Nov 22, 2024 by WO Team
Focal Length
The focal length of a telescope is the distance from the objective lens or mirror to the focal point where the telescope forms an image of an object. This measurement, usually given in millimeters (mm), determines the magnification power and the field of view of the telescope. Assuming the eyepiece is the same, a longer focal length results in a higher magnification.
In a refractor system, the focal length is proportional to the length of the tube, with shorter focal length tubes being more compact and portable. In a catadioptric system, the focal lengths are typically longer than 1000mm, resulting in higher magnification.
Note:
Magnification is not a fixed specification, but rather a relative relationship.
For visual observation, the magnification between the image seen through the telescope and the image seen by the naked eye can be calculated as:
In the field of astrophotography, however, there is no precise standard for calculating magnification. Instead, a widely accepted reference is to consider an image captured with a 50mm focal length lens and a full-frame (36mm x 24mm) sensor as having a magnification of 1x. Using this as a baseline, you can estimate the magnification of your camera and telescope setup. Alternatively, using the Field of View (FOV) as a criterion to choose a telescope can be more practical than focusing on magnification. FOV defines the actual area of the sky visible through the telescope, offering a standardized and meaningful way to gauge the telescope’s performance. Refer to How to calculate FOV?
Focal Ratio
The focal ratio (f/#) of an optical system describes the relationship between the focal length and the aperture diameter. It is commonly referred to as the aperture ratio, which indicates the image brightness performance of a telescope.
A smaller f-ratio means a larger aperture, which enhances the system's light-gathering ability and allows for shorter exposure times in astrophotography.
Diameter
The diameter, also known as the aperture, is the size of the telescope’s primary lens or mirror, measured in inches or millimeters. It is the most critical factor in determining a telescope’s light-gathering ability and resolution. Larger diameters gather more light, allowing faint objects to become visible and providing greater detail in the observed images.
Image circle
The image circle is the circular area over which a telescope can form a good quality image. It is the diameter of the usable image produced by the telescope or lens system on the camera sensor or eyepiece. A larger image circle means that the telescope can produce clear images over a larger area, which is particularly important in astrophotography to ensure the entire camera sensor receives a sharp, well-illuminated image.
In Triplet and Doublet lens systems, flatteners and reducers are commonly used to enhance image quality. A flattener corrects field curvature, ensuring that stars remain sharp at the edges. Meanwhile, a reducer affects the optical path, which, in turn, impacts the telescope’s original specifications, including parameters like focal length and image circle size. For more details, please refer to How Reducer Affects the Light Path.