Understanding the focal length of a telescope or refractor

Understanding the Focal-length of a Telescope or an Astronomical Refractor

When choosing a telescope or refractor, it is essential to understand the different technical aspects of these instruments. One of the most important concepts to master is that of focal length. But what exactly is focal length and why is it so crucial for astronomical observation? In this article we explain everything you need to know about focal length.

What is Focal?

Focal length, or focal length, is the distance between the center of the lens (or primary mirror) and the point where light rays converge to form a sharp image. It is generally measured in millimeters (mm).

Practical Example

Imagine a beam of light entering your telescope. Focal length is the distance that light must travel before being focused to a point where the image becomes clear and defined.

Importance of Focal Length

Magnification

Focal length plays a key role in the magnification of your instrument. Magnification is calculated by dividing the focal length of the telescope by the focal length of the eyepiece used. For example, with a focal length of 1000 mm and an eyepiece of 10 mm, you will obtain a magnification of 100x.

• Long Focal Length: A telescope or refractor with a long focal length will provide higher magnification with the same eyepiece, which is ideal for observing planets and lunar details.
• Short Focus: A short focal length provides a wider field of view, perfect for observing deep sky objects like nebulae and galaxies.

Rapport F/D

The F/D ratio (or f/ratio) is another crucial aspect. It is obtained by dividing the focal length by the diameter of the aperture (the lens or primary mirror).

• Low f/ratio (f/4 - f/6): These telescopes are fast and are excellent for astrophotography because they collect more light in less time.
• f/high ratio (f/10 and above): They offer sharper images and contrast, ideal for observing plants and the Moon.

Choose the Right Focal Length

The choice of focal length depends on what you want to observe:

• Plantary Observation: Opt for a long focal length (1200 mm or more) to obtain detailed images of plants.
• Deep Sky Observation: Prefer a short focal length (600 mm to 800 mm) to benefit from a wide field of vision and capture faintly luminous objects.
• Astrophotography: A telescope with a low F/D ratio (f/4 - f/6) will be more suitable.

Examples of Telescopes and Glasses

• 90/900 Astronomical Refractor: Here, 90 mm represents the diameter of the objective and 900 mm the focal length. It is a good choice for plantar observations.
• Newtonian Telescope 200/1000: With a diameter of 200 mm and a focal length of 1000 mm, this telescope offers an excellent compromise between light collection and magnification.

Conclusion

Understanding the focal length of a telescope or refractor is essential to choosing the instrument that will best meet your observing needs. Whether you're passionate about plants, stars or deep sky objects, the focal length will help you maximize your observing experience. Consider your goals and choose your instrument wisely based on its focal length.

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To find the telescope or refractor with the ideal focal length for your observations, explore our collection on Astronomy Store . Our experts are available to advise you and help you choose the perfect equipment.