Conlin Hill Observatory -- 42 7' N, 71 54' W

Techniques - PEC Training  

What's PEC?

(P)eriodic (E)rror (C)orrection is a technique where keystrokes or guiding inputs are used to teach the mount electronics to compensate for mechanical imperfections in the mount. There are many sources of periodic error, but LX200 PEC training targets the largest source, the worm gear that drives the RA tracking. The LX200 worm takes 8 minutes to perform one full revolution, so PEC training lasts for 8 minutes. Multiple 8 minute training sessions can be made, with the LX200 averaging the input values over the 8 minute period.

Manual PEC Training

PEC training while looking through a reticle eyepiece at the telescope for eight minutes at a time isn't much fun. Instead I used my SAC7 CCD, taking .1 second exposures at high magnification and keeping the star in the on-screen reticle that comes with the AstroVideo software. This allowed me to sit back in a deck chair inside the observatory and comfortably train the worm. Here's a screen shot showing how this looked at the computer, with the training star slightly below the center of the cross-hairs:

Manual PEC Training: Results

KNOW THY EQUIPMENT. Gathering and analyzing lots of data is a really good way to learn about your equipment and it's imperfections. It's also a great way to measure the effectiveness of your efforts to improve things - AND it gives you plenty to do when the moon is bright ;)

I trained PEC manually and measured the amount of error exhibited by my system. To take the measurements, I took a series of 120 10 second images of a star. I then logged the position (in pixels) of the centroid of the star. Finally I converted pixels to ArcSeconds and plotted them as seen in these charts. I developed a handy spreadsheet which acts as a form for gathering data, and which performs all the calculations and generates the graphs. Feel free to email me if you would like a copy.

Automated PEC Training

PEC can also be trained using guiding inputs from an autoguider. The 201XT autoguider works well for this task. Pointing at a magnitude 2-3 star, the 201XT will track with .1 second exposures. By dropping the LX200 into train mode after the 201XT has started guiding, the guiding input from the autoguider is used to perform precise PEC training with multiple updates per second for the entire 8 minute training period. As with manual PEC training, multiple training sessions should be performed to average the results. I performed 4 automated PEC update training sessions which lead to much improved tracking.

A Note About Declination

The LX200 also has a facility for training the DEC drive, but I haven't tried it yet and have not used it in any of the tests presented above. All DEC values shown in this data come from an untrained DEC drive. It occurred to me while analyzing this data that the DEC numbers give an indication of the accuracy of polar alignment. DEC was not trained, and I was using stars along the North-South meridian and near a declination of zero degrees. This is a good position to check for DEC drift associated with errors in polar alignment. My values consistantly come in very near ZERO DEC drift over 20 minutes, indicating that my mount is aligned quite accurately to the celestial pole.

However, the graphs also demonstrate a trend - positive spikes in RA have a corresponding positive spike in DEC, likewise negative RA movements have corresponding negative movements in DEC. This means that changes in RA are causing changes in DEC. The most likely culprit is ME! Having the camera oriented in any way other than perfectly orthogonal to the forks will cause this phemonenon. To prove it, I've run the final test again, this time after carefully aligning the camera. Here are the results.

Animations Highlight the Difference PEC Makes!

I used the raw data to produce animations which dramatically show the value of PEC training.

This animation shows how my LX200 follows a star over 20 minutes with no PEC training. Notice how the star oscillates back and forth.

This animation was made after training PEC with the autoguider. Although there is still noticeable oscillation, the distance from center that the star moves is reduced significantly.

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