Uses a 12.5 mm prism to intercept light from the telescope's optical path Large 48mm clear aperture minimizes vignetting 8 mm of travel on the helical focuser ensures precise guide camera focus Includes a full set of adapters for SCT/EdgeHD, M48, and M42 T-thread connections Corrects for tracking and opto-mechanical errors for pinpoint stars Description FAQ Specifications In the Box Downloads Warranty Celestron Off-Axis Guider (OAG) The Celestron Off-Axis Guider (OAG) eliminates tracking errors in long-exposure astrophotography by sampling the same light path as your main camera through a generous 48mm clear aperture. Its integrated 12.5 mm prism and 8 mm travel helical focuser work together to locate and sharply focus guide stars without introducing the flexure common with separate guide scopes. Weighing just 18 oz (510 g), it provides a rigid, single-path solution for achieving perfectly round stars, even with long focal length instruments. Guiding Through a 48mm Aperture: Eliminating Opto-Mechanical Flexure For long focal length imaging, the most significant source of elongated stars is often not the mount's tracking, but differential flexure between the main telescope and a separate guide scope. The Celestron OAG solves this by using the same optical tube for both imaging and guiding. With a 48mm clear aperture, it passes the full, unvignetted light cone to your main sensor while using an internal prism to pick off a small, peripheral section of the field for your guide camera. Any deviation seen by the guider is the exact same error affecting your main image, ensuring corrections are immediate and precise. The 12.5mm Prism Advantage: Finding Brighter Guide Stars A common frustration with off-axis guiders is the difficulty of finding a sufficiently bright guide star at the edge of the field. Celestron addresses this with a large 12.5 mm prism that intercepts more light and can be adjusted radially. This allows you to position the prism closer to the better-illuminated center of the light path—just outside your main camera's sensor—dramatically increasing the brightness and availability of usable guide stars. 8mm Helical Focuser: Precision Focus without Twisting Achieving perfect, simultaneous focus for both the imaging camera and the autoguider is critical. The Celestron OAG incorporates a fixed-orientation helical focuser with 8 mm of travel, allowing you to make exceptionally fine and smooth focus adjustments to the guide camera without altering its orientation. This non-rotating mechanism prevents the guide star from drifting out of the field of view during focusing and ensures your calibration remains valid. The unit includes a complete set of adapters and spacers for SCT/EdgeHD, M48, and M42 T-thread systems, providing a turnkey solution for most DSLR and dedicated astronomy cameras. Why use the Celestron Off-Axis Guider instead of a separate guide scope? The primary reason is to eliminate differential flexure. A guide scope is a separate telescope with its own mounting rings and focuser, all of which can shift minutely relative to the main telescope as it tracks across the sky. The Celestron OAG uses the main telescope's optics, so any flexure or mirror shift is seen by both the imaging camera and the guide camera simultaneously, allowing the autoguider to correct for it perfectly. How do I achieve focus with both my imaging camera and the Celestron OAG? First, achieve sharp focus with your main imaging camera through the OAG. Then, without changing the telescope's focus, adjust the OAG's 8 mm helical focuser until the stars in your guide camera are also sharp. The goal is to make the light path distance from the prism to the guide sensor equal to the distance from the OAG's center to your imaging sensor. Will the 12.5 mm prism on the Celestron OAG vignette my full-frame camera sensor? No, when properly adjusted. The prism is designed to be positioned just outside the light cone used by the main imaging sensor. The large 48mm clear aperture of the OAG body is sufficient for full-frame sensors, and you can slide the prism stalk radially outwards to ensure it does not cast a shadow on your imaging chip. What adapters are included with the Celestron Off-Axis Guider? The Celestron OAG comes with a comprehensive set of adapters to fit most common imaging setups. Included are: SCT/EdgeHD adapter (female thread to connect to the telescope) M48 and M42 (T-thread) female adapters for the telescope side M48 and M42 (T-thread) male adapters for the camera side Three T-thread extension spacers for achieving the correct back focus A 1.25" adapter for the autoguider port Is the Celestron OAG suitable for imaging planets like Jupiter? No, an off-axis guider is not necessary for planetary imaging. Planetary work involves taking thousands of very short exposures (video), where atmospheric turbulence is the main challenge, not mount tracking over long periods. The OAG is specifically designed for long-exposure deep-sky astrophotography of targets like the Andromeda Galaxy (M31) or the Orion Nebula (M42). Can I use the Celestron OAG with my 8" f/4 Newtonian telescope? Yes, but it can be challenging. Fast optical systems like f/4 Newtonians have a very limited back focus and a more pronounced coma at the edge of the field, where the OAG prism sits. While the included adapters provide flexibility, you must ensure you have enough inward focuser travel to bring the entire imaging train to focus. The large 12.5 mm prism is a significant advantage here, as it helps find brighter guide stars in the comatic outer field. Weight 18 oz (510 g) Clear Aperture 48mm (1.89") Prism Size 12.5 mm Helical Focuser Travel 8 mm Telescope Connection SCT/EdgeHD, M48, M42 T-thread Camera Connection M48, M42 T-thread Autoguider Connection 1.25" adapter, M42 T-thread Off-Axis Guider Body × 1 SCT/EdgeHD Adapter × 1 M48 Adapter (Female) × 1 M42 T-thread Adapter (Female) × 1 M48 Adapter (Male) × 1 M42 T-thread Adapter (Male) × 1 T-thread Spacers × 3 Download Celestron Software Celestron 2-Year Warranty
