ZWO OAG Off-Axis Guider with 8mm Prism

A compact off-axis guider for APS-C and smaller cameras

The ZWO OAG Off-Axis Guider sits between your telescope and your imaging camera and uses a small 8 × 8mm prism at the edge of the light path to feed a guide camera. Because the guide camera sees through the main telescope, there's no separate guide scope to buy, mount or flex. ZWO recommends it for ASI cameras with APS-C or smaller sensors, such as the ASI533MM Pro, ASI294MM Pro and ASI2600MM Pro, with an ASI Mini guide camera. It's machined aluminum and weighs 181g (6.4oz).

Key specs: 8 × 8mm prism · 7.4 × 5mm effective guide aperture · 16.5mm thick · M48 × 0.75 telescope side · M42 and M48 camera-side adapters included · 1.25" guide camera holder · for ZWO ASI Mini guide cameras · 181g

When an off-axis guider makes sense

• Longer focal lengths: on a 612mm ZWO FF90 or 770mm FF110, a small guide scope can't sense small tracking errors well. An ASI220MM Mini on the OAG guides at about 1.35″ and 1.07″ per pixel on those scopes.
• SCTs and other mirror scopes: mirror shift can't be seen by a separate guide scope, but an OAG sees it and corrects it.
• Long narrowband subs: 5- to 10-minute exposures through narrowband filters need steady, flexure-free guiding to keep stars round.
• A cleaner rig: no guide scope, rings or extra dovetail on top of the telescope.

How it works

A small prism sits just outside the main camera's light path and turns a slice of the image up into the guide camera. You focus the guide camera once and lock it, and from then on it stays in focus whenever the main camera is focused.

The prism assembly lifts out of the body, so you thread the OAG onto the telescope first, then put the prism back, attach the guide camera, and mount the imaging camera with the M42 or M48 adapter and the three locking screws. ZWO's example for a 55mm train:

• OAG: 16.5mm (M42 telescopes use an M48-M42 adapter, included with ZWO cooled cameras, which adds no length)
• M42 extender: 21mm (included with ZWO APS-C cooled cameras)
• ZWO cooled camera: 17.5mm
• Total: 55mm

On the guide side, ASI Mini cameras (8.5mm back focus) go in the included 1.25" holder, and ZWO cameras with 12.5mm back focus use the included 5mm T2 extender. Set the guide camera so its sensor is the same distance from the prism as the main camera's sensor, and check that the prism doesn't shadow the main sensor. Both are easiest in daylight on a distant object.

What to add for the best results

• A guide camera (required): ZWO recommends its ASI Mini cameras with the OAG.
  • ASI220MM Mini: very sensitive, with a sensor that fits the 8mm prism well. The one we suggest for most setups.
  • ASI120MM Mini: the budget option, fine at shorter focal lengths.
  • ASI174MM Mini: the largest sensor of the three, though the small prism won't fill all of it.
• ZWO 1.25" Helical Focuser: replaces the slide-in holder with a fine-focus helical, which makes focusing the guide camera much easier and lets you lock it.
• A filter wheel: the ZWO 7 × 36mm EFW for APS-C, or the 8 × 1.25" EFW for smaller sensors like the ASI533 and ASI585.
• ASIAIR Plus: runs guiding, the camera, filter wheel and mount from your phone.

ZWO OAG vs. OAG-L vs. OAG-L-68

See all autoguiders for astrophotography.

What's in the box

• ZWO OAG body with removable prism assembly
• 1.25" guide camera holder
• 5mm T2 extender
• M42 and M48 camera-side adapters
• M48-M42 adapter
• Hex wrench

The guide camera and helical focuser are sold separately.

ZWO OAG FAQ

ZWO OAG or the OAG-L?

Choose the OAG for a 4/3" or smaller camera such as the ASI533 or ASI294, a 1.25" filter wheel, or a telescope with an M42 or M48 back. Choose the OAG-L for a full-frame camera, or an APS-C camera at long focal lengths where you want more guide stars. The OAG works with APS-C cameras like the ASI2600 too.

Which guide camera should I use?

ZWO recommends its ASI Mini cameras. The ASI220MM Mini is our usual pick for its sensitivity. The ASI120MM Mini is the budget choice.

Do I need the helical focuser?

It isn't required, since the included 1.25" holder works, but sliding a guide camera in and out by hand is fiddly. The ZWO 1.25" Helical Focuser makes fine focus easy and holds it.

How does it affect back focus?

It takes 16.5mm. With a ZWO cooled camera (17.5mm) and the 21mm extender that comes with ZWO's APS-C cooled cameras, the total is 55mm. With a filter wheel, use the wheel's thickness in place of part of that extender.

Will the prism shadow my main camera?

Set correctly, no. When you first assemble the train, look through the front or take a test frame, and adjust the prism height if you see a shadow at one edge.

Can I use it on an SCT?

Yes, with an adapter from the SCT's rear thread to M48 or M42. An OAG is the usual way to guide an SCT, since it sees mirror shift that a guide scope can't.

Specifications