CGEM Series Manual - Celestron

7. Point the telescope at the object you want to photograph and center the guide star on the illuminated cross hairs andyou are ready to play back the periodic error correction.8. Once the drive's periodic error has been recorded, use the Playback function to begin playing back the correction forfuture photographic guiding. If you want to re-record the periodic error, select Record and repeat the recordingprocesses again. The previously recorded information will be replaced with the current information. Repeat steps 7and 8 to playback the PEC corrections for your next object.Does the PEC function make unguided astrophotography possible? Yes and no. For solar (filtered), lunar, and piggyback (upto 200mm), the answer is yes. However, even with PEC, off-axis guiding is still mandatory for long exposure, deep skyastrophotography. The optional Reducer/Corrector lens reduces exposure times making the task of guiding a little easier.As you perfect your technique, try specialized films, that is films that are designed or specially treated for celestialphotography. Here are some popular choices:Ektar 125 (color print)Fujichrome 100D (color slide)Tech Pan, gas hypered (black and white print)T-Max 400 (black and white print)There is no exposure determination table to help you get started. The best way to determine exposure length is look atpreviously published photos to see what film/exposure combinations were used. Or take unguided sample photos of variousparts of the sky while the drive is running. Always take exposures of various lengths to determine the best exposure time.Terrestrial PhotographyYour telescope makes an excellent telephoto lens for terrestrial (land) photography. Terrestrial photography is best donewill the telescope in Alt-Az configuration and the tracking drive turned off. To turn the tracking drive off, press the MENU(9) button on the hand control and scroll down to the Tracking Mode sub menu. Use the Up and Down scroll keys (10) toselect the Off option and press ENTER. This will turn the tracking motors off, so that objects will remain in your camera'sfield of view.MeteringThe CGEM Series telescope has a fixed aperture and, as a result, fixed f/ratios. To properly expose your subjectsphotographically, you need to set your shutter speed accordingly. Most 35mm SLR cameras offer through-the-lens meteringwhich lets you know if your picture is under or overexposed. Adjustments for proper exposures are made by changing theshutter speed. Consult your camera manual for specific information on metering and changing shutter speeds.Reducing VibrationReleasing the shutter manually can cause vibrations, producing blurred photos. To reduce vibration when tripping the shutter,use a cable release. A cable release keeps your hands clear of the camera and lens, thus eliminating the possibility ofintroducing vibration. Mechanical shutter releases can be used, though air-type releases are best.Blurry pictures can also result from shutter speeds that are too slow. To prevent this, use films that produce shutter speedsgreater than 1/250 of a second when hand-holding the lens. If the lens is mounted on a tripod, the exposure length is virtuallyunlimited.48