8
Focusing
Focusing
To focus your Travel Scope turn the focus knob located near the rear of the telescope (see Figure 1-1). Turning the knob
counterclockwise allows you to focus on an object that is farther than the one you are currently observing. Turning the knob
clockwise from you allows you to focus on an object closer than the one you are currently observing.
Note:
Remove the front lens cap of the Travel Scope optical tube prior to attempting your observation.
Note:
If you wear corrective lenses (specifically glasses), you may want to remove them when observing with an eyepiece
attached to the telescope. If you have astigmatism
, corrective lenses should be worn at all times.
Calculating Magnification
Calculating Magnification
You can change the power of your telescope just by changing the eyepiece (ocular). To determine the magnification of your
telescope, simply divide the focal length of the telescope by the focal length of the eyepiece used. In equation format, the
formula looks like this:
Focal Length of Telescope (mm)
Magnification =
Focal Length of Eyepiece (mm)
Let’s say, for example, you are using the 20mm eyepiece that came with your Travel Scope 70 telescope. To determine the
magnification you divide the focal length of your telescope (the Travel Scope for this example has a focal length of 400mm) by
the focal length of the eyepiece, 20mm. Dividing 400 by 20 yields a magnification of 20x.
Although the power is variable, every telescope under average skies has a limit to the highest useful magnification. The general
rule is that 60 power can be used for every inch of aperture. For example, the Travel Scope 70
is 2.8” inches in diameter.
Multiplying 2.8 by 60 gives a maximum useful magnification of 168 power. Although this is the maximum useful magnification,
most of your observing will be done at low powers which generate brighter and sharper images.
Note on Using High Powers
– Higher powers are used mainly for lunar and sometimes planetary observing where you can
greatly enlarge the image, but remember that the contrast and brightness will be very low due to the high magnification. When
using the 8mm eyepiece together with the 3x Barlow lens with the Travel Scope 50 gives extremely high power and can be used
on rare occasions – you will achieve the power but the image will be dark with low contrast because you have magnified it to the
maximum possible. For the brightest images with the highest contrast levels, use lower powers.
You can purchase optional eyepieces to give you a range of powers you can observe with. Visit the Celestron website to see
what is available.
Installing & Using the Barlow Lens
Installing & Using the Barlow Lens
Your telescope also comes with a 3x Barlow Lens which triples the magnifying
power of each eyepiece. However, the greatly magnified images should only be
used under ideal conditions – see the Calculating Magnification section of this
manual. To use the Barlow lens remove the diagonal and insert the Barlow
directly into the focuser tube. You then insert an eyepiece into the Barlow lens for
viewing.
Note:
Start by using a low power eyepiece as it will be easier to focus.
