The aperture of a telescope is the diameter of its main, light-gathering lens or mirror.  A bigger aperture means collecting more light and hence having sharper and brighter images that show more detail.

This makes aperture, from all the specifications you will find on a telescope, the most important one.  Whatever you plan to do, whether it is just stargazing or imaging the night sky, it all starts with collecting light.  Unfortunately, telescopes get quickly a lot more expensive when you look into telescopes with bigger apertures.  That is why bigger telescopes typically use mirrors instead. Deciding what telescope you want to purpose has a lot to do with how much you are willing to spend on the aperture.

But aperture should not be taken into consideration on its own.  Aperture needs to be evaluated from the perspective of other specs that will impact a lot what and how you observe the sky.   As you will see in a future post, the aperture will have a big impact on the f/ratio and the resolution of a telescope.

Fun fact

Aperture is not only a specification of a telescope but of any light gathering system.  Cameras and even our eyes have apertures also.

Cameras have adjustable apertures and our eyes have pupils to regulate the aperture of our eyes.  I am sure you have seen how someone’s pupils dilate when they see something or someone they like.

A human eye pupil has a maximum aperture of 7mm