Modifying projector optics

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Hi,

I am trying to modify a normal projector so it can be used on very short throws. As it will be used for rear projection , the position of the projector is not a problem. It can be in the middle of the screen as it will not obstruct the view. Because of this, I thing, there is no need for expensive and complex lens system like the one used by front ultra short throw projectors.

Can anybody give me any instructions as to what lens I should use?
Am I wrong in thinking I could place a concave lens in front of the existing projector lens?

Cheers.
 
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