National Geographic NG 91 User manual

Please retain the information in
this manual for future reference.
Colour, designs and decorations may vary
from those shown in the photographs.
Item no. NG 91
Printed in China
INSTRUCTION
MANUAL
INSTRUCTION
MANUAL
50mm
ASTRONOMICAL
TELESCOPE
50mm
ASTRONOMICAL
TELESCOPE
AGES 8 AND UP
WARNING!
Do not view the sun through
the telescope as serious injury
to the eye may occur.
Telescopes should
always be used under
adult supervision.
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PARTS OF YOUR TELESCOPE
Lens hood
Lens cap
(not shown)
Focus
wheel
Erecting
eyepiece
Tripod
8.3mm eyepiece
Diagonal
mirror
16.6mm
eyepiece
Focus tube
Telescope tube
INTRODUCTION
With proper care and handling, you will enjoy looking at nature’s sights
for many years. This telescope has been designed for both
astronomical and terrestrial viewing, so you can not only gaze into the
night sky, but also view mountains, birds and animals at a distance.
When you stand outside and look at the sky on a clear night, you can
normally see masses of stars without using a telescope. The telescope
is really a light-gathering instrument that also magnifies the view, so
you can see far more detail.
If you only see black or grey when looking through the telescope,
it could be you are using an eyepiece with too high a magnification,
so swap to the lower-power eyepiece and check again. Using the
higher-powered eyepiece will make objects seem darker. This is
because when magnification in a telescope increases, brightness is
reduced.
As well as searching for stars with your telescope, the moon is also a
great target. The best time to look at the moon is when it is less than
half full, because the dividing line between the dark and light of the
moon (called the terminator) shows the best detail in the craters and
mountains.
You should be able to see stars and even planets with your telescope
(weather conditions permitting). The stars will twinkle and the planets,
although as bright, will show as constant lights. Looking at the Internet
with the permission or assistance of an adult will help you find star
maps and charts to help you identify objects in the universe. You can
also buy astronomy magazines that advise the locations of planets as
they change from month to month.
ASSEMBLY
1. Carefully remove all parts
from the box.
2. Stand the tripod vertically
and spread the three legs
fully apart.
3. Fit the telescope tube to the
tripod bracket and secure it
using the bolt and wing nut
provided.

3 4
ABOUT MAGNIFICATION
The magnification of a telescope indicates how much an image is
enlarged or how big and close it appears to the viewer. The focal length
of the eyepiece combined with the focal length of the telescope
determine the magnification. To calculate the magnification of your
telescope with any particular eyepiece, simply divide the focal length of
the telescope (500mm) by the focal length of the eyepiece (indicated in
‘mm’ on the eyepiece collar).
The erecting eyepiece used for viewing terrestrial objects and
landscapes has a focal length of 12 mm, providing a magnification of
41X.
The two eyepieces included
provide the following
magnification:
eyepiece 8.3mm = 60.24X
eyepiece 16.6mm = 30.12X
USING THE IMAGE ERECTOR
For viewing terrestrial
(land-based objects),
remove the diagonal
mirror from the focus
tube by unscrewing
the thumbscrew and
pulling out as shown.
Fit the image erector to the end of the focus tube and secure with the
thumbscrew.
500mm focal length tube = 30.12 X magnification
16.6mm focal length eyepiece
EXAMPLE
5. For greater magnification,
release the thumbscrew
securing the 16.6 mm
eyepiece, remove and
replace with the 8.3 mm
eyepiece.
4. Fit the eyepieces like this:
We suggest you fit the 16.6mm eyepiece first.
The 16.6 and 8.3mm
eyepieces fit like this.
The erecting eyepiece fits
like this.

5 6
USING YOUR TELESCOPE
1. The best way to use your telescope is outside, because viewing
through windows (even if they are open) can be distorted by
reflections from the glass or by air currents (differing
temperatures) passing through a window.
2. Let your telescope adjust to the outside temperature before use.
Your telescope will work better if the lenses and the air inside the
tube are the same temperature as outside, so give it up to
30 minutes to equalise.
3. Try to find a viewing spot that is away from glaring lights. If you live
in a city, your viewing can be reduced by too much lighting, and it's
best to use the telescope where it is as dark as possible.
4) Try to avoid sudden changes of temperature, as this may cause
condensation on your lens. (For example, carrying your telescope in
from a very cold wintry outside to a very warm house interior). If this
occurs, leave the lens cap off, tilt the lens face down and wait till the
telescope reaches room temperature before storing it.
5) When you remove your telescope from its box, the diagonal mirror
and low-power 16.6 mm eyepiece are already fitted. It is
recommended that you start viewing the moon and stars with the
16.6mm eyepiece because it gives you the widest angle with the
brightest and sharpest views. Once you have completed assembly,
adjust the angle of the telescope by first releasing the
bolt and wing nut that secures the telescope tube to the tripod
bracket. Move the telescope as desired, and secure with the bolt and
wing nut, being careful not to over-tighten.
NOTE: When using the diagonal mirror, objects will appear right-side
up in your telescope, but reversed, like a reflection in a mirror. This is
normal and does not indicate a problem. When using the erecting
eyepiece, objects will appear right-side up and in the correct
orientation (not reversed).
TAKING CARE OF YOUR TELESCOPE
The lens and eyepieces in your telescope can get dirty over time, and
utmost care must be taken when cleaning them. Here are some tips for
keeping your telescope in good working order:
1. Keep the dust caps on when not in use or when transporting the
telescope.
2. Condensation can occur when the telescope is not in use. If this
happens, remove the dust caps and allow the moisture to evaporate
naturally. While doing this, keep the telescope pointing downwards
to avoid dust accumulate on. Replace the cap as soon as the
condensation has gone.
3. If the lens looks like it is dusty, try blowing away any particles. If
your lens requires more cleaning then use a moistened lens tissue
or cloth (not included).