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Fine focus for a guide camera on the ZWO OAG
Why we like the ZWO helical focuser:
- Made for the standard ZWO OAG: it threads onto the OAG's M42 × 0.75 guide port, and a ZWO Mini guide camera slides into its 1.25" holder. ZWO sells the OAG without one.
- The camera doesn't spin: turning the knurled ring moves the camera in and out without rotating it, so the cable stays put and the guide star stays where you left it.
- Set it once: a 0.1mm scale and a brass lock screw let you find focus in daylight, lock it and leave it for the season.
- Worth knowing: it gives 6mm of travel, enough to match the guide camera to the main camera's focus, not to fix a big spacing error. The OAG-L and the 30F5 guide scope already have their own focusers.
The ZWO 1.25" Helical Focuser (HF125) is a small non-rotating focuser that sits between an off-axis guider and a guide camera. ZWO designed it for the ZWO OAG with its ASI Mini cameras, such as the ASI120MM Mini, ASI220MM Mini and ASI174MM Mini. It's machined from aluminum, stands 48.5mm (1.9") tall at its shortest and weighs about 80 to 100g (2.8 to 3.5oz).
Key specs: 1.25" (31.75mm) holder · 6mm travel · 0.1mm scale · M42 × 0.75 male top and female bottom threads · 48.5mm minimum height · 46mm diameter · 80–100g
How it works
With an off-axis guider, the guide camera has to sit at the same optical distance from the prism as the main camera's sensor, or the guide stars stay blurry. The helical focuser lets you make that last adjustment.
- Thread the focuser onto the OAG's M42 guide port.
- Slide a 1.25" Mini guide camera into the holder and tighten the thumbscrews.
- In daylight, focus the main camera on something distant. Then turn the focuser's ring until the guide camera is sharp too.
- Tighten the brass lock screw and note the number on the scale.
ZWO notes a focusing tolerance of ±0.3mm, so close is good enough for guiding. A slightly soft guide star often guides better than a pinpoint one.
What to add for the best results
- ZWO OAG off-axis guider: the guider this focuser was designed for.
- A guide camera (required): the ASI220MM Mini is very sensitive and the one we suggest for most setups. The ASI174MM Mini has a larger sensor, and the ASI120MM Mini is the budget choice.
- ASIAIR Plus: runs guiding and your camera from a phone or tablet.
Guide focusing options
| ZWO OAG + HF125 | ZWO OAG-L | ZWO 30F5 guide scope | |
|---|---|---|---|
| Guide focuser | This helical focuser, added | Built in | Built in |
| Guides through | Main telescope | Main telescope | Separate 30mm scope |
| Main camera | APS-C or smaller | APS-C and full frame | Any |
| Best for | Compact OAG trains on 4/3" and APS-C cameras | Large sensors at long focal lengths | Shorter focal lengths and simple setups |
See all autoguiders for astrophotography.
What's in the box
- ZWO 1.25" helical focuser with brass lock screw and 1.25" holder thumbscrews
The OAG and guide camera are sold separately.
ZWO 1.25" Helical Focuser FAQ
Do I need this with the ZWO OAG?
We recommend it. Without it, you focus the guide camera by sliding it in and out of the OAG's 1.25" holder, which is hard to do precisely and easy to bump. The OAG-L and OAG-L-68 already have a focuser built in.
Will it fit non-ZWO guide cameras?
Any guide camera with a 1.25" barrel fits the holder. ZWO notes that it designed the focuser for its Mini cameras on an OAG, so travel and spacing are set up for those.
Does the camera rotate when I focus?
No. Only the ring turns, and the holder moves straight in and out.
How much travel does it have?
About 6mm. If you can't reach focus within that range, adjust the OAG's prism depth or the spacing of the main imaging train first.
Specifications
| Mechanical | |
|---|---|
| Inner diameter | 1.25" (31.75mm), with thumbscrews |
| Travel | 6mm |
| Scale | 0.1mm divisions |
| Focusing tolerance (ZWO) | ±0.3mm |
| Top thread | M42 × 0.75 male |
| Bottom thread | M42 × 0.75 female |
| Minimum height | 48.5mm (1.9 in) |
| Diameter | 46mm |
| Weight | 80–100g (2.8–3.5oz) |
| Material | Aerospace aluminum alloy, CNC machined, anodized; brass lock screw |
| Warranty | 2 years (ZWO) |
| Model / SKU | HF125 / ZWO-HF125 |