Even with the best mounts and polar alignment, most imagers need to guide the mount to ensure near-perfect tracking of the sky. The two most common methods are: 1) a secondary guidescope on top of the main imaging telescope, and 2) a small device attached right next to the imaging camera, i.e., an off-axis guider (OAG).
My preference has always been the off-axis guider because it’s more accurate, especially for reflectors and longer focal length instruments. The primary limiting requirement is having enough back focus with your optical setup. If it does, I’d recommend an OAG over a guidescope.
Here we look at four relatively simple and low-cost options ($100 to $200) from QHY, ZWO, and a less-known option from Hercules.
QHYCCD OAG
The QHYCCD OAGs come in three sizes, priced at $180, $230, and $280, to accommodate most sensors, from the smaller one-inch sensors up to full-frame and even larger. These OAGs bolt onto the camera or filter wheel with the provided screws. Adapters are provided to thread onto the telescope.
Advantages:
- Very slim 10mm body with 3mm of thread, totaling 13mm of required back focus.
- Mono-block (one-piece) design with the body directly attached to the focuser, and thus less flex.
- Can be bolted directly to QHY cameras and filter wheels.
Disadvantages:
- Designed to work with QHYCCD equipment; use with other brands requires minor modifications (screwing threaded adapters to either side).
- The OAG needs to be removed to adjust the prism stalk height.
- The prism is relatively small at about 8x8mm.
ZWO OAG (Standard Version)
The standard OAG made by ZWO is quite versatile, having been designed with M42 / M48 threads on both the camera and telescope sides. It requires 16.5mm of back focus.
The basic version costs only $128 but I highly recommend the helical focuser version ($187) since sub-millimeter focusing precision isn’t possible without a focuser.
Advantages:
- Low cost.
- Easy to adapt to most setups with M42 or M48 threads.
- Prism can be moved in and out without having to disassemble the system.
Disadvantages:
- Threaded connections rather than bolted, with awkward orientation at times.
- Not suitable for heavy cameras and filter wheels.
- Will not work with full frame sensors, not ideal for APS-C.
- The prism is relatively small at about 8x8mm.
New ZWO OAG-L
ZWO recently introduced the larger OAG-L for $199. This model includes a helical focuser, has a much larger prism, and can easily accommodate full-frame sensors.
Similar to the QHY design, the focuser is directly connected to the body, and the prism stalk is adjusted internally. Also similar the QHYs, the ZWO OAG-L is bolted to the ZWO filter wheel and thus not generally compatible with non-ZWO equipment.
A M48 telescope adapter is included, but M54 or M68 adapters must be purchased separately, if needed.
Advantages:
- Mono-block (one-piece) design with the body directly attached to the focuser, and thus less flex.
- Bolted directly to ZWO filter wheels.
- Tilt can be adjusted.
- Large 12x12mm prism.
Disadvantages:
- Designed to work with ZWO equipment; use with other brands requires minor modifications (screwing threaded adapters to either side).
- OAG must be removed to adjust the prism stalk height.
- Slightly thicker than others with 17.5mm body and 5mm of thread, totaling 22.5mm.
Hercules OAG S8239
Hercules a relatively unknown brand but the quality and features of this OAG S8239 were much better than I expected, especially at the reasonable price of $146.
The OAG includes all three popular threaded sizes: M42, M48, and M54 male and female for both sides.
Dovetail-type threaded adapters are then attached to the body with four grub screws, so the orientation of the OAG to the telescope and camera can be adjusted.
The OAG has a CNC body, a heavy-duty focuser (a bit too heavy duty), and a large 12x12mm prism.
Advantages:
- Low cost.
- Versatile, with M42, M48, and M54 threads.
- Dove-tail type threads are much better for adjusting orientation.
- Large 12x12mm prism.
- Comfortably fits APS-C sensor (tested here). Hercules claim full-frame also possible (not tested).
- Prism height can be adjusted externally with thumbscrew.
- Focuser is very solid.
Disadvantages:
- High thickness at 26mm.
- The focuser and prism are held in place by only one thumbscrew.
- Small adjustment range of prism stalk height.
Final Thoughts
When it comes to astrophotography, parts are rarely inexpensive. Fortunately, for about $200, these OAGs enable significant improvements in tracking, and improved image quality as a result. I recommend using an OAG for tracking. The best choice depends on the user’s camera type, sensor size, and focal length.
For typical QHY setups, the QHY OAGs are a very good match. The main limitation is the small prism size, which makes finding suitable guide stars a challenge at long focal lengths (1500mm and up).
The standard ZWO OAG works well with most smaller, lighter imaging trains. With larger cameras and longer focal lengths, the ZWO OAG-L is a better choice better for both its larger prism and clear aperture. Its camera mounting holes are only compatible with ZWO equipment, however.
The Hercules OAG has been a pleasant surprise. It’s not limited to a specific brand, it solidly built, and it provides up to 54mm of clear aperture. Its prism size is generous, and the focuser is the nicest to use of those reviewed here. Its main disadvantage is the 26mm of back focus that’s required.
QHYCCD: https://www.qhyccd.com
ZWO: https://astronomy-imaging-camera.com
Hercules OAG: https://www.aliexpress.com/item/4000557977655.html
Author’s Deep Sky Photography with OAGs: https://www.astrobin.com/users/Rouzbeh/
Editor’s Note: To learn more on why to use an OAG for astrophotography and how to set it up, see this article also by Rouzbeh Bidshahri:
Improve Your Images with an Off-Axis Guider (OAG): Steps to Get Started
