6
MAGNIFICATION
To determine the magnification of a telescope and eyepiece
combinantion, divide the telescope focal length by the eyepiece
focal length.
Magnification (x) = Telescope Focal Length (mm)
Eyepiece Focal Length (mm)
Ex: 20mm Eyepiece with a 50mm x 360mm telescope
Magnification = 360mm/20mm
Magnification = 18X
FOCAL RATIO
To determine the focal ratio of a telescope, divide the focal length of
the telescope by the aperture.
Focal Ratio = Telescope Focal Length (mm)
Aperture of Telescope (mm)
Ex: Focal Ratio of a 50mm x 360mm telescope
Focal Ratio (f/x)= 360mm/50mm
Focal Ratio (f/x)= f/7.2
LIMITING MAGNITUDE
To determine the limiting magnitude of a telescope, use the aperture
in the following formula for an approximation.
Limiting Magnitude = 7.5 + 5 LOG (Telescope Aperture in cm)
Ex: Limiting Magnitude of a 50mm x 360mm telescope.
Limiting Magnitude = 7.5 + 5LOG (5cm)
Limiting Magnitude = 7.5 + (5 x 0.70)
Limiting Magnitude = 11.00
RESOLVING POWER
To determine the resolving power of a telescope under ideal
conditions, divide the aperture into 4.56.
Resolving Power = 4.56
Aperture (inches)
Ex: Resolving Power of a 50mm x 360mm telescope.
Aperture (in.) = 50mm/25.4 = 1.97 in.
Resolving Power = 4.56/1.97 in.
Resolving Power = 2.31 seconds of arc
ASTRONOMY FORMULAS FOR YOUR
ZHUMELL TELESCOPE
As you set out to begin viewing, one of the easiest and most
enjoyable objects to check out is the moon. Finding the moon
and adjusting to view it is a good way to acquaint yourself with
the movements of your telescope.
Practice using the azimuth and altitude adjustments to bring the
moon into the center of your view. Focus your view by turning
the knobs located on the smaller end of the OTA. Once you’ve
located the moon and successfully focused your telescope on it,
experiment with focusing and your different eyepieces. This will
help familiarize you with the different results you can get from
your telescope.
THE MOON
COOL VIEWS WITH YOUR
ZHUMELL TELESCOPE
THE PLANETS
Once you’ve used your telescope to view the moon, you should
be familiar with the basic telescope movements and adjustments
you need. Next stop: the planets. Not all the planets are visible
from one area at one time, so you’ll need to do a little research
before you begin. There are a number of online resources helpful
for discovering what planets and objects should be visible in your
area on any given night.
To find a planet, you must first locate it with the naked eye. Once
you’ve got its general location, point your telescope in that direction
and center the planet in the crosshairs of your finderscope. Once
the planet is lined up in the finderscope, begin to view the planet
through your telescope using the lowest power (longest focal
length) eyepiece. You may need to make slight adjustments to
the telescope aim and you will need to focus the eyepiece to
bring the planet into full view.
For a closer look at the planet, replace the low-powered eyepiece
with a higher-powered one and refocus your telescope.
