Tele Vue's model TRF-2008 0.8x Reducer / Flattener converts TV-85 to 480mm f/5.6 and TV-76 to 380mm f/5.0 for flat field, fast photography. While optimized for TeleVue's TV-85 telescope, it is quite effective with all doublet or triplet refractors with a focal length range of 400mm - 600mm.
Fully multi-coated 3-element unit inserts directly into 2" focuser on the telescope side. It accepts standard T-rings (M42x0.75 thread) for DSLR cameras.
Installation is simple. Just thread your camera's T-ring (not included) onto the 0.8x Reducer/Flattener. Then, attach the T-ring to your DSLR camera. Remove the diagonal from your telescope’s focuser and replace it with the flattener/camera assembly. Your telescope’s focal length and f/ratio will now be 0.8 times their normal figures.
"This accessory is a must-have for someone planning to use the TV-85 photographically" - review in Sky & Telescope, July 1999.
Manufacturer part number: TRF-2008
User Manual: Tele Vue 2" 0.8x Reducer / Field Flattener Manual (PDF format, 3.94 MB)