Introduction to Reflector Telescopes

There are generally two types of telescopes; reflector telescopes and refractors telescopes.

The main difference between them is that the reflector telescope uses a mirror and the refractor telescope uses a lens for gathering and focusing the light.

We all know that light from all celestial objects in the sky comes to us in parallel direction. This is because of the fact that these celestial objects are too far from us. This is reason behind the parabolic shape of a reflector’s telescope mirror.

Because the shape of the mirror of a reflector telescope is parabolic, it directs all the parallel rays of light, coming to it, at a single point.

Reflector telescopes offer more advantage as compared to refractor telescope.

This is the reason most of the astronomers today prefer to use reflector telescope rather than refractor telescope.

One of the main advantages of using a reflector telescope rather than a refractor telescope is that the reflector telescopes are not affected with chromatic aberration which is the case in refractor telescopes.

The reason behind this advantage is that all the wavelengths are reflected from the mirror in the same manner.

Another important advantage of reflector telescopes over refractor telescope is that they are relatively cheaper even if it is of the same size of a refractor telescope.

Light is reflected, not penetrated through the mirror which means that only one side of the mirror should always be perfect. This is not the case in refracting telescopes because light penetrates through it so it needs to be perfect from both sides.

Although reflecting telescopes are preferred by most of the astronomers, even then there are few disadvantages of reflector telescopes. For instance, alignment is a very difficult task in case of a reflector telescope.

Since reflector telescopes have a tube that is open from the outside, it requires regular cleaning of different optical instruments being used in it. Otherwise it can cause in a blurred image.

Sometimes it also happens that a secondary mirror has to be used in order to again direct the light from one spot to another spot. The use of secondary mirrors can affect the quality of the image.

One problem with both refracting and reflecting telescope they make an inverted image of the distant object. This problem is resolved by the use of a prism or a relay that are placed between the image of the object and the eyepiece.

The prism or the relay acts as an inverter and they invert the inverted image made by the telescope; hence inverting the inverted image make a real image of the object that is in upright position.

As mentioned earlier, a reflector telescope uses a secondary mirror to change the spot of view. This secondary mirror is quite small as compared to the primary mirror.

Sir Isaac Newton preferred a secondary mirror that was flat at 45 degree angle in order to reflect the light to the eyepiece at one side of the telescope near the top of the tube. This arrangement, often called as Newtonian design, is still in use.

Another interesting fact about reflector telescope is that most of them use a special light path based on the Cassegrain design in order to reflect the light, coming from the object of interest, back to the primary mirror.

This helps in placing the eye-piece conveniently. Most modern telescope including the most famous Hubble Space Telescope have this design and most of the astronomers today prefer, for various reasons, the reflector telescope.