Celestron Advanced VX 8" EdgeHD Telescope Advanced VX Manual - Page 31

Big Dipper LiƩle Dipper N.C.P Pointer Stars Polaris (North Star) Cassiopeia Many methods of polar alignment require that you know how to find the celestial pole by identifying stars in the area. For those in the northern hemisphere, finding the celestial pole is not too difficult. Fortunately, we have a naked eye star less than a degree away. This star, Polaris, is the end star in the handle of the Little Dipper. Since the Little Dipper (technically called Ursa Minor) is not one of the brightest constellations in the sky, it may be difficult to locate from urban areas. If this is the case, use the two end stars in the bowl of the Big Dipper (the pointer stars). Draw an imaginary line through them toward the Little Dipper. They point to Polaris. The position of the Big Dipper changes during the year and throughout the course of the night. When the Big Dipper is low in the sky (i.e., near the horizon), it may be difficult to locate. During these times, look for Cassiopeia. Observers in the southern hemisphere are not as fortunate as those in the northern hemisphere. The stars around the south celestial pole are not nearly as bright as those around the north. The closest star that is relatively bright is Sigma Octantis. This star is just within naked eye limit (magnitude 5.5) and lies about 59 arc minutes from the pole. Long Exposure Prime Focus Photography This is the last form of celestial photography to be attempted after others have been mastered. It is intended primarily for deep sky objects, that is, objects outside our solar system including star clusters, nebulae, and galaxies. While it may seem that high magnification is required for these objects, just the opposite is true. Most of these objects cover large angular areas and fit nicely into the prime focus field of your telescope. The brightness of these objects, however, requires long exposure times and, as a result, are rather difficult. There are several techniques for this type of photography, and the one chosen will determine the standard accessories needed. The best method for long exposure deep sky astrophotography is with an off-axis guider. This device allows you to photograph and guide through the telescope simultaneously. In addition, you will need a T-Ring to attach your camera to the Radial Guider. Other equipment needs include an autoguider, which is a small camera that attaches to the radial guider and keeps your guide star centered while you are imaging with your main camera. Here is a brief summary of the technique. 1. Polar align the telescope. For more information on polar aligning, see the Polar Alignment section earlier in the manual. 2. Remove all visual accessories. 3. Thread the Radial Guider onto your telescope. 4. Thread the T-Ring onto the Radial Guider. 5. Mount your camera body onto the T-Ring the same as you would any other lens. 6. Set the shutter speed to the "B" setting. 7. Focus the telescope on a star. 8. Center your subject in the field of your camera. 9. Using your autoguider, find a suitable guide star in the telescope field. This can be the most time-consuming part of the process. 10. Open the shutter using a cable release. 11. Monitor your guide star for the duration of the exposure using the buttons on the hand controller to make the needed corrections. 12. Close the camera's shutter. Periodic Error Correction (PEC) Periodic Error Correction, or PEC for short, is a system that improves the tracking accuracy of the drive by reducing the number of user corrections needed to keep a guide star centered in the eyepiece. PEC is designed to improve imaging quality by reducing the amplitude of the worm errors. Using the PEC function is a three-step process. First, the Advanced VX mount needs to know the current position of its worm gear, so it has a reference when playing back the recorded error. Next, you must guide using an autoguider for at least 10 minutes during which time the system records the correction you make. (It takes the worm gear 10 minutes to make one complete revolution). This "teaches" the PEC chip the characteristics of the worm. The periodic error of the worm gear drive will be stored in the PEC chip and used to correct periodic error. The last step is to play back the corrections you made during the recording phase. Keep in mind, this feature is for advanced astrophotography and still requires careful guiding since all telescope drives have some periodic error. Using Periodic Error Correction Once the telescope has been properly polar aligned, select PEC from the Utilities menu and select the Record option. Here's how to use the PEC function: 1. Find a bright star relatively close to the object you want to image. 2. Insert the autoguider into the eyepiece holder of your telescope. Orient the guider so that one axis of the sensor is parallel to the declination axis while the other is parallel to the R.A. axis. 3. Focus the telescope, and study the periodic movement. I 31