I'm using the Orion 2" FF and it works a treat with my ED80 and DSLR.
At only $149 from Bintel, it's a steal.
* Eliminate field curvature in short, optically fast refractor telescopes with this handy 38mm clear-aperture, fully multi-coated field flattener
* Designed for use with 400mm to 650mm focal length refractors with f/5 to f/7.5 focal ratios
* Optimizes your fast refractor telescope for astrophotography use and visual performance
* Preserves focal length while flattening the field of view
* Requires 55mm of refractor telescope backfocus, weighs 7.5 oz.