Covering 24 × 36 mm detectors of G3 cameras or even 36 × 36 mm detectors of G4 cameras requires good quality optics, large focusers and field-flatteners or correctors. There is usually only a little space within a field of view to which a mirror, reflecting light to the guiding camera, can be placed. This is why there are two modifications of large OAG adapters, one for G3 cameras and another for G4 cameras.

• G3 variant places the mirror closer to the optical axis, because the shorter side of G3 detectors never exceeds 24 mm and the elongated shape of G3 detectors allows placing of the mirror well into the image circle, created by the used optics.
• Situation is somewhat more complicated in the case of G4 cameras, because of the square shape of the used detectors. Typically the well illuminated field of view has a diameter only slightly exceeding 50 mm, very close to the CCD diagonal dimension. Amount of light as well as image quality rapidly degrade beyond this circle. The reflecting mirror is placed close to the CCD detector edge to be as much inside the field of view circle as possible, but on the other side as far from the optical axis not to cover the detector when f/4 or faster optics is used.

Common 2-inch barrel or M48 thread adapters are no option for both G3 and G4 OAG adapters. They are simply too small and the OAG mirror would be shielded from incoming light. So both G3 and G4 OAG types use the same M68 × 1 threaded adapter, which is enough even for G4 OAG. Also the back focal distance is the same 61.5 mm from the M68 adapter front.

The mirror used to deflect light to guiding camera has 10 × 7 mm front cross-section (mirror dimensions are 10 × 10 mm, but the mirror is angled at 45 degrees). This is enough for guiding cameras with very large 2/3" format detectors without any significant vignetting (smallest detectors used in G0/G1 guiders is of 1/3" format, but numerous guiding cameras use 1/4" or even smaller chips).