What does the objective inscription “160” mean?
The 160 identifies a finite tube-length objective, with a distance of 160mm from the nosepiece (where the objective screws in) to the top of the observation tube (where the eyepiece inserts). When you lengthen this distance by inserting accessories in the light path above the nosepiece, spherical aberrations will occur unless the accessories include the proper optical corrections.
This number identifies a finite tube length objective. 160 millimeters is the distance from the opening of the nosepiece (where the objective is screwed in) to the top of the observation tube (where the eyepiece is inserted). If this distance is lengthened, e.g. by insertion of accessories in the light path above the nosepiece, spherical aberration will result, unless optically corrected lenses are included in the accessory.
