![]() So if you think of a 4-inch aperture as 100 mm, you will be close enough for this discussion. For convenience, you can round that to 25. Telescope aperture is often reported in units of inches or millimeters. For the purpose of this discussion, we will speak of telescopes and optical tubes interchangeably as we can assume the optical tube will be on some kind of mount. Just know that there are a variety of styles of mounts and either type of optical tube can go on any kind of mount. This discussion will be focused on the optical tube, refractor or reflector. When we say telescope we are really talking about two parts – the optical tube assembly and a mount. ![]() Focal Ratio – Focal length/aperture usually expressed as F# such as F5 or F10.Focal Length – The length of the path that the light travels in the optical tube.Aperture – The diameter of the front lens or the rear mirror in inches or millimeters.I am going to address that question and hopefully enable you to purchase the best telescope with confidence.įirst, let’s define some terms that I will be using. Which is better for a first-time telescope buyer? What’s the difference between reflector and refractor telescopes? Isaac Newton in the mid-1600 and so it is often referred to as a Newtonian or Newtonian reflector. The mirror sits in the bottom of a tube and the eyepiece sticks out the side. They start their shopping only to discover that there are also reflector telescopes that are based on mirrors rather than a lens. This is the type that was made popular by Galileo in the early 1600s. Most people, looking for their first telescope, will have an image in their mind of a long tube with a lens on one end and an eyepiece in the other.
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