What is Periodic Error Correction and How to Use It

While astrophotographers do their best to set up their equipment for perfect alignment, there are mechanical issues that are inevitable during long exposure imaging like periodic error.  

What is Periodic Error

Periodic error is an error that happens in the right ascension of your mount’s worm gear during deep space imaging. It is called periodic error because it happens in rotation every few minutes or so.  

How to Fix Periodic Error

Periodic Error is fixable however so read along as we overview how you can fix it. You can counteract the error using computer-controlled mounts in three general ways: 

  • Using the PEC (Periodic Error Correction) option on your hand controller if it is included with your mount  
  • Autoguiding to allow your mount to make these adjustments automatically 
  • PEC tools like astronomy software  

periodic error correction

How to Achieve Periodic Error Correction with a Hand Controller 

  1. When using your hand controller for PEC, first index or reference a point on the worm gear.  
  2. Guide your scope for at least a full rotation of the worm gear. Your mount will record the corrections you are making and memorize them in the PEC function of your hand controller. 
  3. Once you have trained your scope for a full rotation, start using your telescope and playback the recorded training to correct any errors missed. 

Note: If you are planning to use autoguiding, you won’t need to worry about using your hand controller for PEC as autoguiding will do these corrections automatically.   

How to Achieve Periodic Error Correction with Autoguiding  

Start by autoguiding your set up to track a star so your mount can see where it is tracking slowly or too fast. Your mount will memorize this and compensate by speeding up or slowing down.  

Autoguiding works by using a guide camera to watch a star, and if that star begins to drift by even a pixel, it can communicate with your tracking mount to make the necessary corrections. 

How to Achieve Periodic Error Connection with Tools  

Astronomy software can help you keep a record of more details of the Periodic Error Correction for your mount like Celestron’s PEC Tool. Celestron’s PEC Tool can record multiple cycles of your worm gear, obtain the average course corrections for optimal use, and upload it to your mount.  

What is the best way you have found to achieve Periodic Error Correction? Let us know in the comments below. 

New to astrophotography? Check out our tips on how to get started in astrophotography to learn more. 


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