Cold Mirror

[bigteks]

By the way is an achromatic lense really important? In searching a couple of places today (Surplus Shed & Anchor Optics) I see there are lots of plain old double-convex lenses but very few achromatics ...

40 mm seems nowhere near big enough, I'm thinking I will need a lense around 75 mm or larger.

Originally posted by Guy Grotke
You need an achromat with a large negative focal length, and it has to be wide enough to not obstruct the cone of light coming out of your existing lens.

Achromat so you don't add a lot of chromatic aberration to your screen image. Negative focal length to make the effective focal length of your lens larger.

I just looked through the lens list at surplus shed. They do have achromats in the -1000 mm to -4000 mm fl range, but mostly they are small diameter (32-37 mm wide). If you supplied some parameters like the focal length of your existing lens, the throw and image size you have now, and the throw and image size you want, then we could calculate exactly the fl of the lens you need.

But keep in mind, the resulting image will not be quite as sharp as the one you have now.