Celestron EdgeHD 11 Optical Tube Assembly EdgeHD Optics Manual - Page 7

Focusing Your telescope's focusing mechanism controls the primary mirror which is mounted on a ring that slides back and forth on the primary baffle tube. The focusing knob, which moves the primary mirror, is on the rear cell of the telescope just below the star diagonal and eyepiece. Turn the focusing knob until the image is sharp. If the knob will not turn, it has reached the end of its travel on the focusing mechanism. Turn the knob in the opposite direction until the image is sharp. Once an image is in focus, turn the knob clockwise to focus on a closer object and counterclockwise for a more distant object. A single turn of the focusing knob moves the primary mirror only slightly. Therefore, it will take many turns (about 30) to go from close focus to infinity. For astronomical viewing, out of focus star images are very diffuse, making them difficult to see. If you turn the focus knob too quickly, you can go right through focus without seeing the image. To avoid this problem, your first astronomical target should be a bright object (like the Moon or a planet) so that the image is visible even when out of focus. Critical focusing is best accomplished when the focusing knob is turned in such a manner that the mirror moves against the pull of gravity. In doing so, any mirror shift is minimized. For astronomical observing, both visually and photographically, this is done by turning the focus knob counterclockwise. Figure 2-3 -The emblem on the end of the focus knob shows the correct rotational direction for focusing your telescope. Mirror Support Clutches The EdgeHD Optical tube is equipped with mirror tension clutches to help support and minimize lateral movement of the primary mirror during astrophotography. To use the mirror clutches: 1. Use the focus knob to adjust the primary mirror to the desired focus. 2. Once in focus, turn the two mirror lock knobs clockwise until both are very tight and can be turned no further. Mirror Lock Knobs Warning! Once the mirror is locked down, do not turn the focuser knob without loosening the mirror locks first. Figure 2-4 - Mirror Support Clutches Although turning the focus knob should not damage the telescope, undue stress can be placed on the focus mechanisms causing excessive image shift while focusing. Calculating Magnification You can change the power of your telescope just by changing the eyepiece (ocular). To determine the magnification of your telescope, simply divide the focal length of the telescope by the focal length of the eyepiece used. In equation format, the formula looks like this: Focal Length of Telescope (mm) Magnification Focal Length of Eyepiece (mm) Let's say, for example, you are using the 40mm Plossl eyepiece. To determine the magnification you simply divide the focal length of your telescope (the C8 for example has a focal length of 2032mm) by the focal length of the eyepiece, 40mm. Dividing 2032 by 40 yields a magnification of 51 power. 7