The first thing to know is that the magnifying power of a telescope is limited by the amount of light that the telescope aperture can collects. The two other important values for accurately calculating the magnification are the focal length of your telescope and the focal length of the eyepiece. These two values are indicated on a label placed on both items. The magnification formula is quite simple:
The telescope FL divided by the eyepiece FL = magnification power
Example: Your telescope FL is 1000 mm and your eyepiece FL is 20 mm. 1000/20= 50x! The image seen in your eyepiece is magnified 50 times!
Being able to quickly calculate the magnification is ideal because it gives you a more:
- Flexibility, because each celestial object has a different brightness (magnitude) and you may want to quickly increase or decrease the magnification when changing targets.
- Responsiveness, because the weather conditions can greatly affect your viewing and you want to be able to adjust your magnification quickly and accordingly.
Note: The focal length of your telescope is a fixed value that cannot be changed, so the only way to increase or decrease the magnification is to change the eyepiece or Barlow lens.
Lowest Useful Magnification: You can calculate your telescope lowest magnification by multiplying your aperture (in inches) by 3 or 4 times. For a 4-inch telescope, the minimum useful magnification is between 12x and 16x. At this power, you will have a wider field of view (FOV) and a brighter image, which is ideal for large celestial objects such as galaxies and nebulae.
Highest Useful Magnification: Calculating the maximum useful magnification of your telescope is quite simple. If you work in inches, you multiply this value by 50. So, for an 8-inch telescope, the maximum magnification you can use is x400. If you exceed this value, you will get an overly magnified image.