Astronomy 101: Tips & Tricks
  • Magnification
  • Add on Aug. 3, 2017, 1:50 p.m.

Often asked question for beginners to astronomy                                           

Is high magnification better?

How do I calculate magnification power?

How much magnification can I use with my telescope?


Usable Magnification


Some telescope advertisements include phrases about the very high magnification or power that their instruments can achieve. These telescopes usually have about 60mm (2.4") diameter apertures, and claim magnifications of 600x or more. It is true that their images can be magnified that much but what they end up magnifying is all the turbulence in the air between the telescope and the subject. When you are looking at astronomical objects, you are looking through a column of air that reaches to the edge of space and that column seldom stays still. Similarly, when viewing over land you are often looking through waves of heated air radiating from the ground, houses, buildings, etc. A good rule of thumb is that the usable magnification of a telescope is about 50x per inch (2x per mm) of aperture under good conditions. Values of 3x per millimeter or higher are often quoted for ideal conditions, but these conditions are usually very rare. The final resolution that an astronomical telescope can achieve depends on the amount of light that it can capture. The bigger the aperture, the higher the resolution and therefore the better the image. However, there are times when the earth's atmosphere is so unsettled that a smaller aperture will give better results because it sees fewer turbulent zones. A telescope cap with a smaller opening which acts as a mask, can prove to be a useful accessory under these conditions. Sky conditions are usually defined by two atmospheric characteristics, seeing, or the steadiness of the air, and transparency, the clarity of the air due to the amount of water vapour and particulate material present



How much magnification can I use with my telescope?

Every telescope is different, but a rough rule of thumb is 30-50X per inch diameter of the objective. A good refractor may, however, use 100X/inch on bright objects, so this is not a hard rule. You can always increase the magnification above these limits, but it is pointless if you're not seeing more. This rule breaks down for larger instruments, as the distortion of the atmosphere limits practical magnification to 300X. See Usable Magnifications.

Is Higher magnification better?

Only for some objects, although undermagnification is often a problem, even for experienced observers. The penalty for increased magnification is reduced field of view and brightness; faint objects grow fainter as the magnification is increased This is why larger aperture telescopes are so effective on faint objects; they provide enough light to stimulate the eye at high magnifications. For example, a 4-inch telescope will only view a globular cluster effectively at 80X, and it will appear as a blob. A 6-inch will resolve the outer stars at 130X, an 8-inch will resolve further in at 200X. 10 and 12.5-inch telescopes will make them glitter to the core at 300 and 400X.


How do I calculate the magnification power?

a general formula for calculation of the magnification of telescope to eyepiece
Focal Length of the scope ÷ Focal Length of Eyepiece = Magnification Power




Table of magnification Eyepiece to Telescope

As google spreadsheet doesn't support fixed row and column, please
Click this to View in google sheet for easier viewing


Green cells are the most recommended eyepieces to scope

Red cells are the not recommended eyepieces to scope