Few amateur astronomers can have not noticed a new type of telescope mount called a Harmonic Drive (or Strainwave) mount. They are very light and portable compared to standard equatorial mounts and do not need counterweights except for the heaviest telescopes. Their cost lies between that of typical (less than £1,500) and premium (more than £4,000) equatorial mounts and range from £1,665 for the ZWO AM3 up to £3,395 for the iOptron HEM44EC.
They are built around Strainwave gears which have zero backlash and very high torque – which is why counterweights are not usually needed. This is because there are always many the teeth within the gear in mesh – as opposed to perhaps 4 in a worm drive. The drive gears have three main parts: an outer fixed ring with internal teeth, a flexible ring (flexispline) inside with, typically, two fewer teeth than the fixed ring – these face outwards to engage with the teeth on the inside of the outer ring. The elliptical wave generator is abutted into the flexispline. As it rotates within the flexible ring, it forces the teeth on opposing sides of the ring into the outer gear causing the flexispline to turn at a very slow rate – giving very high reduction ratio. The rear of the flexispline is, however, very rigid and coupled to the RA or Dec axes. [It rotates in the opposite direction to the input drive. They have reduction ratios in the range 100-150. Most Astronomy systems used have a x300 or x500 total reduction with a belt drive from the stepper motor providing the remaining reduction.
They do have one downside: their periodic error is typically a few times larger than a good equatorial mount, perhaps up to +/- 20 arc seconds. Sadly, the period error is not constant so periodic error correction is not feasible which means that, for astrophotography, they are usually autoguided to bring their tracking down to second or sub arc second levels. However, the most expensive in the partial list of those available below has encoders as part of the RA drive to automatically correct for the periodic error so can easily be used for unguided astroimaging.
They can be used in either Equatorial or, for visual observing, in ALT/AZ mode without then needing to polar align. The manufactures make great play of the fact that they can be used without counterweights but I do not like the fact that mount/telescope can become very unbalanced and I, along with many others, do use counterweights except with the lightest loads.
Some common Harmonic Drives.
ZWO AM3 £1,665 Head weight 3.9 kg Load, 8kg – with counterweight 13kg
ZWO AM5 £2,260 Head weight 5 kg Load 13kg – with counterweight 20kg
Pegasus NYX-101 £2,995 Head weight 6.5 kg Load 20kg – with counterweight 30kg
iOptron HEM44EC £3,395 Head weight 6.2 kg Load 20kg – with counterweight 25kg
Note 1: iOptron mounts have GOTO nova hand controllers but can use Apps. Others require a laptop program or smartphone or tablet. The Pegasus NYX-101 now has a hand controller.
Note 2: All will require a tripod. These are usually made of carbon fibre costing £275 for the iOptron and £308 for the ZWO tripods. One is sometimes warned that, with heaviest loads, a more sturdy tripod might well be needed.
Note 3: If, for example, a long focal length refractor was to be used, it can be wise to purchase a tripod extension which lifts up the mount head and make clashes with the tripod legs unlikely.
My tripod
Many years ago, I happen to have bought a wonderful ash Berlabach tripod (now costing ~£400) which, modified slightly, mated with my Pegasus drive. It is highly stable and could support any load that the mount could handle. [It could be better to purchase one of these rather than the suggested lightweight Carbon-Fibre tripods.]
Bending Moment
If one multiplies the mass of the tube assembly in kg by the distance from the RA axis to the centreline of the telescope in metres one gets a measure of the bending moment in Newton-metres. As an example, that of my inch f/4 astrograph is around 5 kg x 0.1 metres giving a bending moment of 0.5 kg – quite low. But if, say, an 11 inch Schmidt-Cassegrain were employed with values of 13 kg x 0.15 m one gets ~2 Newton-metres. This might even cause a lightweight tripod to topple over with disastrous results. So, my advice is to use a counterweight to partly balance this out and perhaps add some wrights suspended below the tripod centre line to make the tripod more stable.
The Pegasus NYX-101
Probably the most common mount currently sold is the ZWO AM5 which has a very good performance for its price. To be honest, particularly if you have a ZWO camera and so can use an ASI AIR there is no reason not to buy an AM5. Its performance is identical to the mount that I did buy. As I do not have a ZWO camera so (so “locked” out of the ASI AIR), I was less compelled to go in this direction and chose the Pegasus NPX-101 (designed and built in Greece) as it has a larger load capacity than the AM5 and has some nice features such as a mounting point for my QHY Polemaster polar alignment camera and can be driven by an excellent app, hand set or laptop program. The 12 volt supply is, pleasingly, through a GX12-2 screw on coupling. It measures the atmospheric pressure and temperature so can provide a tracking compensation for atmospheric refraction al lower elevations. It includes an electronic compass to find true north and sensors to level the mount – nice. Its Swiss RA harmonic drive has a high 500 to 1 reduction using a larger gear than the others – which is why it can support larger loads. It comes is a beautiful armoured case. Along with most such mounts, it has a RA, power off, brake to prevent the telescope swinging rapidly down in the event of power loss.
There were two non-technical reasons I came to like it. I think that it is, perhaps, the most beautifully crafted Harmonic drive on the market and I was pleased to be able to purchase a European designed and manufactured mount.
It has not been on the market for as long as the AM5 and there are only a couple of video reviews. I hope that this will change and the NTX-101 is seen to be one of the very best harmonic drive mounts available – as I honestly believe it to be. An unguided imaging experiment described below indicates that its periodic error may well be less than others – which cannot be bad and will help guided exposures – or, even as I have shown, unguided ones.
The laptop control screen allows one to slew at differing rates in 9 directions and to the home position. It also allows for a ‘spiral search’. Pleasingly, within ‘settings’ one can choose what will happen as a source reaches the meridian. It can automatically do a meridian flip or, great for astroimaging, can travel past the meridian for up to 2 hours – providing, of course, that the telescope can clear the tripod.
The laptop screen does not give access to source catalogues and Stellarium is used select objects. However when using a tablet to control the mount, a full range of catalogues is provided. [ password 12345678]
Other software
I use Stellarium to select the source to be observed.
Sharpcap Pro to use plate solving to help find faint objects and centre them in the field of view.
PHD2 for autoguiding.
Sharpcap Pro for image capture.
Unguided imaging.
Unguided astroimaging is, in principle, a no-no with harmonic mounts as their periodic error – perhaps up to +/- 20 arc seconds – is very high as compared to my Astro-Physics equatorial mount which has just +/- 3.5 arc seconds periodic error. Though not recommended, I do believe that it might be just possible with this mount (and perhaps others). A typical Pegasus periodic error graph shows that its cycle is ~7 minutes long and is actually only +/- 10 arc seconds). Pegasus state that it changes rapidly for 2 minutes, is then relatively stable for 3 minutes before changing rapidly again. It is then theoretically possible that, if taking unguided 30 second exposures, those taken during the
‘stable’ period would be acceptable but those when the change is rapid will need to be rejected. Happily, the ASTAP stacking program will analyse all the captured frames and give a HPBW for the stars in each image. One can then untick those where the tracking is poor before stacking the remainder. Another good program to use is SharpCap Pro to ‘Live Stack’ a captured file. As the frames are analysed, their angular size is shown on a graph. Having reviewed all the frames, a filter can be set to eliminate those which are poor. This does, of course, reduces the time collecting photons from the sky.
Test Image
The image is a composite using a Micro 4/3 sensor of a star field and insert of the heart of Orion taken at full Moon. Both were unguided and no frames had to be discarded. This does show that the mount is capable of carrying out unguided astroimaging
