For deep-sky astrophotography enthusiasts aiming to capture stunning, long-exposure images, autoguiding is often a vital technique. It significantly enhances the tracking accuracy of your equatorial telescope mount, allowing for sharper images. An off-axis guider (OAG) is a key component in many advanced autoguiding setups.
Autoguiding addresses common issues that arise when tracking the night sky, such as periodic errors in the equatorial telescope mount, wind disturbances, and even minor polar alignment inaccuracies. In essence, autoguiding empowers you to capture longer tracked exposures devoid of star trailing.
What is an Off-Axis Guider (OAG)?
An off-axis guider (OAG) employs an internal pick-off prism to divert starlight to your guide camera, utilizing light from the periphery of the telescope’s axis.
The primary advantage of an OAG is its ability to use your main imaging camera and telescope for autoguiding purposes, eliminating the necessity for a separate guide scope.
I’ve personally used off-axis guiders like the Lumicon Easy Guider in conjunction with Starlight Xpress filter wheels to autoguide my Sky-Watcher EQ8-R Pro telescope mount. This system, coupled with a Starlight Xpress Lodestar X2 guide camera, has delivered exceptional results.
Starlight Xpress Lodestar X2 guide camera used with an off-axis guider for precise autoguiding.
The most challenging aspect was achieving the correct focus position for the guide camera. Minor adjustments to the camera’s position can dramatically impact the sharpness of the guide stars, determining whether they are suitable for guiding or completely out of focus.
This guide will focus on how to effectively use the Lumicon Easy Guider for autoguiding your equatorial telescope mount in astrophotography.
Using the Lumicon Easy Guider for Astrophotography
Off-axis guiding offers distinct advantages over using a small auxiliary telescope as a guide scope. Let’s examine the specific OAG, the Lumicon Easy Guider, that I use with my William Optics RedCat 51 refractor telescope.
The Lumicon Easy Guider is positioned between my ZWO ASI294MC Pro camera and the telescope’s 2″ adapter. This OAG features an internal pick-off prism that redirects a portion of the light passing through the telescope to the connected guide camera.
In my setup, I utilize the ASI290mm Mini from ZWO as a monochrome guide camera. This camera integrates seamlessly with the Lumicon Easy Guider and can be controlled via the ZWO ASIair app alongside my main imaging camera.
When autoguiding with the iOptron SkyGuider Pro and the ASIair, ensure that you select “On-Camera-ST4” in the telescope settings for proper communication.
Given that I employ a one-shot-color astronomy camera, a filter wheel isn’t necessary, unlike configurations with monochrome cameras. In this case, the spacer included with the Lumicon Easy Guider is used to achieve the correct spacing.
Understanding How an OAG (Off-Axis Guider) Functions
At the core of the off-axis guider lies a prism. This prism allows light to pass through to the primary camera sensor while simultaneously diverting starlight to the guide camera. This clever design enables the use of the same star field for both imaging and guiding adjustments.
The design of the pick-out prism ensures that the exposure captured by the primary imaging camera remains unaffected.
Moving from a traditional guide scope setup to an OAG was a significant shift for me. OAGs eliminate the need for a separate guiding telescope mounted on top of the primary imaging telescope, which reduces weight and potential misalignment issues.
For another perspective on OAGs, Chuck Ayoub has a helpful video on his YouTube channel discussing the Celestron Off-Axis Guider. This is an excellent option to consider, particularly if your setup resembles Chuck’s.
Chuck’s Astrophotography demonstrates the Celestron OAG.
Advantages of Using an Off-Axis Guider
The primary advantage of an off-axis guider compared to a small auxiliary guide scope is that it utilizes the same focal length as the imaging telescope. Given that your main imaging telescope typically possesses a larger aperture than a small guide scope, it gathers more guide stars, resulting in superior guiding precision.
Another advantage of OAGs is that they eliminate differential flexure between the imaging telescope and the guide scope. Telescopes requiring extended focuser drawtube settings often exhibit some degree of “give” or wiggle, creating a discrepancy between what the camera is capturing and what the guiding telescope, secured to the optical tube, is seeing.
An OAG detects flexure within your imaging train and compensates with necessary guiding adjustments based on the movement of your camera sensor.
Connecting the ZWO ASI294MC Pro camera to the Lumicon Easy Guider.
Potential Challenges and Recommended Practices
For an OAG system to function effectively, it requires sufficient starlight to be picked up by the prism positioned in front of your imaging camera. The addition of strong filters in front of the OAG can diminish the starlight signal received by your guide camera.
A weak signal makes it more difficult to identify a suitable guide star. Therefore, positioning the off-axis guider before any filters in the optical path is recommended to maximize the signal strength.
The Lumicon Easy Guider OAG might not be the best solution for optical systems with small apertures or slow focal ratios. You’ll need sufficient aperture to gather a guide star within the relatively small field of view captured by the prism.
Cameras with large image sensors may require the OAG to be positioned farther away to prevent it from obstructing the optical path. This can result in stars that are not perfectly pinpoint, so it’s essential that your guiding software can accurately identify the center of the star even when it’s not perfectly round.
Most guiding software, such as PHD2 or Maxim DL, can handle stars that aren’t perfectly round. In my tests, the autoguiding tool within the ZWO ASIair effectively used the guide stars identified with the 290mm Mini in the Easy Guider.
Helpful Resource: OAG vs. Guidescope (Astrojolo)
My Setup Using a Small Refractor
As mentioned, I’m using the William Optics RedCat 51 refractor. This compact imaging APO (Petzval Quad) features a 51mm diameter and an f-ratio of F/4.9. My experience with this telescope is about as small and slow as I would recommend for using an OAG system.
Using this telescope, I could find a usable guide star in any part of the night sky I was imaging. This is largely due to the use of a highly sensitive monochrome guide camera. The ASI290mm Mini excels at capturing a significant number of stars despite the telescope’s limited aperture.
Finally utilizing the autoguiding feature of the iOptron SkyGuider Pro was rewarding. This mount is incredibly easy and quick to set up. To learn more about the SkyGuider Pro, refer to my detailed review.
When autoguiding with the iOptron SkyGuider Pro, remember that it does not have a Declination (DEC) motor. You must disable DEC guiding in your guiding software to properly calibrate the mount.
Final Thoughts on Off-Axis Guiding
A piggybacked guide scope and guide camera mounted to your primary imaging telescope can certainly work for deep-sky astrophotography. In many cases, this configuration is ideal, particularly when using a small refractor telescope.
However, the mechanics of the system can introduce issues. Any flexure in the telescope focuser or optical tube can create a mismatch between the autoguiding system and the imaging train. This is less likely to be a problem with a compact refractor that has a secure and robust focuser but may be an issue with a large Newtonian with a stock focuser.
Your guide camera needs to be sensitive enough to reveal a useful guide star in any imaging location. If you’re using a color guide camera or a slower optical system, an OAG might struggle to detect a guide star in some of the more “empty” regions of the night sky.
Guiding graph in PHD2 Guiding using the OAG with the iOptron SkyGuider Pro, showcasing guiding precision.
Conclusion: Is an OAG Right for You?
I appreciate the concept of eliminating potential error sources in my optical system. I believe I’ve encountered the adverse effects of differential flexure in my astrophotography setup in the past, though I didn’t initially understand the cause of the poor guiding.
Ultimately, an OAG offers an excellent solution for enhancing autoguiding in specific setups, while it might be unnecessary or impractical in others. If you’re considering switching from a secondary guide scope to an OAG setup, remember the adage: “If it ain’t broke, don’t fix it.”
If you suspect that differential flexure is impacting your autoguiding, an off-axis guider could be the solution you’ve been seeking.
My experience with the Lumicon Easy Guider has deepened my understanding of astrophotography and the potential of OAG systems.
