Discovery Astrophotography with ZWO ASTRO

10 things you need to know about the custom AM5’s PE Test Report provided by ZWO

1. Why doesn’t the ZWO AM5 Mount need a counterweight?

The AM5 is different from traditional equatorial mounts that are driven by worm gears and require counterweights to balance.  The AM5 adopts a high-torque strain wave gear reducer customized for astrophotography.  There is no need to balance the mount or use a counterweight.  Just install the telescope on the mount, start imaging simply and quickly.

Strain wave gear reducers are widely used in industrial applications.  The pressure per unit area is relatively small, while the whole load capacity is relatively large.  Even without a counterweight, the AM5 can easily lift a weight up to 13kg.

Strain wave gear -2


2. How is the stability of the AM5 without a counterweight?

We understand people who are experienced astrophotographers with traditional mounts may have a cautious attitude towards the AM5 because it does not need a counterweight.  The benefits of losing the weight of counterweight are obvious – the weight of the entire setup is reduced and there is no need to carry a heavy counterweight all the way out to a remote location for astrophotography.  The AM5 works out of the box!

Without a doubt, the heavier the gear is, the more stable it will be.  But instead of only pursuing weight, normally astrophotographers have to face one question.  “How do I keep the balance between weight, mechanical precision and personal strength?” The AM5 itself is able to keep good stability and precision as long as the load is no larger than 13kg when no counterweight is attached.  The included weight sling comes with the TC40 tripod to further increase the stability.  Portable batteries, buckets, stones or anything else you can find can all be put in the weight sling to keep the setup as stable as possible.

weight bag

3. What is the difference between the AM5’s strain wave gear reducer and the ordinary ones?

There are many kinds of strain wave gear reducers used in the industrial field on the market, but they cannot be directly used for astrophotography when it comes to accuracy.  Just like cameras, there are so many industrial cameras available, but we cannot directly use one of them to do planetary and deep space imaging.  Thus, we developed astronomical dedicated cameras.

ZWO has been deeply involved in the field of astrophotography for over 10 years. The company was founded based on the passion for astrophotography, so we have a clear view of amateur astrophotographers’ points of concern.  We need the strain wave gear reducers that have small periodic errors, large transfer ratio, large torque, small size, are lightweight, and can also be used in regions around the world with various temperatures.  Among these strict requirements, the most important one is to keep the periodic error in the reasonable range of +/-20 arc seconds, which is a maximum total of 40 arc seconds.  This is very hard to achieve for industrial strain wave gear reducers.

ZWO worked together with a well-known and professional manufacturer in China to develop the strain wave gear reducer dedicated for astrophotography, making sure the guiding accuracy of each mount keeps stable at 0.5″ ~ 0.8″, which is fine enough for most amateur astrophotgraphers’ telescopes and their seeing conditions.

astronomy dedicated strain wave gear

astronomy dedicated strain wave gear PE report vs industrial strain wave gear PE report

4. How does ZWO ensure the quality of their strain wave gear reducer?

To ensure quality, we work hard on the test procedure.  Normally, companies producing mounts also have reports on their reducers’ partial periodic errors.  At the very beginning, ZWO had the same thought as them and create a report on 1~2 single periodic errors of the mount which would be satisfactory.  However, after deep research from our development team, we found the periodic errors of a strain wave gear mount are different from that of a worm gear mount.  When we say PERIODIC ERRORS of a strain wave gear, the errors are actually not that “PERIODIC”.  It seems the error of each gear is different from another.  Such a discovery surprised us, and we decided to test the full periodic errors of each mount.

all kinds of periodic errors

All kinds of periodic errors

A full period goes 360° in one reducer rotation in 24 hours.  To scrutinize the full periodic errors of each mount, it can be a very laborious task.  But we eventually did it.  Now when you receive your AM5 package, a custom report will be found in the box, giving a clear view of the reducer of your mount.

5. Where can I find the PE report of my mount?

As previously mentioned, the paper PE report is found in the AM5 box with the mount’s Quick Guide.  Scan the QR code on the report, there will be an online testing report to download and share with friends.  The reports will be stored in ZWO’s server permanently.

PE report in the box

6. Can you explain the PE report?

ZWO sets a standard for each AM5 mount that its maximum periodic error must lie within the range of +/-20 arc seconds, but due to the workmanship difference, which is basically unavoidable, each mount has its own unique periodic error curves, bringing different maximum periodic error values. Some users might find the maximum periodic error of their AM5 mount is extremely small compared to others.  But as long as the total periodic error is under 40 arc seconds, the AM5 is within our acceptable range, and you can also easily get a good guiding accuracy between 0.5″ ~ 0.8″ with a 1-second guide exposure.

The smaller the periodic error is, the better tracking accuracy the mount can achieve without guiding.

Please view the reports below, the left report shows a maximum periodic error of 35.9 arc seconds with an approximate tracking accuracy of 0.21”/s.

0.21″/s is the mount deviating 0.21 arc seconds per second when tracking the object. If guiding is enabled, it can be also considered as the theoretical best guiding accuracy the mount can get under maximum periodic error, assuming effects of seeing and other conditions are not taken into consideration and a 1 second guide exposure* is used.

The report on the right shows a smaller maximum periodic error of 15.9 arc seconds and therefore a better tracking accuracy of 0.074″/s.  Such high accuracy allows the mount to reach better theoretical guiding accuracy.

That being said, we all know that the theoretical best guiding accuracy is almost impossible to achieve due to seeing, winds, and all other aspects which would affect the mount’s final guiding performance.  Especially when looking at the seeing and the guiding accuracy of a mount, the seeing greatly affects the final guiding performance.

Periodic error reports_AM5 

7. How to understand the full periodic error chart and partial zoom of the max periodic error chart?

The full periodic error chart represents the errors the strain wave gear reducer produced in different positions during its whole operating period at 360° of one reducer rotation. Looking at the two reports above as examples, the left one has a maximum PE of 35.9 arc-second and a minimum PE of 25.3 arc-second. The right one has a maximum PE of 15.9 arc-second and a minimum PE of 3.8 arc-second.

To test the periodic error of one strain wave gear mount, we must not just test one certain position. Obviously if we do so, the result we get will be very limited.

larger max periodic error vs smaller max periodic error

The partial zoom of the max periodic error chart selects one single period with maximum error from all periods.  When zoomed in, it can be seen how quickly the error changes and if it is small enough, the best theoretical guiding accuracy is achieved.

Periodic error reports_AM5-1

The duration from the low to high points in the left chart is 172.8 seconds, so with the formula we can calculate the maximum error per second of the reducer, which is 0.21″.  Assuming 1 second guide exposure, which is correcting the error per second, even if the error can be corrected, there will still be a deviation of 0.21″.  Considering all other factors during imaging, we can ensure the guiding accuracy is between 0.5″ ~ 0.8″.

The smaller the periodic error is, the better the guiding results.  But as mentioned above, seeing is an important factor that can not be ignored.  Seeing makes it very difficult for us to get an even better guiding accuracy smaller than 0.5″.


8. Why do you give a PE test report with each AM5 mount?

The uncertainty and irregularity of the periodic error of strain wave gear make it necessary to carry out a full-period test for each gear to ensure its maximum error falls in the range of +/-20 arc seconds.  No testing or only testing of single periods instead of the full period may allow unqualified products to come into the market.  It also gives very bad experiences to our customers.

See the sample below, in some periods, the error can be as low as 30.9 arc-seconds, while in some periods, the error reaches 92 arc-seconds. Obviously, it is not up to the standard.

Unqualified case


9. What relations do periodic error, guiding accuracy and star sizes have?

The period error has effects on guiding accuracy. The guiding accuracy determines whether the star can be round enough. In most cases, you won’t get star trailing in your images if the guiding accuracy is less than 1/4 of the star size.  Take a look at the average star size in your images, then you shall know how much guide accuracy you need.

For example, if the guiding accuracy you get is 0.5″, then it can basically support a star size around 2″ and above.

One of the most important influencing factors of star size is seeing. The better the seeing is and the larger aperture your telescope has, the smaller the star size you can get, which as a result giving higher requirements for the guiding accuracy.

star size 

10. Why isn’t the ZWO AM5 integrated with PEC?

PEC represents periodic error correction.  It records the error curve in some periods, then plays it back when photographing an object to allow the mount to correct errors.  In the past, most worm gear mounts have the PEC function.  PEC helps a lot for astrophotography without guiding. But since guiding is used while imaging deep space objects, PEC is not a required function.  Also, as we mentioned above, when we say PERIODIC ERRORS of a strain wave gear, the errors are actually not that “PERIODIC”. The changes are irregular, so it cannot help us correct errors by playing it back.

Irregular PE curve


There are no two identical leaves in the world, nor are there two identical strain wave gear reducer error curves.  The AM5 PE error curve received by each AM5 user is unique and guaranteed to be qualified by us. Enjoy your astrophotography journey with the AM5, and feel free to ask us any questions.  We are always glad to help!


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