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  • Mount Periodic Error Correction

    Reggie Jones

    Mount Periodic Error Correction is a common feature for motorized Go-To mounts but to someone new to using one of these mounts, it might not be readily understood what it is.  If you are a new user of a mount with this feature, hopefully, I’ll try to provide some useful information that helps you understand what this feature is and how to use it.

    Just about every Go-To mount have transmission gears to move the mount in the 2 axis we’re concerned about in astronomy and astro imaging - declination and right ascension.  The transmission gears used to do this are either toothed gearboxes, belt drives or worm gears.  Even with the tight manufacturing tolerances these mounts are constructed with, they aren’t absolutely perfect and contain a small amount of error in them.  Since these gears rotate, the error in the gear rotates around and causes changes in the angle of rotation at the same point; as a result, the error repeats with each rotation at the same point and hence it is called the periodic error.

    Periodic error is related to the manufacturing accuracy of the mount.  The manufacturing accuracy of the mount is also unfortunately related to the price of the mount; the more expensive the mount, the smaller the periodic error.  As a general rule, entry level mounts have a periodic error somewhere between 30 arc seconds and as much as 100 arc seconds of error.  Intermediate level mounts will have a periodic error of 30 arc seconds or less.  Prime, top level advanced mounts like the Software Bisque Paramount MEII priced at $17K have a periodic error listed in their specifications of 7 arc seconds or less.

    The period of the error usually equals the worm gear rotation period which is around 600 seconds or 10 minutes.  For an intermediate mount, 30 arc seconds may not seem like a lot but if you’re imaging a target such as the Orion Nebula with a long focal length telescope, the error could be a significant issue, especially if the exposure time is longer that a couple of minutes.

    To combat this type of tracking error, there are 2 tools to use - Periodic Error Correction which is available on intermediate and advanced mounts and Guiding.  Periodic Error Correction measures the error on your mount over time to calculate a Periodic Error Curve.  With this data, the mount will make corrections to the tracking of the mount to compensate for the error before it happens.  Guiding many of us are already familiar with; it too will compensate for the periodic error but the problem here is that guiding will correct for the error only after it has happened.  To really compensate for the periodic error of your mount, you should use both periodic error correction and guiding.

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