For those who are not familiar with mounts that use Strain Wave gears (trademarked as Harmonic Drive[tm] by the original patent holder), they are not new. Hobym (Korea) sold one of the first commercial hobby mounts based on strain wave gears, RainbowRobotics (manufacturer of Robotics arms) created a subsidiary called RainbowAstro, just to sell mounts that are based on strain wave gears, SS One (Japan) had a low volume production mount, etc.
The strain wave gears have been used in many applications that benefit from high torque. The NASA Mars Rover wheels are driven by motors which are connected with Strain Wave gears.
https://en.wikipedia.org/wiki/Strain_wave_gearing
The new comers to using Stain Wave gears (the ads call them called "Harmonic Drive" even though they may not be built with gears manufactured by the US company -- we don't know yet) are SharpStar (Sharpstar Mark III) and ZWO (AM5).
In the past, the gears have been very expensive since only Harmonic Drive (and a Japanese licensee) could sell them. The gears for a single axis alone in the RainbowAstro RST-135 is close to about $1000 US.
The main attractiveness of strain wave gearing is that the gear ratios are quite high (even reaching 200:1) which makes then a good replacement for worm gears. Worm gears, even with the large gear ratio, have terrible torque since there is only a single tooth of the worm that is in touch with the spur gear. With a strain wave gear, you typically have a dozen teeth that are meshed.
As a result, you can drive very heavy payloads without the use of a counterweight. As such, they are perfect for travel, since not only do you not have to carry a counterweight and counterweight shaft, the mount itself is small and light. The RainbowAstro RST-135 itself weighs 3.3 kg (not a typo; three point three kilos), while being able to toss around a 13.5 kg payload like it is a rag doll. The larger RainbowAstro RST-300 weighs 8.5 kg, and it can carry a payload of 30 kg. The weight ratio of the ZWO mount is not as good (they probably use rougher manufacturing).
If you add a small counterweight, the strain wave gear mounts can take even higher payloads. The payload capacity of the RST-135 for example goes up from 13.5 kg to 18 kg when you add a small counterweight. Most of the time, the counterweight is necessary not because the high torque gears cannot handle it, but because the tripod would topple over without the counterweights!
Again, for people who are not familiar with strain wave geared mounts, not only do you not have to balance the mount, you cannot balance the mount. The RST-135 uses a 100:1 ratio gear and it is impossible to move the RA and declination axis by hand. The only way to do it is to apply power and control the motors electronically.
Another good part with having all that torque is you can easily use a dual saddle plate (I do) and not have to worry about the "3rd axis" balance problem.
The bad part (for astronomy use) is that they have very large period error (they were not invented for that purpose). The worst case for the RST-135 is about 70 arc seconds peak to peak -- most people see 40 to 60 peak to peak arc seconds on their mounts (while even a small legacy German mount as the Takahashi EM-11 has a peak-to-peak error of just 7 arc seconds). Looking at the specs, the ZWO AM5 also has a high PE. The graph shown in ZWOs product page claims 40 arc seconds peak-to-peak (they call it +/- 20 arc seconds, but most mounts specify peak-to-peak, since the PE may not be symmetrical.
In fact, the error graph shown in the ZWO product page shows a highly non-sinudoidal curve, together with a slower drift term. The drift term probably make it impossible to have a Periodic Error Correction -- PEC. We shall see if I am right, i.e., if ZWO can create a PEC for their mount.
(It looks like the ZWO AM5 may need to use very fast autoguiding feedback loops to counter the periodic high slopes, perhaps even faster than the 2 FPS that I use for my RST-135.)
The periodic error of the RST-135 is very sinudoidal, with a small 3rd harmonic term. The ZWO is quite different, I include ZWO's curve from the AM5 product page here:
The reason I bring this up is that one of the ability to guide well is not really the error, but the slope (first derivative) of the error. That is what guide pulses will need to keep up with. When a curve is not sinusoidal, the slope of parts of the curve is unnecessarily large. The reason why an Avalon M-series mount can be guided so well even though it too has a large error is that the curve is very smooth (i.e., small slope) (The Avalons do not use strain wave gears.)
Already, you can see from the above graph that the three cycles of the periodic error are not the same even though they immediately follow one another. To do good PEC, all cycles (except for noise term) will need to be identical.
I currently use two RainbowAstro RST-135. Bought my first one (serial number 13 from the first production run) in the Spring of 2019. Both are in use. In fact, one of them is outdoors all the time under just dry bags, in rain and snow. There is no grease to freeze or melt -- I don't know if ZWO's strain wave gears need grease.
If you just have a home observatory, with no need to travel, it may not be worth the addition cost of the strain wave gears. However, once you are spoilt by the light weight and the lack of need to balance, you will never want to use the legacy German mounts again. I bought a second RST-135 not just because I wanted two mounts, but I could not see myself using my EM-11 again, in case the first RST-135 fails.
Chen
