ZWO AM5 Strain Wave Mount Review — Loran Hughes Astrophotography

Note: I bought the ZWO AM5 for personal use soon after pre-orders opened in December 2021 and received it in July 2022. This is totally on my dime.

The ZWO AM5 is in a new breed of portable astrophotography mounts that don’t require balancing or counterweights (within limits). Strain wave drives are traditionally used in industrial applications and robotics. These are also commonly known as Harmonic Drives® (a registered trademark of Harmonic Drive, LLC). This video provides a great explanation of how strain wave drives work:

Harmonic Drives Explained

When Is Periodic Error Not Periodic Error?

The AM5 Periodic Error Report For My Mount

Strain wave drives are different. Really different. They have no backlash and error isn’t periodic, so you can’t PEC train the mount. ZWO provides a “periodic error” report with each mount, but the numbers really aren’t that important. The report simply means that the mount met ZWO’s specifications for astrophotography. There is currently no encoder version of the AM5, so forget unguided imaging - no matter how good you get the polar alignment.

According to ZWO, the AM5 will work best at focal lengths under 900mm with short, 1 second guide pulses. However, I’ve seen images taken with a Celestron 8” EdgeHD on the AM5 that look great. I’m currently shooting with a William Optics GT-81 with 0.8x reducer/flattener (approximate focal length of 382mm) and I’m getting round stars in 10 minute subframes. Do not let these periodic error reports scare you!

Phantom Backlash?

PHD2’s Guiding Assistant measured an average DEC backlash of 2.5 seconds on my AM5, but I’m not convinced this is a true measurement for this type of mount. Between the strain wave gear reduction and the synchronous belt drive, backlash should be quite low.

According to this post, PHD2 may be measuring a backlash that isn’t really there.

Internals

I was hoping someone would do a tear-down video and behold, someone did! It’s in Russian, but a picture is worth a thousand words in any language. You can see that each stepper motor and strain wave gear is a single unit, with synchronous belts driving the RA and DEC.

The Important Stuff

I’m not going to list all the specifications for the AM5. You can find those here. These are the things you need to know.

It’s light and robust. The mount weights just 5.5 kg (12.1 lbs). The build is solid and doesn’t feel cheap.
The AM5 can handle a payload of up to 20 kg (44 lbs), although you’ll want to use a counterweight with loads over 13 kg (28.6 lbs).
The carbon fiber tripod is very light and surprisingly stable. It weighs just 2.3 kg (5 lbs) and has a load capacity of 50 kg (110 lbs)! A counterweight bag for stability is included.
The tripod is short and I mean kid-short. You’ll want the optional pier extension to bring your scope up to a normal height and to keep longer scopes from hitting the legs.
The AM5 can operate in either equatorial or alt-az mode.
You don’t need to balance your payload, nor is there any way to really do that. The AM5 just doesn’t care.
Out of the box support in the ASIAIR and ASCOM for Windows.
It’s about as portable as you can get for a DSO astrophotography rig! The AM5, tripod, and pier extension come in under 10 kg (22 lbs).

AM5 and Tripod Packed in Bags — The tripod doesn’t come with a case, but I found one here that fits the tripod plus pier extension. There’s plenty of room left over for snacks, too.

Quirks

First and foremost, make sure you have a stable power supply to the mount. It will stop tracking if the voltage drops somewhat below 11.5 volts and relentlessly “beep” its displeasure (the beep somehow sounds louder in the middle of the night).

The AM5 does not currently track past the meridian, so plan your meridian flips accordingly. You may need to stop imaging for a few minutes, wait for your target to move past the meridian, then flip.

For guiding, you’ll need to use short guide pulses - on the order of one second.

If you are using the ASCOM driver and N.I.N.A., the left/right azimuth adjustments during the 3-point polar alignment routine are reversed. For example, if N.I.N.A. tells you to adjust the azimuth left, you’ll need to loosen the left knob and turn the right.

The Proof Is In The Pudding

All of this is interesting, but how well does it perform in real life?

For my first test run, this 4-panel mosaic of the Cygnus Loop was taken with the AM5 and William Optics GT-81, as seen above. I was able to achieve 10-minute subframes and maintain round stars.

Finally, the Lobster and Bubble nebulas, shot with 6-minute subframes.

Conclusion

The ZWO AM5 fits several use cases. It’s light, portable, and more than capable for astrophotography. I can envision this being used for outreach, camping, or as a primary mount at home. Overall, I’m very satisfied with this mount!