OAG Off Axis Guider Adapter for Moravian Instruments G2 Cameras - T-threaded version
Guiding of a telescope mount is a necessity for really long exposures even if a high-quality mount is used and mass produced “cheap” mounts require guiding even for short exposures. The guiding camera repeatedly images the selected star or a star pattern. The computer calculates differences in the measured position and performs slight corrections to the mount to compensate for mechanical irregularities and other factors, e.g. refraction. A guiding camera can be attached to a separate guiding telescope or a off-axis guider can be used to feed the guiding camera with the light gathered by the main telescope, used for imaging. If the guider camera uses the same optics as the main imaging camera, it is crucially important to divert the light for it before the light reaches the filters. Numerous filters have so low light transmission (e.g. photometric B or UV filters, narrow-band filters etc.), that they effectively disable guiding with the exception of a very few very bright stars.
The G2 off-axis guider (OAG) is designed to allow attachment of any guiding camera with 1.25" eyepiece nose, although the nose must be somewhat shorter to achieve focus. G0 cameras are designed to achieve focus with G2-OAG and a special shorter version of 1.25" nose with C-mount thread is available for all G1 cameras. Any camera with CS-mount (short version of C-mount standard) can use this adapter to be attached to G2-OAG.
The mirror used to deflect light to guiding camera has 10 × 7 mm area (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).
The G2OAG is equipped with standard M42 × 0.75 (T-thread) or M48 × 0.75 on the telescope side in proper 55 mm back focal distance. This means G2 camera with OAG can be used on numerous optical systems, including Newtonian coma-correctors, refractor field flatteners etc.
The back focus distance is 55 mm for both G2 OAG variants.
G2 OAG is manufactured in two variants with different front thread. Variant with T-thread (M42 × 0.75) allows attachment to many adapters, this thread is used by a lot of astronomical equipment manufacturers. But the thread diameter is only 42 mm and the mirror is slightly shielded close to the upper edge. Adapters with M48 × 0.75 thread are less common, but offer more space and the mirror is completely illuminated. More and more manufacturers of correctors and adapters move to the M48 × 0.75 thread standard.
It is possible to use G2 OAG with all G2 cameras with internal filter wheel. If the OAG has to be used with camera equipped with external filter wheel, the smaller variant of external filter wheels (marked with S letter, e.g. EFW-2S-10) must be used. Older external filter wheels did not allow usage of OAG because of their greater outer dimensions.
G2 OAG is supplied with a spacer, which compensates different distances between front plate of the camera with internal filter wheel and front plate of the external filter wheel and the detector. If the OAG is mounted to external filter wheel, this spacer should be omitted. In the case of camera with internal filter wheel the spacer must be inserted between OAG and the camera body. This preserves the proper distance 55 mm to the detector inside the camera.