Telescope Magnification

Want to stay on top of all the space news? Follow @universetoday on Twitter

Just what exactly is telescope magnification? If you’ve never used a telescope before, it’s easy to fall for department store advertising hype that sounds like “magnifies up to 1000X!”. At first, this might sound like what you’re looking for… but it’s not. The magnifying power of a telescope is in its inherent design and the eyepieces. Why are there so many different eyepieces? The answer is magnifying power. The magnifying power of any eyepiece is a simple equation expressed in millimeters: divide the focal length of the telescope by the focal length of the eyepiece and your answer is the amount of magnification your telescope will deliver. There’s a lot of different variables that go into a telescope’s magnification power, so let’s learn…

The Focal Length of a Telescope Helps Determine a Telescope’s Magnification

To help understand magnification, first you must understand focal length and f-ratio. The focal length of a telescope is the length of the light path to form a real image. These numbers help to determine how wide the visual field can be achieved with a particular eyepiece The f-ratio of a telescope is the ratio between the focal length of the telescope and the diameter of the primary light gathering source. Low f-ratios, such as f/4 indicate wide fields of view with low practical magnification limits, while high f-ratios, such as f/10 indicate restricted fields with high practical magnification limits.

Eyepiece Focal Length and a Telescope’s Magnification

When it comes to understanding eyepiece focal length, it’s best to know this term is nearly synonymous with “magnifying power”. The magnifying power of any telescope is its eyepiece and a simple equation expressed in millimeters: divide the focal length of the telescope by the focal length of the eyepiece. Long focal length eyepieces such as 32mm and 25mm are lower magnification, while lower numbers like 10mm and 5mm are magnifying powerhouses.

So let’s talk about magnifying powers! Low magnification such as 20-50X are ideal for wide-field views of extended deep-sky objects under dark skies, while 20-100X are great for helping to locate objects and most deep sky observing. 75-200X is suitable for the Moon, planets and more compact objects such as globular clusters or wide double stars, while more than 200X truly requires steady skies to make the additional magnification work for you.

Telescope Magnification and Practical Limits

While it would be tempting to use as much magnification as possible, all telescopes (and the human eye) have practical limits. If you look through your telescope with two different focal length eyepieces, you’ll see you have the choice of a small, bright, crisp image or a big, blurry, dimmer image – but why? A telescope can only gather a fixed amount of light. When using high magnification (lower focal length number) you’re only spreading the same light over a larger area. Even the best telescope can only deliver a certain amount of detail and magnifying beyond a telescope’s limits only makes for empty magnification.

Telescope Magnification and the Exit Pupil

To make a particular focal length eyepiece work well with your telescope, you’ll need to determine exit pupil. Before you panic at the thought of more math, exit pupil is merely the focal length of the eyepiece divided by the f-ratio. For example, an f/5 telescope with a 25mm eyepiece would deliver 5mm of exit pupil. Why is this important? At low magnification the exit pupil must be smaller than your eye (5-7mm) or the extra light is simply lost. For high magnifications, the exit pupil must be between .5 and 1.0mm of exit pupil to avoid empty magnification. Using our f/5 example, a 5mm eyepiece would deliver 1.0mm of exit pupil and be right at the practical limits of magnification.

How To Choose Your Telescope’s Magnification Power

Now that you know a little bit more about choosing an eyepiece by focal length, your telescope’s focal length and f ratio, let’s take a very brief look at what each magnification and eyepiece is well suited for! Remember, these are just general applications. 2mm-4.9mm Eyepieces produce very high magnification and will work best on long focal length refractors and standard Schmidt-Cassegrains. Unless you have very steady seeing, this range more than likely will produce too much magnification on other telescope styles. 5mm – 6.9mm Eyepieces make good planetary detail and double star eyepieces for long focal length telescopes and will work satisfactorily in shorter focal length telescopes with steady seeing conditions. 7mm – 9.9mm Eyepieces are right in the optimum ballpark for high magnification for shorter focal length telescopes and serve as good planetary, double star and lunar details units. 10mm – 13.9mm Eyepieces work across all focal lengths and offer great background darkening capabilities for studying planetary nebula, small galaxies, planetary details and lunar details. 14mm – 17.9mm Eyepieces are a great mid-range magnification for all focal lengths and will help resolve globular clusters, galaxy details and spot planetary nebulae. 18mm – 24.9mm Eyepieces will work as nicely on long focal length telescopes to show wide field and more extended objects. Shorter focal length telescopes will enjoy great mid-range magnification for objects like galaxy clusters and large open clusters. 25mm – 30.9mm Eyepieces are extended field eyepieces for longer focal length – good for large nebula and open clusters. For shorter focal length, they are fantastic for large objects such as the Orion nebula, views of the full lunar disc, large open clusters and more. This multi-purpose magnification range also makes for good “locator” eyepieces in all focal lengths. 31mm – 39.9mm Eyepieces are well suited to shorter focal length telescopes for extended views while 40mm Eyepieces and Above are exclusively the domain of shorter focal lengths. This magnification range is superb for showing large, starry vistas… extended nebula with star fields, etc. Zoom Eyepieces are very nice for terrestrial applications .

About Tammy Plotner

Tammy is a professional astronomy author, President Emeritus of Warren Rupp Observatory and retired Astronomical League Executive Secretary. She’s received a vast number of astronomy achievement and observing awards, including the Great Lakes Astronomy Achievement Award, RG Wright Service Award and the first woman astronomer to achieve Comet Hunter's Gold Status.