Secondary mirrors - a key component of every reflector
A secondary mirror is an indispensable part of every reflector telescope. It redirects the light collected by the primary mirror to the focal point, where the image is observed through an eyepiece or captured by a camera. The quality of the secondary mirror directly affects the sharpness and contrast of the resulting image. Without a properly ground and precisely positioned secondary mirror, even the best primary reflector cannot reach its full potential. That is why choosing the right secondary mirror is one of the most important decisions when building or upgrading a telescope.
Who are secondary mirrors intended for
Secondary mirrors for telescopes are suitable for two main groups of customers. The first group consists of enthusiasts and astronomers who want to improve the performance of their existing reflector - for example, replacing a worn or lower-quality mirror with a better one. The second group includes builders and hobbyists who are constructing their own telescope from scratch. Building your own telescope is a wonderful and educational project that brings a deep understanding of astronomical optics. For both types of users, we offer mirrors in various sizes and shapes to meet different design requirements.
Types and parameters of secondary mirrors
The most commonly used type of secondary mirror is an elliptical flat mirror designed for Newtonian reflectors. The size of the mirror is specified by the length of its minor axis and must correspond to the diameter of the primary mirror and the parameters of the optical tube. A smaller secondary mirror causes less central obstruction, which has a positive effect on image contrast. A larger mirror, on the other hand, allows a wider field of view to be illuminated, which is advantageous for astrophotography. Choosing the right size is therefore a compromise between visual observation and astrophotography.
Optical surface quality and surface coating
The accuracy of the optical surface of a secondary mirror is expressed as a fraction of the wavelength of light, typically lambda/10 or lambda/8. The more precise the surface, the sharper and more contrasty the image produced by the telescope. The mirror surface is usually coated with a reflective aluminium layer with a protective overcoating, which increases the durability and resistance of the mirror. Some premium mirrors have a silver or dielectric coating for maximum reflectivity. When selecting a mirror, we recommend paying attention to both the surface accuracy and the quality of the coating.
How to choose the right secondary mirror
Selecting a secondary mirror depends primarily on the diameter of the primary mirror, the focal length, and the intended use of the telescope. For visual observation, it is recommended to choose a mirror with as little central obstruction as possible, typically 20-25% of the primary mirror diameter. For astrophotography, a slightly larger mirror is preferable to illuminate the entire sensor field. When replacing a mirror in an existing telescope, compatibility with the secondary mirror holder must be maintained. If you are unsure about the selection, we are happy to help you calculate the optimal size.
Mounting and collimation of the secondary mirror
Proper mounting and collimation of the secondary mirror are just as important as its quality. The mirror must be precisely centred in the tube and tilted at the correct angle relative to the primary mirror. Adjustable holders with collimation screws are used for this purpose. Regular inspection and collimation of the telescope will ensure consistently optimal performance of the optical system. We also recommend protecting the mirror from dust and mechanical damage, which would reduce its reflectivity and accuracy.
