the planet to be photographed and the ISO of the film being used.
“Piggybacking” Photography
The Moon and planets are interesting targets for the budding astrophotographer, but what’s next? Literally thousands of deep-sky objects can be captured on film with a type of astrophotography called “piggybacking.” The basic idea is that a camera with its own camera lens attached rides on top of the main telescope. The telescope and camera both move with the rotation of the Earth when the mount is polar aligned and the motor drive is engaged. This allows for a long exposure through the camera without blurring of the object or background stars. In addition to the motor drive, an illuminated reticle eyepiece is also needed (Orion part #8481 is recommended). The t-ring and camera adapter are not needed, since the camera is exposing through its own lens. Any camera lens with a focal length between 50mm and 400mm is appropriate.
On the top of one of the tube rings is a piggyback camera adapter. This is the black knob with the threaded shaft pro- truding through its center. The tube ring with the piggyback adapter on it should be closest to the front of the telescope. Remove the tube rings from the equatorial mount and swap their positions, if necessary. Now, connect the camera to the pig- gyback adapter. There should be a 1/4"-20 mounting hole in the bottom of the camera’s body. Thread the protruding shaft of the piggyback adapter into the 1/4"-20 hole in the camera a few turns. Position the camera so that it is parallel with the telescope tube and turn the knurled black knob of the piggyback adapter counterclockwise until the camera is locked into position.
Aim the telescope at a deep-sky object. It should be a fairly large deep-sky object, as the camera lens will likely have a wide field of view. Check to make sure that the object is also centered in the camera’s viewfinder.Turn the motor drive on. Now, look into the telescope’s eyepiece and center the brightest star within the field of view. Remove the eyepiece and insert the illu- minated reticle eyepiece into the telescope’s star diagonal. Turn the eyepiece’s illuminator on (dimly!). Recenter the bright star (guide star) on the crosshairs of the reticle eyepiece. Check again to make sure the object to be photographed is still centered with- in the camera’s field of view. If it is not, recenter it either by repositioning the camera on the piggyback adapter, or by moving the main telescope. If you move the main telescope, then you will need to recenter another guide star on the eyepiece’s crosshairs. Once the object is centered in the camera, and a guide star is centered in the eyepiece, you’re ready to shoot.
Deep-sky objects are quite faint, and typically require expo- sures on the order of 10 minutes. To hold the camera’s shutter open this long, you will need a locking shutter release cable (part #5231). You will also need to set the camera’s shutter to the “B” (bulb) setting for the locking shutter release to work properly. Depress the release cable and lock it. You are now exposing your first deep-sky object.
While exposing through the camera lens, you will need to monitor the accuracy of the mount’s tracking by looking through the illuminated reticle eyepiece in the main tele- scope. If the guide star drifts from its initial position, then use the hand controller of the motor drive to “bump” the guide star
back to the center of the crosshairs. The hand controller only moves the telescope along the R.A. axis, which is where most of the corrections will be made. If the guide star appears to be drifting significantly along the Dec. axis, then the mount’s slow-motion control cables can be carefully used to move the guide star back onto the crosshairs. Any drifting along the Dec. axis is due to imprecise polar alignment. If the drifting is significant, you may need to polar align the mount more accu- rately.
When the exposure is complete, unlock the shutter release cable and close the camera’s shutter.
Astrophotography can be enjoyable and rewarding, as well as frustrating and time-consuming. Start slowly and consult out- side resources, such as books and magazines, for more details about astrophotography. Remember . . . have fun!
8. Terrestrial Viewing
The Skywatcher 90 may also be used for long-distance view- ing over land. For this application we recommend substitution of an Orion 45° Correct-Image Diagonal (#8790) for the 90° star diagonal that comes standard with the telescope. The cor- rect-image diagonal will yield an upright, non-reversed image and also provides a more comfortable viewing angle, since the telescope will be aimed more horizontally for terrestrial subjects.
For terrestrial viewing, it’s best to stick with low powers of 50x or less. At higher powers the image loses sharpness and clar- ity. That’s because when the scope is pointed near the horizon, it is peering through the thickest and most turbulent part of the Earth’s atmosphere.
Remember to aim well clear of the Sun, unless the front of the telescope is fitted with a professionally made solar filter and the finder scope is covered with foil or some other complete- ly opaque material.
9. Care and Maintenance
If you give your telescope reasonable care, it will last a life- time. Store it in a clean, dry, dust-free place, safe from rapid changes in temperature and humidity. Do not store the tele- scope outdoors, although storage in a garage or shed is OK. Small components like eyepieces and other accessories should be kept in a protective box or storage case. Keep the cap on the front of the telescope when it is not in use.
Your Skywatcher 90 telescope requires very little mechanical maintenance. The optical tube is aluminum and has a smooth painted finish that is fairly scratch-resistant. If a scratch does appear on the tube, it will not harm the telescope. If you wish, you may apply some auto touch-up paint to the scratch. Smudges on the tube can be wiped off with a soft cloth and a household cleaner such as Windex or Formula 409.
Cleaning the Optics
A small amount of dust or a few specks on the glass objective (main) lens will not affect the performance of the telescope. If dust builds up, however, simply blow it off with a blower bulb, or lightly brush it off with a soft camel hair brush. Avoid touch-
