Many people have a misconception about the focal length of their telescope and focal length in general. Most people think that the focal length is simply the length of the optical tube itself. This is incorrect. What is focal length then if not the length of the tube itself?
Contrary to popular belief, the focal length is the distance, measured in millimeters, from the center of the aperture to the point where the image is formed. This point in the telescope is where the eyepiece connects and takes in the light from the aperture. Unlike other lenses, the focal length of a telescope is usually the same or at least very similar to the length of the optical tube, but not always.
This is an important distinction to make because, while related, the two are not always the same.
Optical Tube Length Versus Focal Length
The first thing to note is that the length of the optical tube must be equal to or greater than the focal length. You cannot have a focal length that exceeds the length of the optical tube.
You can, however, have an optical tube that exceeds the focal length. There could be extra space inside the optical tube that is not contributing to the focal length. This could be done for better balance, weight distribution, or other similar design choices that has the focal length in a position where it is shorter than the overall optical tube.
In the vast majority of cases, this will not be an issue but it is definitely something to keep in mind. If you see a telescope that has a focal length of 600mm (roughly 24”) and automatically assume that will be the length of the tube only to find out that the tube is actually 30” long, it could affect your plans for travel and storage.
Focal Length’s Effect On Imaging
The general rule for focal length is the shorter the length, the wider the image. This plays a huge role in determining the focal ratio and goes into choosing a telescope that has an image angle that fits your style and goals or astronomy.
While the focal length is only one part of the focal ratio, it is an important part, along with aperture. Generally, though, a shorter focal ratio will always provide a squatter image angle. That is why reflectors usually have a very short focal ratio compared to refractor telescopes. Reflectors thrive on wide angle shots while refractors perform better with narrow angle shots.
This will affect the image quality depending on what kind of image you are trying to process with your telescope.
Other Ways Length Influences Telescopes
While it is a bad habit to immediately equate the focal length with the optical tube length, they are similar. That means that a telescope with a longer focal length is, by necessity, going to have to have a longer optical length.
If you have any sort of constraints on the size of your telescope, whether that be from a desire to want to travel with it or specific storage constraints that you may have regarding your vehicle or closet, then the overall size of the optical tube is going to have an effect on your quality of life.
Long telescopes can be hard to store, hard to travel with, and hard to handle in certain situations. If you are looking for a squat or more petite telescope you are going to need to pay attention to the focal length. If you are looking at a telescope comparison table.
Remember, the optical tube is always going to be equal to or greater than the focal length. It will never be shorter than the length listed.
Millimeters to Inches
All focal lengths are given in millimeters so it is good to roughly know how that translates to inches. An inch is roughly 25mm. That means that you can divide all of your focal lengths by 25 to get a rough approximation in inches. This can help when visualizing how long the focal length actually is for anyone who is not fluent in the metric system.
A focal length of 500mm is going to net you roughly 20” of length in imperial measurements which is less than two feet. These are all good things to keep in mind when reading about focal length.
