Innovative manufacturers have been offering astronomical mounts with a special drive design for a few years now. You can find these in the shop under the Harmonic / Strainwave link.

These mounts are equipped with a so-called strain wave gear. It is also known under the protected brand name Harmonic Drive.

The advantages are:

• High load capacity with low dead weight
• High torsional rigidity: no counterweights required
• Relatively inexpensive, as industry standard
• No swivelling error (“backlash”)
• Very high reduction ratio
• Compact dimensions
• Maintenance-free and backlash-free in the long term

The strain wave gear is used for the axes of industrial robots, among other things. It is therefore a standard and does not need to be specially developed and built for astronomical mounts.

Although the periodic gearbox error is also high for strain wave gears, the error typical of worm shafts, which occurs when the direction of rotation changes (“backlash”), is eliminated. This means that the strain wave gear reacts immediately to the control impulses, so that the periodic error can be compensated very well by autoguiding.

For large mounts, such as the Skywatcher EQ-8, the mounts from 10Micron or the CGX-L from Celestron, a worm shaft is still the first choice, because optics with a long focal length require a very low periodic gear error and a counterweight cannot be dispensed with for heavy optics, even with a strain wave gear. In addition, the advantages of strain wave gears in terms of compactness, weight and price are lost as soon as a mount with a high load capacity is required.

There is therefore an ideal range of load capacity in which mounts with a strain wave gear can play to their strengths. The requirements for mobile astrophotography with small to medium-sized optics fall within this range. The harmonic mounts from brands like iOptron, PegasusAstro, RainbowAstro and ZWO are designed precisely for these requirements.

You can find all mounts with strain wave gear under the Harmonic / Strainwave link in the shop.

Typical strain wave gear to industry standard