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The Ultimate Guide to Buy a Good Telescope

Before you buy the right equipment it is good to know a few things about the theory behind telescopes and binoculars.

People have long been amazed by the night sky because of its alluring mystery. For generations, observers had relied solely on their eyes to conduct their observations until 1608, when a German-born Dutch eyeglass maker made the first telescope. His gadget was not employed for astronomical purposes, but rather for military purposes. The Galilean invention of his first telescope in 1609 is an event that people credit the most today. The first Galilean optical tube was extremely basic, magnifying objects only three times. The scientist was able to achieve better optical power after multiple changes. He was able to observe many phenomena: the Venusian phases, the surface of the Moon, and 4 major Jovian satellites: Io, Europa, Ganymede and Callisto. These observations had a significant impact on the cosmological models of the time as they demonstrated arguments against a geocentric system.

The following are the primary functions of a telescope:

•To increase the angular separation between objects by gathering as much light radiation as possible

•To Create a sharp and clear image of a specific object

We have now reached a high technological level that allows us to build massive telescopes (and even send many of them outside the protective layers of the atmosphere of the Earth, that blocks many of the key wavelengths of radiation such us UV radiation, Gamma rays and X-rays) that are capable of reaching distant regions of the Universe and making significant discoveries. Examples of successful space telescopes include the HST and the JWST.

Components of a telescope:

The following are the major components of any telescope:

• The primary lens (for refracting telescopes), which is the device's most important component. The larger the lens, the more light a telescope can collect and the fainter the objects that may be seen.

• Primary mirror (in reflecting telescopes), which functions similarly to the primary lens in refracting telescopes.

• An eyepiece that enlarges the image.

• Mounting, which holds the tube in place and allows it to revolve.

Refractors and reflectors are the two types of telescopes. The simplest sort of a telescope is a refractor, which consists of two lenses at the tube's ends. The primary lens - the objective – is the major component of this sort of telescope.

The objective of a refractor is a concave lens. It specifies how small items can be seen. At the opposite end of the tube is the eyepiece, which is also a concave lens. The magnification of an object observed is defined by this term.

A reflector is a telescope that reflects light with a mirror. This is a later type of telescope that was first proposed by Giovanni Francesco Sagredo (1571–1620), who suggested that a curved mirror instead of a lens may be employed. However it was Isaac Newton who invented the first reflector in 1669.

These telescopes use a parabolic mirror to collect light from far away space objects, as well as many auxiliary mirrors to route the light path to the viewer. The figure above depicts a reflecting telescope (Newtonian design). The light from a source converges at a focal point of a system's primary lens (or primary mirror). Light enters the eyepiece (refractors) or is reflected outside a main tube towards the ocular lens, depending on the design (reflectors). A longer focal length will produce a larger image and a shorter focal length will produce a smaller image. The length of a telescope is equal to the focal length of the eyepiece plus the focal length of the objective lens. The focal length f of a lens is the place where the light rays converge, as it is illustrated in the images below.

D is the power of a lens and D=1/f. This means that stronger lenses have a smaller focal length f, and they will magnify the image more.

Refractors have shortcomings like chromatic aberration, glass defects, and distortion by weight and hence reflectors are most used in modern astronomy. Optical imperfections, often known as aberrations, impair the quality of views generated by optical telescopes. Chromatic aberration affects refractors more than reflectors. The wavelength of red light is refracted more than the wavelength of blue light. A star is surrounded by multicoloured concentric rings of light as a result of this phenomenon. To eliminate chromatic aberration, a corrective lens is put on top of the primary objective. Monochromatic aberrations are another form of aberration. Spherical aberration is caused by a unique property of spherical surfaces: they focus on a line rather than a point. The use of parabolic mirrors corrects this.

A note for slightly more advanced readers:

If the focal length of the objective is F and the focal length of the eyepiece is f, then the magnification of the image is given by M= F/f. Note also that: sinα = 138/D where D is the diameter of the main lens or the mirror of the telescope (measured in mm) and α is the limit resolution of the telescope measured in arcseconds. Finally the formula M= 2.7+ 5logD gives the limiting magnitude M (a dimensionless quantity), that is visible from an aperture of diameter D (measured again in mm).

Mountings are divided into two categories: altitude-azimuth and equatorial.

The equatorial mount.

The polar axis is aligned with the equatorial mount. The benefit of this setup is that it only requires the tube to spin along one polar axis to follow the movement of the celestial sphere and hence you can track celestial objects easily and you can easily use a camera with your telescope. It is important to buy an equatorial telescope if your goal is astrophotography.

The Alt-azimuth mount.

The biggest advantage of an alt-azimuth mount is the simplicity of its mechanical design. The primary disadvantage is its inability to follow astronomical objects in the night sky as the Earth spins on its axis. However computerized equatorial mounts and alt-azimuth mounts both track the motion of the stars and celestial objects, but they do so in different ways.


Buying a telescope will depend on your personal preferences and your budget. Now that you understand the theory behind telescopes and binoculars you can click this link to buy a telescope from our selection of the best telescopes & binoculars and support our educational articles.

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