Specifications

Chapter 4

Periodic Error Correction (PEC)

Periodic Error Correction, or PEC, is a system that improves the tracking accuracy of the drive.

PEG is designed to improve photographic quality by reducing the amplitude of the worm gear

errors. Periodic error is a slight oscillation in right ascension caused by imperfections in all drive

gears. The cycle of the periodic error is equal to one rotation of the worm gear, which is four

minutes for the G-11 and

two

minutes for the G-8 and CI-700. No matter how precise, all

telescope drives will have some periodic error. Using the PEC function is a two-step process.

First, you must guide for at least four minutes (two minutes for the G-8 and CI-700), keeping the

guide star centered on the cross hairs of your guiding eyepiece, during which time the system

records the correction you make. It takes the worm gear four minutes to make one complete

revolution, hence the need to guide for four minutes. The second step is to play back the

corrections you made during the recording phase. The microcomputer inside the electronic

console does this automatically after one revolution of the worm gear. Keep in mind that this

feature is for advanced astra-photographers and requires careful guiding. Here's how to use the

PEG function most effectively.

1. Find a bright star relatively close to the object you want to photograph.

2. Insert a high power eyepiece with illuminated cross hairs into your telescope. Orient

the guiding eyepiece cross hairs so that one is parallel to the declination axis while

the other is parallel to the R.A. axis.

3. Center the guide star on the illuminated cross hairs, focus the telescope, and study

the periodic movement.

4. Take a few minutes to practice guiding. This will help you familiarize yourself with the

periodic error of the drive and the operation of the hand controller.

5. Enter the "PEC" mode and select 'RECORD' then press 'ENTER'. 'RECORD' will

flash once a second for four minutes. For best results, the star should be centered on

the cross hairs for a few seconds before activating the PEC - RECORD function.

6. Guide for two or four minutes depending on your telescope mount. Try not to

overshoot corrections in right ascension. Ignore drift in declination.

After fourltwo minutes, the system will automatically switch to the PEC - ON mode to playback

the corrections made during the first four minutes.

Once you have used the PEC function for a while you may mistake its operation for the way the

drive normally operates. The

best

way to see how well the PEC function works is to turn if off and

note the change in tracking. PEC results improve with practice and patience.