: the light will not go through the sides of the ridges. You'll definitely see the rings this way, which will definitely look horrible. So, if you use two fresnel lenses, one will collimate the light, then one will focus it, and no rings appear. This is also why the ridged sides of the fresnels must always face each other - so the light doesn't go through the sides of the ridges.

I'm sure the experts will chime in and correct, but I think fused fresnels retain ideal placement (centers perfectly aligned) and as such might represent a slight advantage over separated fresnels placed in approximate alignment. The only downside is that many have reported a hotspot in the center of the projection when using fused fresnels. Splitting them seems to correct this.

Lots of people have separated fresnels. Its not really that hard. You can take a very thin blade and carefully (and repeated) slice them at the seam until you reach the glue point just inside the outer edge, or you can simply cut them down to size (which you may have to do anyway, depending on the size LCD you are using), which will likely cut beyond the glue point anyway. The downside to this is that you are then introducing dust into the grooved side (which is murder to completely clean) and sometimes the fusing is keeping the fresnels straight and separating them introduces a bow.

Also fused fresnels tend to show fresnel rings.

LOL, sorry, i hadnt realised there were two fresnels in it. I thought it was just wedged inside 2 plastic sheets for protection.

And yeah i can see visible fresnel rings.. im guessing no way to solve this unless i take appart the fresnel?

This isn't really a problem. I know friends who made their projectors from old OHP and they have wonderful results.