All Takahashi accessories that modify the optical properties of the instrument on which they are added, except eyepieces, are grouped in this category. Included here are focal reducers and extenders, field flatteners, finders for pointing celestial objects and diagonals. The quality of these optical parts must be impeccable so as not to degrade the high performance of the objectives. They are often designed specifically for a model of telescope; at Takahashi, the optical design does not tolerate approximation.
