How to get large focal length?

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Hey guys. Planning my projector and ran into a major problem. To get the image size I want, I need a projection lens with a focal length around 910mm. I can't seem to find any lenses this big.

What can I do about this?

I know DIYPC sells a lens with 455mm fl. Is there a way I can use it and put in something else to effectively double its fl?

Any ideas would be greatly appreciated.
 
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