REFRACTORS

They consist of 2 lenses :

• the aperture or objective lense concentrates the light in a focal point
• the eye piece lens projects this image on the retina of the eye

You can calculate the magnification of the telescope :

focal length (of the objective lense) / eye piece (focal length)

--> For example : a refractor with a focal of 700mm and an eye piece of 25 mm can magnify 700/25=28 times.

Though, there is minimal magnification. This is due to the diameter of the pupil that can reach at most 7-8mm when fully dilated in the dark.

Minimal magnification = aperture (diameter of the objective lens) /7 (diameter of the pupil) .

Maximal magnification = 2 to 2.5 times the aperture. More or less, the image is blurred.

So, you cannaot increase the strength of a telescope by just decreasing the eye piece. You need to have larger apertures for higher magnifications and longer focal lengths. This is why amateur refractors have limited magnification, elsewhere, the tube would be too long to be practical in use.

Refractors, since they refract light when it passes through their aperture, cause chromatic aberration : you see a rainbow of colors around the observed object, such as in this photo taken with our refractor of the planet Venus :

Associating 2 lenses together decreases this aberration.
There are also apochromatic refractors manufactured with high quality lenses with low refraction indice , but they are highly expensive.

Refractors have many advantages :

1. They give a sharp image. They are great for observing planets, the moon and double stars. For the same aperture size, refractors are, while observing, 1 inch superior to reflectors. Since light is partly filtered through the lenses, refractors are not ideal for observing dark objects.
2. They do not fear vibrations, their lenses stay aligned, which is why they are more practical for trips.