From SBIG's website:
FR-237 FOCAL REDUCER LENS threads directly into the 1.25" nosepiece of an ST-5C, ST-237, STV, PixCel 255 or PixCel 237. When the focal reducer lens is used on an f/10 SCT it will increase the field of view and reduce exposures by several times. It can provide unvignetted operation at f/5.95 or f/3.75 by selecting a fixed mechanical extension tube. The same lens when used with the eFINDER ASSEMBLY extension tube turns the FR-237 lens into an ultra compact f/4 telescope of 100mm focal length for wide field views or autoguiding without an additional optical tube. (The focal reducer lens only may also be used with the ST-4 and old model ST-5 cameras).
Price: $130, and I'll pay shipping in the lower 48.