The Tele Vue RFL4087 0.8x Field Reducer and Flattener is meant for use with Tele Vue's TV-102 Telescope, but works for all non-flat-field 800-1000mm refractors which have 2" focusers. The RFL4087 has a 2.4" diameter female-threaded front to work with their own IS system focuser, but you simply screw the RFL4087 reducer into the back of Tele Vue's RAD1074 nosepiece adapter to use with the TV-102 or any other normal 2" focusers.
This reduces the focal length and speeds up the apertue of projected image by 20% for sensors up to 43mm (full-frame DSLRs), and flattens the field for better overall sharpness.
You can even add 48mm filters to the optical path by placing Tele Vue's AFT1105 filter adapter in front of this reducer (AFT1105 does add 0.25" of spacing, so you'll need 0.25" of focuser in-travel length).
Canon EOS SLR users should use the CWT-1070 adapter + a Canon EOS T-mount. Nikon, Pentax, Sony, and Olympus Four Thirds mount users should buy the TRG1072 adapter plus their system's specific T-mount.
If you're using the SBIG STL11000 Research Series CCD camera, the STL1071 adapter is needed. Additionally, some CCD cameras are shallower than the 55mm specified depth of the normal T-mount; in this case, Tele Vue's spacers would be placed between this reducer and the CCD camera.