Mastering Off-Axis Guiding

When it comes to astrophotography and long-exposure imaging, one of the most critical factors for capturing crisp and detailed images is accurate guiding. Off-axis guiding is a technique widely used by amateur and professional astronomers to maintain precise tracking of celestial objects during long-exposure imaging.
In this blog, I will go into some detail about off-axis guiding, explore the equipment needed to use it effectively and provide examples of products that can enhance your astrophotography experience.

Understanding Off-Axis Guiding (OAG):

Off-Axis Guiding – How it works

OAG involves the use of a secondary imaging camera, typically mounted adjacent to the primary imaging camera on a telescope, to track and guide the telescope’s mount. By using a small portion of the telescope’s field of view, OAG eliminates the need for a separate guide scope, reducing the complexity of your telescope rig and improving accuracy.

Look for OAGs that offer precise alignment and robust construction to minimize flexure.

Flexure in a telescope refers to the undesired bending or distortion of the optical system due to mechanical or thermal factors. It can negatively affect the pointing accuracy and image quality of the telescope.

Product Examples: 

Introducing the QHYCCD Off-Axis Guider (OAG), designed to complement QHY cameras equipped with APS-C, APS-H, and 36 x 24mm sensors. Here are the key features:


Lightweight Design: The OAG is available in three sizes—Small (127g), Medium (145g), and Large (149g).

Prism: Equipped with 8mm x 8mm multicoated prisms for optimal performance.

Easy Connection: Seamlessly connect your main imaging camera and guide camera using six 3mm screws, eliminating the need for a guide scope.

High-Quality Focuser: Boasting a non-rotating helical 1.25″ focuser of exceptional quality, providing a 6mm range for precise focusing.

Extensive Compatibility: Compatible with most QHY cameras that use M42 and M48 adapters, ensuring versatility and flexibility in your astrophotography setup

Enhanced Adjustability: Specifically designed for the QHY5II and QHY5III models, the OAG allows for further adjustments to accommodate a wide range of back focus requirements, going the extra mile to meet your specific needs.

QHY Off-Axis Guider – Small – $289.95
QHY Off-Axis Guider – Large – $449.95

ZWO Off-Axis Guider 

This OAG allows you to Autoguide in SCTs and Refractors only adding 16.5mm to the optical path.
The package includes M48 and T2 adapters for attaching the scope, along with M42 and M48 adapters for connecting the camera, ensuring minimal back focus.


1. This OAG is Lightweight
2. Large prism: 8mm x 8mm
3. Easily connect the main imaging camera and the guide camera. No guide scope is required!
4. Flexible adaption to your camera and telescope, such as M42 and M48 fittings
5. Fully compatible with all ZWO ASI cameras
6. Good stability and rigid connection to the telescope to minimize flexing of the imaging train

ZWO Off-Axis Guider – Small – $189.95
ZWO Off-Axis Guider – Large – $359.95

Recommended Guiding Cameras
A high-quality guiding camera is crucial for capturing guide stars accurately. Look for cameras with high sensitivity, low read noise, and large pixel sizes to maximize the number of stars available for guiding. Monochrome cameras are preferred for their higher sensitivity.

Product Example:

ZWO ASI290MM Mini – This compact monochrome camera offers excellent sensitivity, low read noise, and a pixel size of 2.9μm. With its high quantum efficiency and fast frame rates, it is a popular choice among astrophotographers.
ZWOASI290MM mini – $449.95

ZWO290MM mini

Mount with ST-4 Autoguider Port:
To connect the guiding camera to the telescope mount, ensure your mount has an ST-4 autoguider port. This port facilitates communication between the guiding camera and the mount, allowing for real-time adjustments to maintain precise tracking.
The trend nowadays is to move away from the ST-4 cable and by using a USB cable connecting to a computer running ASCOM or something like an ASIAIR. The USB cable has the ability to send pulses to the mount more efficiently than the ST4 cable. (This is called pulse guiding)

Tips for Effective Off-Axis Guiding:
a. Star Selection: Choose guide stars that are bright, steady, and located near the edge of the field of view. This ensures that the guide star’s motion can be detected accurately, minimizing errors in tracking.

b. Calibration: Perform calibration routines to determine the relationship between the guide camera’s movement and the telescope’s tracking. This calibration is essential for accurate guiding and can usually be done through the guiding software.

c. Guiding Software: Utilize guiding software that supports off-axis guiding and provides advanced features such as guiding algorithms, dithering, and automatic guide star selection. Popular guiding software options include PHD2 Guiding, Nina, Sequence Generator Pro, and the ZWO ASIAIR.

Remote Guiding:
Remote observatories often rely on off-axis guiding due to limited access to the equipment. In such cases, remote guiding solutions enable astronomers to guide their telescopes remotely, providing flexibility and convenience.

Product Example: ZWO ASIAIR – The ASIAIR combines the functions of a computer, power distribution, and remote control in a single package. It allows for remote monitoring and guiding, eliminating the need for a dedicated computer at the telescope site.

Why would you consider OAG compared with Standard autoguiding?

Wide field of stars to choose fromAdds weight to your setupUsing stars in your telescopes FOVTakes up critical backspace on a refractor telescope
Can be easily transferred to multiple telescopesCan get unaligned from your imaging scopeEasier guiding for larger focal-length telescopesHarder to find guide stars in narrowband if its behind a filterwheel
Independent focusCan cause your main scope to chase your guide star on longer focal lengthsLess expensive than a guidescope systemCan provide only a few stars to guide on.
Focus relies on your main scope.
If the main scope loses focus so does your guide star.

Shadows cast if the OAG is not centred properly

Off-axis guiding is an invaluable technique for astrophotographers and astronomers aiming to capture stunning, long-exposure images of celestial objects. By investing in essential equipment, such as an OAG and a high-quality guiding camera, and exploring advanced options like adaptive optics and remote guiding solutions, you can enhance your astrophotography capabilities significantly. Remember to carefully select the products that suit your specific needs and enjoy the wonders of the universe through your guided telescope setup.

Leave a Reply

Your email address will not be published. Required fields are marked *