The Antares SCTFR is the standard thread-on F/6.3 4-element fully multi-coated reducer used on F/10 Schmidt-Cassegrain telescopes. It is actually only a reducer, but it does increase true field at the expense of some increase in spherical aberration and some vignetting (dimming) at the edge of the field.

Note that the reducer is marked as 0.7x but the actual reduction is determined by the distance between the reducer and the sensor.  To get 0.63x reduction the spacing would 80mm.

The adapter has M48 threads on both ends