Focal length is the distance between the telescope’s primary lens or mirror and the point where the light rays come together in focus. This is typically expressed in millimeters and is used to calculate key metrics such as focal ratio.
How does Focal Length in Telescopes Affect Astronomy?
Focal lengths determine the magnification and field of view of your telescope. Shorter focal lengths yield a wide angle of view of space. This results in lower magnification and large fields of view, which is ideal for astrophotography. Longer focal lengths have higher magnification and smaller fields of view. This allows you to focus on singular objects at great distances.
How does Focal Length in Telescopes Affect Magnification?
Magnification can be calculated by dividing the focal length of the telescope by the focal length of the eyepiece. For example, if you have a telescope with a focal length of 1000mm and an eyepiece with a focal length of 40mm, the telescope’s magnification would be:
Magnification = 1000mm / 40mm = 25x
What is a Short Focal Length in Telescopes?
Short focal length telescopes are typically under 500mm and have a wider field of view, so they can capture a larger area of the sky. However, they provide less magnification.
Advantages of Short Focal Length in Telescopes
Shorter focal lengths, typically under 500 mm, are best suited for viewing star clusters or galaxies. This is because shorter focal lengths yield a wider field of view. Telescopes with short focal lengths are also generally cheaper, making them ideal for beginners.
Disadvantages of Short Focal Length in Telescopes
Short focal length telescopes typically have a lower magnification compared to longer focal length telescopes. This can make it difficult to observe small or distant objects in detail. They are also more prone to optical aberrations because of the smaller aperture. This can cause images to appear blurry or discolored.
Which Telescopes Have Short Focal Lengths?
Telescopes with shorter focal lengths are typically more cost-efficient and allow for a broader view of the night sky. This makes the following telescopes ideal for beginners.
- Orion 10034 GoScope II 70mm Refractor Telescope has a focal length of 400mm.
- Celestron 21035 70mm Travel Scope has a focal length of 350mm.
- Sky-Watcher BK 70mm AZ Refractor has a focal length of 400mm.
- Meade 70mm Adventure Scope has a focal length of 400mm.
- Gskyer 70mm AZ Refractor has a focal length of 400mm.
What is a Long Focal Length in Telescopes?
In telescopes, “long focal lengths” refer to telescopes with a focal length that is typically greater than 1000mm. Long focal-length telescopes have a narrower field of view, which means they can capture a smaller area of the sky. However, they have stronger magnification, allowing viewers to observe details at great distances.
Advantages of a Long Focal Length in Telescopes
Longer focal lengths, typically ranging from 1000 mm to over 3000 mm, are common in deep space astronomy. This is because they provide high magnification and a narrow field of view, making them ideal for viewing distant objects, like planets or stars.
Disadvantages of a Long Focal Length in Telescopes
Long focal length telescopes are typically larger and heavier compared to short focal length telescopes. This can make them more difficult to transport and set up, and they may require a larger and sturdier mount to support the weight. They also have a narrower field of view, making it more difficult to locate and track objects in the sky. Another disadvantage is that they are more expensive than shorter focal length telescopes. This is due to the use of larger and higher-quality optics, as well as the need for a stronger and more stable mount.
Which Telescopes Have Long Focal Lengths?
Powerful telescopes have longer focal lengths to allow for deep space observation. The James Webb Space Telescope boasts a 131,400mm focal length, allowing it to magnify objects by a magnitude of thousands. Unfortunately, backyard space telescopes can’t support these incredible focal lengths, but many can still achieve magnifications up to 400x.
- Celestron NexStar 8 SE has a focal length of 2032mm.
- Meade LX200 Classic has a focal length of 2000mm.
- Orion SkyQuest XT12i IntelliScope has a focal length of 1200mm.
- Sky-Watcher Evostar 72ED DS Pro has a focal length of 900mm.
- Takahashi TOA-130 has a focal length of 1300mm.
How to Calculate Focal Length in Telescopes?
Focal length is calculated by multiplying the focal ratio by the diameter of the diameter of its primary optic. For example, if you have a telescope with a focal ratio of f/6 and an objective lens of 40mm, the telescope’s magnification would be:
Focal Length = 6 x 40mm = 240mm
The formula for focal length is: Focal Length = (Aperture) x (Focal Ratio)
How to Increase Focal Length in Telescopes?
Because magnification is a product of focal length and the length of eyepiece, increasing the focal length is a common way to improve your telescope’s power. The most common way to do this is by using Barlow lenses. Barlow lenses are concave lenses, placed before the eyepiece, that multiply the focal length by its set magnitude. Most barlow lenses increase your focal length by a factor of 2 or 3, but some designs yield over 10x magnification.