Moravian instruments, Inc., source:, printed: 23.5.2017 8:46:18

Main page  Products  CCD cameras

Off-Axis Guider adapter for G2 CCD cameras
 Guiding of 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. Guiding camera repeatedly images a selected star or a star pattern. Computer calculates differences in measured position and performs slight corrections to the mount to compensate for mechanical irregularities and other factors, e.g. refraction. Guiding camera can be attached to separate guiding telescope or 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 filter has 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.

Both ways of using of guider camera have advantages and disadvantages.

  • Standalone guiding telescope often offers shorter focal length compared to imaging telescope, so it provides wider field of view and makes finding of bright-enough guiding star easier. On the other side guider telescope focal length must be long enough to provide seeing-limited star centroid positions not to limit guiding quality. Another advantage is that the guiding camera on stand-alone telescope uses the center of the telescope field of view, where the image is of the best quality—all optical aberrations of all telescopes increase with the distance from the optical axis.

    The problem is the guiding camera in fact follows the star in guiding telescope and not in the main telescope. When both telescope attachment is not rigid enough, optical axes of both telescopes slightly moves as environment temperature changes, telescope and the mount orientation changes as the telescope tracks the sky etc. This means even if the guiding star remains on the same pixel of guiding camera, image often slowly drifts over the imaging camera detector.

  • Guiding camera on off-axis guider acquires light from the main telescope, so no separate guider scope is necessary, which saves money as well as the weight attached to the mount. Off-axis guider is mechanically rigid, guider camera follows the same mechanical distortions of the optical tube and focuser as the main camera and guiding actions compensate for these distortions.

    But if the main telescope has long focal length, guider camera field of view is small (guider cameras have relatively small detectors) and bright enough guiding star may not be available within field of view. Finding appropriate guiding star may require altering of the telescope position. Also off-axis guider takes the light from the edge of the field of view, where many telescopes provide distorted star images. Distortion can limit the brightness (star image is spread over more pixels compared to properly focused image) and also affects star centroid calculations.

Either way, with addition of off-axis guider for G2 cameras the user has both options available. 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 G2OAG 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 G2OAG.

Off-axis guider on the G2 camera offers T-thread (M42 × 0.75) and M48 × 0.75 in proper 55 mm back focal distance. Guider input is standard 1.25" barrel.

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).

CAD rendering of G0 (left) and G1 (right) cameras attached to G2 OAG

G0 (left) and G1 (right) cameras on G2 camera attached to OAG

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.

Back focus distance is 55 mm for both G2 OAG variants

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.

G2 OAG with T-thread (M42 × 0.75) and with M48 × 0.75 thread (left), G2 imaging camera with OAG and guiding camera G1 on telescope (right)


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.