Off-Axis Guiders

How an Off-Axis Guider Works

A small prism projects into the edge of the light cone, ahead of the sensor, picking off a star from outside the imaged field and sending it to a guide camera.

• No differential flexure: Guide and imaging light travel the same optical path, so any movement of the telescope affects both identically. This is the fundamental advantage.
• Prism size and position: A larger prism catches more stars but risks intruding into the imaged field. Radial adjustment lets you find a star without vignetting the sensor.
• Back-focus consumption: An OAG typically uses 15 to 20 mm of the spacing budget, which must be planned alongside the filter wheel and camera.
• Guide camera sensitivity: You cannot choose your guide star, so a sensitive small-pixel guide camera matters more than with a guide scope.
• Guide camera focus: The guide camera must be focused independently to the same plane as the imaging sensor, which is a one-time setup step people often skip.
• Placement relative to filters: Positioning the prism ahead of the filter wheel means guiding is unaffected by narrowband filters cutting your guide star's light.

Why Choose David Astro for Off-Axis Guiders

An OAG is straightforward once installed and fiddly to install, and the fiddly parts are predictable.

• Full back-focus plan supplied: We specify the OAG, wheel, camera and spacers together so the corrector still lands on its design distance.
• Prism placement advice: We help set radial position and depth so you find guide stars without intruding on your imaging field.
• Guide camera matched: We recommend a sensitive guide camera appropriate to off-axis use rather than assuming a guide scope camera will do.
• Authorized dealer: Genuine products with manufacturer warranty, in Canadian stock.
• Ships from Canada: No duties or brokerage fees for Canadian orders.

Off-Axis Guiders in Canada — Frequently Asked Questions

When should I switch from a guide scope?

Around 800 mm to 1000 mm of imaging focal length, or sooner if you see star elongation that your guiding numbers do not explain. That mismatch is the signature of differential flexure.

Is it hard to find guide stars off-axis?

Less than it used to be. Modern sensitive guide cameras find usable stars in most fields, and rotating the OAG or the camera assembly gives you another set of options if a field is sparse.

Does the prism appear in my images?

Not if it is set correctly. The prism should sit just outside the illuminated field of your sensor. If it does intrude, it appears as a shadow in one corner and the radial position needs adjusting.

How do I focus the guide camera?

By adjusting the guide camera's position in its holder until its stars are sharp at the same telescope focus as your imaging camera. It is a one-time setup that many people never complete properly.

Guiding at long focal length?

Send us your camera, wheel and corrector and we will plan the spacing. Ships from Canada.