understanding how power, or magnification, is calculated when using a telescope
will require the understanding of a relationship between two independent
optical systems - the telescope itself and the eyepiece you are using. to
understand this we must first understand the term
focal length is the distance measured in millimeters (mm) in an optical
system from the lens or primary mirror to the point where the telescope
is in focus. this point is called the focal point. the longer the
focal length of the telescope, generally the more power it has, the larger
the image and the smaller the field of view. for example, a telescope with
a focal length of 2000mm has twice the power and half the field of view
of a 1000mm telescope.
to determine power in a telescope, divide the focal length of the telescope
by the focal length of the eyepiece. by exchanging an eyepiece of one focal
length for another, you can increase or decrease the power of the telescope.
for example, a 25mm eyepiece used on a telescope with a 1000mm focal length
would yield a power of 40x (1000 / 25 = 40) and a 10mm eyepiece used on
the same telescope would yield a power of 100x (1000 / 10 = 100). since
eyepieces are interchangeable, a telescope can be used at a variety of powers
for different applications.
there are practical lower and upper limits of power for telescopes. these
are determined by the laws of optics and the nature of the human eye. as
a rule of thumb, the maximum usable power is equal to 60 times the aperture
of the telescope (in inches) under ideal conditions. powers higher than
this usually give you a dim, lower contrast image. for example, the maximum
power on a 60mm telescope (2.4” aperture) is 142x. as power increases, the
sharpness and detail seen will be diminished. the higher powers are mainly
used for lunar, planetary, and binary star observations.
be very cautious of manufacturers who advertise a 375 or 750 power telescope
which is only 60mm in aperture, as this is false and misleading. many department
store brand telescopes know that customers are not informed how telescopes
operate. these manufacturers of telescopes market their products to the
misconception that magnification is the most important feature on a telescope.
most of your observing will be done with lower powers – 6 to 25 times the
aperture of the telescope (in inches). with these lower powers, the images
will be much brighter and crisper, providing more enjoyment and satisfaction
with the wider fields of view.
there is also a lower limit of power which is between 3 to 4 times the aperture
of the telescope at night. during the day the lower limit is about 8 to
10 times the aperture. powers lower than this are not useful with most telescope
and a dark spot may appear in the center of the eyepiece in a catadioptric
or newtonian reflector telescope due to the secondary or diagonal mirrors
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most telescopes will come with 1 or more eyepieces. it is good to have a
wide selection of eyepieces for various types of observation. to truly get
the most out of your telescope it is a good idea to have a wide selection
of eyepieces. you will find that most telescopes will come with a 25mm (sometimes
20 or 26 depending on the telescope) because this is one of the most common
eyepieces focal lengths to use to get the optimum magnification and field
of view from your telescope. it is good to spread out your selection of
eyepieces. a good selection would be a 5mm – 10mm – 15mm – 25mm – 32mm as
an example. you will find that many companies offer kits of eyepieces that
will give you a wide selection in one package.
when choosing an eyepiece it is good to remember this rule of thumb. the
telescope is only as good as the eyepiece. you could have the most amazing
telescope quality, but if you use a poorly manufactured eyepiece, you are
not getting the advantage of the telescope. there should be a balance between
the quality of your telescope and the quality of your eyepiece. if you have
a top-of-the-line telescope, it is wise to spend the extra money on a suburb
here is a list of manufacturers that make exceptionally high quality eyepieces.
although these eyepieces are amazing in quality - some of the prices for
an eyepiece alone could purchase a complete telescope!
here is a list of manufacturers that make some very good eyepieces at a
» hardin optical
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credits - "telescope 101"