I am starting a study on automatic telescopes and the correction of their periodic error. I found two ways of correcting it : with a CCD camera and manually by recording the error. I was wondering if it could be possible to model the error by a periodic signal in order to add it to the entry signal then correct it with a low-pass filter or an other filter ?
Quoting the most relevant parts from the Richard McDonald's Wiki on Astrophotography Mounts: Periodic Error Correction:
Periodic Error can be reduced to an acceptable level using a variety of techniques, only some of which are in the range of a beginner or mid-level astrophotographer.
Throw Money at the Problem
Very high-end mounts for astrophotography have very small periodic error because of the time and money spent on manufacturing high-precision gears. You can also buy higher-precision gear upgrades for some mounts. For the beginner, let's call this impractical. Buying a \$1000 replacement worm gear for your \$1000 mount is probably not a good balance.
Don't Use Gears
This is really an extreme case of the "throw money" solution, mentioned just for fun. There are some very-high-end experimental drive systems showing up on the market that don't use worm gears, and that don't exhibit periodic error. Examples include direct drive systems and harmonic drives. 'Way out of our price range.
A second guide camera and computer can be used to make frequent small corrections to the mount's pointing, and this can reduce or eliminate Periodic Error if the onset of the error is not too sudden. This is the subject of a separate article.
Periodic Error Correction Feature
Training PEC with hand-control Most mounts intended for astrophotography include a feature called Periodic Error Correction (PEC), which can be used alone, or in conjunction with Autoguiding. PEC is used in two phases:
- Training. During this phase you use the control panel or menus to say "Hey! Pay attention to this!" to the mount, then you manually keep a star perfectly centred for several worm periods. You do this by centering a star with a high-magnification eyepiece that includes a cross-hair reticle. You then stare at the star for 10 to 15 minutes and use the mount's hand controller to manually make the small adjustments necessary to keep it perfectly centred on the cross hair. The mount records the error corrections you supply, remembering where in the worm position each one was needed. By training through more than one worm cycle, the mount can record an average correction, in case you reacted slowly or over-reacted.
- Playback. Once you have trained the mount, you can turn on PEC. The mount will "play back" the recorded error correction information by slightly changing the speed of the Right Ascension drive to move ahead or back each time your manual error corrections did the same. This will cancel the periodic error, resulting in a smoother track.
Training PEC with hand-control: Periodic Error Correction (PEC) button on the control panel of an equatorial mount.
The manual training phase is quite tedious and a more modern alternative is to use a camera and computer - usually the same one you will use for auto-guiding - to track a star while the mount records the training information.
There is even specialized software available to help collect Periodic Error data, analyze and smooth it, and upload it to the mount. You do not need such software, as the manual techniques mentioned above will work just fine. However, it makes a tedious job simple and pleasant, and you may find it a worthwhile investment. I use PEMPRO and am very impressed with it. It's not free, but it is inexpensive and works very well (and includes another feature to help achieve perfect polar alignment).
Quoted text and photograph source, copyright and courtesy of Richard McDonald, no copyright infringement intended. I would also highly recommend reading same author's extensive article on Autoguiding.