When selecting eyepieces for a 6-inch Dobsonian telescope, it’s important to consider the magnification and the practical limits imposed by atmospheric conditions and optical design.
The magnification of a telescope is determined by dividing the focal length of the telescope by the focal length of the eyepiece. For a 6-inch Dobsonian with a typical focal length of around 1200mm, using a 10mm eyepiece would provide a magnification of 120x (1200/10), while a 5mm eyepiece would yield 240x (1200/5).
A general guideline for maximum useful magnification is about 2 times the diameter of your telescope’s aperture in millimeters. For a 6-inch (150mm) telescope, this results in a theoretical upper limit of around 300x magnification under ideal conditions. However, atmospheric stability often limits practical magnification to much lower levels, usually around 150x to 250x for most nights, depending on seeing conditions.
A 10mm eyepiece generally offers a comfortable magnification that can be effectively used most nights. A 5mm eyepiece, while useful for achieving higher magnification, could lead to a dimmer and sometimes blurrier image due to reaching the limits of atmospheric stability and the telescope’s capabilities. Therefore, while a 5mm is not “too small,” it will primarily be useful on nights of excellent seeing if the telescope and eyepiece quality are sufficient to resolve detail at such high magnification.