The reflecting telescopes and their advantages and disadvantages

Reflecting Telescopes

The reflecting telescopes use the mirrors instead of lenses to focus the light , The convex mirror is used to gather the light and reflect it back to a focal point  to get the light out of the telescope , another mirror is used to direct the light to an eyepiece .

The reflecting telescope uses a mirror to collect the light instead of a lens , This overcomes the problems inherent in supporting the lens in a refractor telescope and the light losses due to the light passing through thick pieces of glass , And the mirror of a reflector is at the bottom end of the telescope tube .

The curved mirrors can bend the light and make parallel light rays converge to a focus , This focus is directly in the path of the incoming light , So , there are several ways of making images from the mirror visible , One is called a Newtonian reflector where a flat mirror is used to point the light rays out to an eyepiece .

There are many types of reflectors that solve the issue of where to focus the light in different ways such as Cassegrain reflectors which have a convex secondary mirror and a hole in the middle of the primary mirror .

Reflector telescopes

Reflector telescopes

Prime focus telescopes have no secondary optics and the observer or camera observes the image from near the focal point , Coude telescopes use a convex secondary mirror like a Cassegrain and an angled mirror like a Newtonian reflector to move the light rays to a focal point away from the telescope , This arrangement is useful when optical equipment is being used that is too heavy to mount directly on the telescope .

The reflectors are used not only to examine the visible region of the electromagnetic spectrum but also to explore both the shorter- and longer-wavelength regions adjacent to it , The primary mirror reflects the light back to a focus instead of refracting it , The primary mirror usually has a concave spherical or parabolic shape ,  as it reflects the light , it inverts the image at the focal plane .

Advantages of Reflecting Telescopes

The reflecting telescopes are not subject to the chromatic aberration because the reflected light does not disperse according to the wavelength , And all the wavelengths will reflect off the mirror in the same way .

The telescope tube of a reflector is shorter than that of a refractor of the same diameter which reduces the cost of the tube  , and they are easier and cheaper to build large ,  They are also easier to mount because the back of the mirror can be used to attach to the mount .

The mirrors in modern telescopes area made in various shapes to correct for the errors , Some telescopes use a combination of mirrors and lenses , Schmidt-Cassegrain telescopes use a spherical mirror with a correcting plate that corrects the focus .

The reflector telescope uses a mirror to gather and focus the light , All the celestial objects ( including those in our solar system )  are so far away that all of the light rays coming from them reach the Earth as parallel rays .

As the light rays are parallel to each other , the reflector telescope’s mirror has a parabolic shape , The parabolic shaped mirror focusses the parallel lights rays to a single point , All modern research telescopes and large amateur ones are of the reflector type because of its advantages over the refractor telescope .

The reflector telescopes are fairly compact and portable which are great for transportation and storage , They are also generally more affordable ( particularly as the  mirrors are cheaper to produce than the lenses ) , And the  mirrors can produce less optical aberrations than the lenses .

The reflector telescopes are cheaper to make than refractors of the same size , Because the light is reflecting off the objective rather than passing through it , only one side of the reflector telescope’s objective needs to be perfect .

Disadvantages of Reflecting Telescopes

It is easy to get the optics out of alignment , The  reflector telescope’s tube is open to the outside and the optics need frequent cleaning , Often a secondary mirror is used to redirect the light into a more convenient viewing spot , The secondary mirror and its supports can produce diffraction effects , The  bright objects have spikes  .

Different reflectors use different shapes of mirrors , The parabolic mirrors will focus all incoming light rays to a single point , The images from a parabolic mirror will have a defect called coma , where images far from the center of the field of view are elongated ,  The spherical mirror surface is relatively easy to make but different parts of a spherical mirror have slightly different focal lengths , so , The  images will be fuzzy .

The mirrors may need regular re-alignment as they can easily slip out of alignment due to knocks and bumps or even the temperature changes , They can be susceptible to the spherical aberrations which can interfere with the image quality , unless the mirrors and other optics are kept at the same temperature as the outside air , There will be the air currents inside the telescope that will cause images to be fuzzy .

How Reflecting Telescopes Work

The reflecting telescopes are similar to a refracting telescopes but just a little bit more complex , They are called a reflecting telescopes because They use the mirrors to reflect the light internally .

the light enters at one end , It is then reflected by a concave shaped primary mirror at the other end towards a smaller mirror known as the secondary mirror , This mirror reflects the light into an eyepiece  which is usually mounted on the side of the telescope , And the eyepiece can be adjusted to alter the sharpness of focus .

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