ZWO OAG-L M68 OFF AXIS GUIDER

The ZWO OAG-L M68 has been discontinued.

The ZWO OAG-L M68 off axis guider is specifically designed for the ZWO ASI461MM Pro medium format camera and EFW 7 x 50mm square filter wheel. 

The OAG-L has a much larger prism than the ZWO M68 OAG at 12 x 12mm. The bigger clear aperture results in a larger field of view for guiding, particularly useful for long focal length imaging. For telescopes with long focal lengths, a guiding camera with a large sensor is recommended such as the ASI174MM Mini.

Because of the large prism, it is recommended to pair the OAG-L-M68 with telescopes whose clear aperture is M48 or larger. It has an optical thickness of 17.5mm plus 5mm with tilt plate, for a total of 22.5mm. The guide camera interface has an effective light-passing aperture of 11.7 x 8.3mm. It is only compatible with ASI mini cameras. The OAG-L comes with an M48 sensor tilt plate which adds 5mm of optical thickness. 

This off-axis guider offers the best solution for guiding as it eliminates the problem of differential flexure. This effect can arise when a guiding scope is used: while a camera is capturing an image with the main telescope, the guiding scope can flex to a different amount causing a guiding error.

In other words, the guiding telescope does not compensate the mechanical movement of the main telescope during the exposure and as a result, the image can be blurred. An off-axis guider is installed on the main telescope before the camera so this problem does not occur as the guiding and imaging telescope are actually the same instrument. 

The off-axis guide uses a prism that deviates part of the light from the main telescope and redirects it to the guiding camera. However, it will not block any light to the sensor or create any shadow if oriented the right way.