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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
10
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-
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