Ultraviolet "Barlow" lens?

I have a need for an achromatic "barlow" lens to effectively convert a 20" diameter F/4 Newtonian telescope (parabolic mirror) to F/8 and extend it's focal plane location a few inches. It needs to have reasonably low chromatic aberration and good transmission from 300 to 600nm. The Edmund 50.8mm x -500mm fl PCV fused silica lens would work were it not for the focal length shift versus wavelength. . Any ideas or sources for such a lens? I can afford up to about $1k. The lens also needs to act as an entrance window so it can't be a delicate material.
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