An astronomical telescope is a refractiong telescope that consists of two convex lenses - the objective lens and the eyepiece.
The objective lens gathers light from a distant object and the image formed, is viewed through the eyepiece.
When the final image is formed in infinity, the rays enter the eyes through the eyepiece in parallel with each other, which in turn minimize the strain on the eyes; the telescope is said to be in the normal adjustment.
When the telescope is in the normal adjustment, the distance between the two lenses - or the length of the telescope - is the sum of the focal lenghts.
The magnification is eqaul the ratio of the two focal lengths.
By moving the eyepiece slightly towards the objective lens, the final image can be made to appear at a distance of 25 cm from the eye too. This, however, can lead to straining eyes.
One of the setbacks of the astronomical telescope is the fact that the final image is inverted. Therefore, it is not good for viewing terrestrial objects; the issue does not rise, while viewing celestial objects.
The real product shown below is an astronomical telescope, although it is difficult to imagine such a simple arrangement of two lenses is inside, when your really handle it. I have one with me!
This is an astronomical telescope that buyers are happy with - from Amazon:
You can practise the astronomical telescope with the following interactively.
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