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.
