CGEM Series Manual - Celestron

3M 1000 (color slide) Scotchchrome 400 T-Max 3200 (black and white print) T-Max 400 (black and white print)Periodic Error Correction (PEC)PEC for short, is a system that improves the tracking accuracy of the drive by reducing the number of user correctionsneeded to keep a guide star centered in the eyepiece. PEC is designed to improve photographic quality by reducing theamplitude of the worm errors. Using the PEC function is a three-step process. First, the CGEM needs to know the currentposition of its worm gear so that it has a reference when playing back the recorded error. Next, you must guide for at least 8minutes during which time the system records the correction you make. (It takes the worm gear 8 minutes to make onecomplete revolution). This “teaches” the PEC chip the characteristics of the worm. The periodic error of the worm gear drivewill be stored in the PEC chip and used to correct periodic error. The last step is to play back the corrections you madeduring the recording phase. Keep in mind, this feature is for advanced astrophotography and still requires careful guidingsince all telescope drives have some periodic error.Using Periodic Error CorrectionOnce the telescope has been properly polar aligned, select PEC from the Utilities menu and select the Record option. Here’show to use the PEC function.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 crosshairs so that one is parallel to the declination 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. Before actually recording the periodic error, take a few minutes to practice guiding. Set the hand control slew rate toan appropriate guide rate (rate 1 = .5x, rate 2 = 1x) and practice centering the guide star in the cross hairs for severalminutes. This will help you familiarize yourself with the periodic error of the drive and the operation of the handcontrol. Remember to ignore declination drift when programming the PEC.Note: When recording PEC only the photo guide rates (rates 1 and 2) will be operational. This eliminates the possibility ofmoving the telescope suddenly while recording.5. To begin recording the drive's periodic error, press the MENU button and select PEC from the Utilities menu. Usethe Up/Down scroll buttons to display the Record option and press ENTER. Once ready to begin recording, press theENTER button again to begin. You will have 5 seconds before the system starts to record. The first time eachobserving session that PEC record or play is selected, the worm gear must rotate in order to mark its startingposition. If the rotation of the worm gear moves your guide star outside the field of view of the eyepiece, it willhave to be re-centered before the recording begins.HelpfulHintOnce the worm gear is indexed, it will not need to be positioned again until the telescope is turned-off. So, togive yourself more time to prepare for guiding, it may be necessary to restart PEC recording after the worm gearhas found its index.6. After 8 minutes PEC will automatically stop recording.47