5
(refer to
Figure 7
), then take hold of the pan handle and gently
move it left or right.
To move the telescope in altitude, first twist the pan handle
counterclockwise, then move the telescope up or down to the
desired position. Then twist the pan handle clockwise to lock
that position. You may be able to find a suitable azimuth and
altitude axis tension to allow the telescope to be moved freely
without having to make any adjustments to the tension every
time you move the telescope.
Eyepiece Selection
Magnification, or power, is determined by the focal length of
the telescope and the focal length of the eyepiece being used.
Therefore, by using eyepieces of different focal lengths, the
resultant magnification can be varied. It is quite common for
an observer to own five or more eyepieces to access a wide
range of magnifications. This allows the observer to choose the
best eyepiece to use depending on the object being viewed and
viewing conditions. Your StarBlast 90mm refractor comes with
25mm (E) and 9mm (F) Kellner eyepieces, which will suffice
nicely to begin with. You can purchase additional eyepieces
later if you wish to have more magnification options.
Magnification is calculated as follows:
For example, the StarBlast 90mm has a focal length of 500mm,
which when used with the supplied 25mm eyepiece yields:
The magnification provided by the 9mm eyepiece is:
The maximum attainable magnification for a telescope is direct-
ly related to how much light it can gather. The larger the aper-
ture, the more magnification is possible. In general, a figure of
50x per inch of aperture is the maximum attainable for most
telescopes. Going beyond that will yield simply blurry, unsatis-
factory views. Your StarBlast 90mm refractor has an aperture
of 90mm, or 3.5 inches, so the maximum practical magnifica-
tion would be about 175x (3.5 x 50). This level of magnification
assumes you have ideal atmospheric conditions for observing
(which is seldom the case).
Keep in mind that as you increase magnification, the brightness
of the object viewed will decrease; this is an inherent principle
of the laws of physics and cannot be avoided. If magnification is
doubled, an image appears four times dimmer. If magnification
is tripled, image brightness is reduced by a factor of nine!
So start with low power by using the 25mm eyepiece, then try
switching to the 9mm eyepiece later if you want to boost the
magnification.
Focusing the Telescope
To focus the telescope, turn the focus wheels
(Figure 7)
for-
ward or back until you see your target object in the eyepiece.
Then make finer adjustments until the image is sharp. If you’re
having trouble achieving initial focus, rack the focuser drawtube
all the way in using the focus wheels, then while looking into
the eyepiece slowly turn the focus wheels so that the drawtube
extends outward. Keep going until you see your target object
come into focus. Note that when you change eyepieces you
may have to adjust the focus a bit to get a sharp image with the
newly inserted eyepiece.
Figure 6.
Insert an eyepiece into the 45-degree diagonal and secure
it with the thumbscrew.
Figure 7.
The telescope is now mounted and ready for action!
= Magnification
Telescope Focal Length (mm)
Eyepiece Focal Length (mm)
= 20x
= 56x
500 mm
25mm
500 mm
9mm
Eyepiece
45
°
Correct-
image diagonal
Thumbscrew
Alignment thumbscrews
Finder scope eyepiece
Compass
Pan handle
Altitude tension
Focus
wheel
Azimuth
tension
knob
