Beginner instruments most often have a diameter between 60 and 150 mm (60 to 80 mm for a telescope, 100 to 150 mm for a telescope). After a few years, deep sky enthusiasts begin to dream of large diameter telescopes (250 mm or even 400 mm) to hunt the most distant galaxies or the pale nebulae that barely stand out from the sky.
- Simulation of an observation of the Moon with two instruments of very different diameters (60 mm and 300 mm). The diameter of a telescope affects both clarity and level of detail. This simulation was carried out with the Stelvision simulator which you can use yourself to evaluate what it is possible to see in a telescope according to its diameter.
As for the focal , it has an influence on:
The magnifications obtained, stronger with a long focal length (but be careful, we will see later that a strong magnification is not always desirable).
The size of the instrument
A conventional telescope is always quite long : its focal length is around 10 to 13 times its diameter, for example 700 mm for a diameter of 60 mm. It is a condition for providing good images within a reasonable budget. Indeed, most glasses suffer from a defect called chromatic aberration which causes the appearance of colored borders at the edge of the image and a loss of sharpness . This phenomenon is negligible in the case of long glasses and more marked in the case of short glasses (except for certain very expensive models which use a sophisticated optical design). Now with the Best Telescope For Viewing Planets And Galaxies you can find the best deal here.
A Newton type telescope can have a short focal length for example 5 or 6 times its diameter, for example 1200 mm focal length for a diameter of 200 mm, as long as its main mirror is a true parabolic mirror – and not a spherical mirror found on inexpensive models.
Let us first clarify that magnification is not a real criterion for choosing a telescope or an astronomical telescope . Indeed, the magnification depends on the eyepiece used. An instrument is always delivered with several eyepieces and additional eyepieces can also be purchased separately to complete its range of magnifications. We can therefore consider the magnification as an adjustable parameter.
Then, it is important not to believe that an astronomical instrument should grow as much as possible. Because low or moderate magnifications can be much more appropriate than high magnifications in many cases. Indeed:
- High magnification makes the image less bright
- It makes you lose sight of everything by narrowing the field of vision
it may lead to a blurred image due to the limited resolution of the instrument; this limitation is inversely proportional to the diameter, so that in general we avoid any magnification greater than 2 times the diameter of the instrument in millimeters; often, a magnification limited to 1 or 1.5 times the diameter is preferable.