I just finished building a projector, mainly from stripping an OHP, and at first try the image seems focused, but if I project white letters over a black screen, the leters apears with a halo (ghosted). It happens on any image, but it's more obvious on these case.
It somehow improves if I tape the projection lens borders (square doublet with mirror in between).
I'll appreciate any help.

Based on my limited expertise gleaned from these forums the fact that stopping down the lens with a crude aperture or iris indicates to me the halo effect you are experiencing is a result of limitations in the components you are using. I suspect it will be difficult to completely eliminate the effect and you will just have to accept the haloing. A more technical explanation may be offered by someone to follow if you're interested. Many things might contribute to the haloing and since you're using a OHP retrofit you may or may not find it tough to make the adjustments necessary to reduce the haloing. Got pictures and details of your build?

Thank's for your reply.
Can't upload pictures yet, but I'll try.
Basic setup is:
- 400W lamp.
- No reflector.
- No condenser.
- Unsplit fresnel (took from the OHP).
- 15" lcd (samsung 540n).

Is there a way to narrow down wich part could be the one responsable for the halo?
Could any other thing like alignment, inside reflections or leakeage be a reason for this?

See if the fresnels are actually dead centered with each other. A bit of an effort to callibrate but might be a culprit in your case.

They are centered because I havent split them, though they seem to be bowed. Could that be the source of the problem?

Thank's a lot for your help.

Well, switching from a "large" arc lamp to a "small" arc lamp made all the difference for me. But others have shown what appear to be totally sharp projections with "large" arc lamps. Is your haloing symmetrical all around the image or is it skewed to one side? Does the skewing (if any) vary with where on the projection screen you are looking? Is the halo NOT in the center but rather at the edges? Supply as many details as possible and that will make it much easier for the true experts to make a diagnosis.