Telescope mirrors and lenses need to be aligned to give a nice, sharp image. The process of getting them aligned is known as collimation – which you can think of as an essential step, like having to tune a guitar before you play it. Collimating is usually only essential with telescopes that have mirrors in them, such as reflectors or catadioptric telescopes.

A great way to see if collimation is the problem is to point the telescope at a bright star and adjust the focus so that the star is out of focus and showing as a big blob. Look closely and you will see a bright outer ring surrounding a dark central disc. If the dark disc is not in the center of the bright ring, then the optics are not aligned, and you need to collimate.

The steps to achieve this depend on the telescope you have, so check out the manual or find the guide for your telescope online – and you should then be able to get nice sharp images.