The Tele Vue NPR1073 0.8x Field Reducer and Flattener is meant for use with Tele Vue's NPis or NP telescopes. It has a 2.4" diameter female-threaded front to work with their own NPis scopes, but you simply screw the NPR1073 reducer into the back of Tele Vue's RAD1074 nosepiece adapter to use with the NP 101 and NP 127 (which have 2" focusers).
This reduces the focal length and speeds up the apertue of projected image by 20% for sensors up to ~30mm (15x22mm 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.