The maximum useful magnification of a refracting telescope isn't a fixed number, but rather depends on the telescope's aperture (diameter of the objective lens). The practical limit is generally considered to be around 50x per inch of aperture, or about 2x per millimeter.
Understanding Maximum Useful Magnification
While you can theoretically achieve very high magnifications with a telescope by using eyepieces with very short focal lengths, the resulting image will likely be dim, blurry, and lacking in detail. This is because:
- Diffraction: Light waves bend around the edges of the objective lens (diffraction), blurring the image. Higher magnification only enlarges this blurred image.
- Atmospheric Seeing: Turbulence in the Earth's atmosphere distorts the image, especially at high magnifications. This is a major limiting factor.
- Optical Imperfections: Even the best telescope optics have minor imperfections. High magnification exaggerates these flaws.
- Light Gathering: Increasing magnification spreads the light collected by the objective lens over a larger area, resulting in a dimmer image.
Rule of Thumb: Exit Pupil Size
A more practical way to determine the maximum useful magnification is by considering the exit pupil, which is the diameter of the beam of light exiting the eyepiece.
- Optimal Exit Pupil: The optimal exit pupil size is often cited as being around 0.7mm to 0.8mm. This matches the typical diameter of the human eye's pupil in dark conditions.
- Calculating Magnification: To calculate the magnification that produces this exit pupil, divide the telescope's aperture (in mm) by the desired exit pupil size (in mm).
Example:
A telescope with a 100mm aperture:
- Maximum useful magnification ≈ 100 mm / 0.7 mm = ~143x
- Maximum useful magnification ≈ 100 mm / 0.8 mm = ~125x
Therefore, for this telescope, a magnification of roughly 125x to 143x would likely be the upper limit of useful magnification.
Factors Affecting Magnification
Keep in mind that these are guidelines. The "best" magnification for a given night and target will depend on:
- Atmospheric Conditions (Seeing): On nights with poor seeing, lower magnifications will provide sharper images.
- Object Brightness: Fainter objects require larger apertures and may benefit from lower magnifications to maintain image brightness.
- Telescope Quality: Higher-quality optics will generally allow for higher magnifications.
- Observer Experience: Experienced observers may be able to extract more detail at higher magnifications than beginners.
In Summary
While there's no hard and fast "maximum magnification," a good rule of thumb is to aim for a magnification that produces an exit pupil between 0.7mm and 0.8mm. Ultimately, the best magnification is the one that provides the most detailed and pleasing view of the object you're observing.
