Terrestrial Viewing; Calculating Magnification (Power); Care And Maintenance - Orion SHORTTUBE 80-A Instruction Manual

is important that you find an observing site well away from is important that you find an observing site well away from
light pollution. Take plenty of time to let your eyes adjust
to the darkness. Don't expect these objects to appear like
the photographs you see in books and magazines; most
will look like dim gray "ghosts." (Our eyes are not sensitive
enough to see color in deep-sky objects except in few of the
brightest ones.) But as you become more experienced and
your observing skills improve, you will be able to coax out
more and more intricate details.

Terrestrial Viewing

Many people enjoy using their telescope to view objects
such as boats, birds and wildlife. The ShortTube 80-A is an
excellent instrument for observing during the daylight hours.
For terrestrial applications, we recommend mounting the
ShortTube 80-A on an altazimuth-style telescope mount
or camera tripod, which allow simple vertical (altitude) and
horizontal (azimuth) motions. We also recommend using a
45° correct-image prism diagonal instead of the supplied
star diagonal. A correct-image prism diagonal is designed
for daytime use and yields a properly oriented view (same
as the naked eye).
Calculating Magnification
(Power)
It is desirable to have a range of eyepieces of different focal
lengths, to allow viewing over a range of magnifications. To
calculate the magnification, or power, of a telescope, simply
divide the focal length of the telescope by the focal length of
the eyepiece:
Telescope F.L. ÷ Eyepiece F.L.=Magnification
For example, the ShortTube 80-A, which has a focal length
of 400mm, used in combination with the supplied 20mm
eyepiece, yields a magnification of
400 ÷ 20 = 20x.
Every telescope has a useful limit of magnification of about
45x-60x per inch of aperture, which is 140x-180x for the
ShortTube 80-A. Claims of higher power by some telescope
manufacturers are a misleading advertising gimmick and
should be dismissed. Keep in mind that at higher powers,
an image will always be dimmer and less sharp (this is a
fundamental law of optics). The steadiness of the air (the
"seeing") can also limit how much magnification an image
can tolerate.
Always start viewing with your lowest-power (longest focal
length) eyepiece in the telescope. After you have located
and looked at the object with it, you can try switching to a
higher-power eyepiece, like the supplied 9mm Expanse eye-
piece, to ferret out more detail. If the image you see is not
crisp and steady, reduce the magnification by switching to a
longer-focal-length eyepiece. As a general rule, a small but
6
well-resolved image will show more detail and provide a more
enjoyable view than a dim and fuzzy, overmagnified image.
Photography with the ShortTube 80-A
With an optional camera adapter, the ShortTube 80-A
becomes a 400mm f/5 telephoto lens for a single-lens reflex
camera. For long-distance terrestrial or astronomical pho-
tography, you need only a T-ring for your particular camera
model. The T-ring attaches to your particular camera model
and threads onto the ShortTube's focuser drawtube, cou-
pling the camera body to the telescope (Figure 3).
Focus lock thumbscrew
Figure 3.
The ShortTube 80-A with 35mm SLR camera The ShortTube 80-A with 35mm SLR camera
attached.
For closer-range (less than 1/4-mile) terrestrial photography,
you need both a T-ring for your camera and a T-adapter
(#5264), which provides the extra focus travel needed.
Remove the nosepiece from the camera adapter; the body
of the adapter will not be needed here. Connect the T-ring
to your camera body (remove any camera lenses first), and
connect the nosepiece of the camera adapter to the T-ring.
Now, insert the entire assembly into the focuser's drawtube
(remove the eyepiece and diagonal), and secure with the
thumbscrew on the drawtube. Be sure to tighten the thumb-
screw, or your camera could fall to the ground!
Use the camera's viewfinder to frame the picture. Use the
telescope's focuser to focus the image. Tighten the focus
lock thumbscrew on the telescope's focuser to make sure
the camera does not slip out of focus.
You may want to consider using a remote shutter release
instead of the shutter release on the camera. Touching the
camera can vibrate the system and blur the resulting photo-
graphic image. Also, be sure to use a solid tripod.
Care & Maintenance
Give your telescope reasonable care and it will last a life-
time. Store it indoors or in a dry garage. Do not leave the
telescope outside except when using it. The optical tube is
aluminum and has a smooth painted surface that should
resist scratches and smudges. If a scratch does appear
on the tube, it will not harm the telescope. If you wish, you
can apply some white auto touch-up paint to the scratch.
Smudges on the tube can be wiped off with standard house-
hold cleaning fluid. hold cleaning fluid.