Antares Optical Antares 2" Focal Reducer for Eyepieces and Cameras 0.5x - Camera Concepts & Telescope Solutions
Amazon.com : Astromania 0.5X Reducer for Photography and Observing - 1.25" Filter Thread (28.5x0.75MM) on Both Sides - Reduces The Focal Length for Visual and Photographic Use : Electronics
What can a field flattener or focal reducer do for my telescope? | BBC Sky at Night Magazine