ZWO OAG-L-68 Off-Axis Guider for ASI461MM Pro

The off-axis guider built for ZWO's ASI461 medium-format system

The ZWO OAG-L-68 Off-Axis Guider sits between a large-format flattener and ZWO's 7 × 50mm square filter wheel and uses a 12 × 12mm 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 flex, which matters at the long focal lengths medium-format rigs often run. ZWO designed it for the ASI461MM Pro and recommends ASI Mini guide cameras. It weighs 277g (9.8oz).

Key specs: 12 × 12mm prism · 11.7 × 8.3mm effective guide aperture · 17.5mm thick (22.5mm with the built-in M68 tilt plate) · M68 × 1 telescope side · bolts to the ZWO 7 × 50mm square EFW · built-in helical guide focuser · for ZWO ASI Mini guide cameras · 277g

When an off-axis guider makes sense

• Long focal lengths: medium-format imaging usually means a large refractor or astrograph, where a small guide scope can't sense small tracking errors. An ASI174MM Mini on the OAG-L-68 guides at about 1.2″ per pixel at 1000mm.
• Heavy imaging trains: an ASI461, a 7-position 50mm wheel and a big flattener put a lot of weight behind the focuser, and an OAG sees any sag or shift that a guide scope can't.
• Long narrowband subs: 5- to 10-minute exposures through 3nm to 7nm filters need steady, flexure-free guiding to keep stars round across a large sensor.
• 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 with the OAG-L-68's helical focuser and lock it, and from then on it stays in focus whenever the main camera is focused.

The OAG-L-68 has no inner threads. The telescope side takes the built-in 5mm M68 sensor tilt plate, and the camera side bolts directly to the ZWO 7 × 50mm EFW with screws. ZWO's current connection chart for the ASI461 system:

• M68 tilt plate: 5mm
• OAG-L-68: 17.5mm
• ZWO 7 × 50mm EFW: 16mm
• ASI461MM Pro: 17.5mm

Check that total against the back focus your flattener or reducer needs, and add or remove spacing on the telescope side as needed.

To set the prism height, take the OAG off the telescope, loosen the M4 screw through the hole in the tilt plate, slide the prism up or down by its sides (without touching the glass), and retighten. Set it as close to the main sensor's light path as you can without casting a shadow, then match the guide camera's focus to the main camera's. 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.
  • ASI174MM Mini: its large 1/1.2" sensor and 5.86µm pixels make the most of the big prism, and it's the one we suggest for long focal lengths.
  • ASI220MM Mini: very sensitive, a good all-round choice at a lower price.
• The ASI461MM Pro and the ZWO 7 × 50mm EFW: we don't list these online, so contact us about ordering them.
• ASIAIR Plus: runs guiding, the camera, filter wheel and mount from your phone.

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

See all autoguiders for astrophotography.

What's in the box

• ZWO OAG-L-68 off-axis guider with built-in helical guide camera focuser and 5mm M68 sensor tilt plate
• Mounting screws and hex key

The guide camera, filter wheel and main camera are sold separately.

ZWO OAG-L-68 FAQ

OAG-L-68 or OAG-L?

They share the same prism and guide aperture. The OAG-L-68 has a square opening, an M68 tilt plate and a bolt pattern for ZWO's 50mm square filter wheel, to suit the ASI461MM Pro's medium-format sensor. For any APS-C or full-frame camera with a 2" or 36mm wheel, choose the OAG-L.

Which guide camera should I use?

ZWO's ASI Mini cameras fit the guide focuser. The ASI174MM Mini has the largest sensor and suits the large prism best. The ASI220MM Mini is very sensitive at a lower price.

Will it fit my telescope?

The tilt plate has an M68 × 1 thread, which fits many large-format flatteners and reducers. For other threads you'll need a matching adapter or a different tilt plate. Ask us before you order if you're not sure.

Will the prism shadow my main camera?

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

Is it hard to focus the guide camera?

Do it once in daylight. Focus the main camera on a distant object, then adjust the OAG-L-68's helical focuser until the guide camera is sharp too, and lock it.

Specifications