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Telescope Power Calculators

Ah! The question on everyone’s mind… How powerful is your telescope? You would think this is a simple question to answer. Telescope power is just magnification right? Not exactly. It’s a bit more complexe than that.

Focal Length

Let’s look at some telescope math. Every telescope has a stated focal length, which is effectively the distance from the primary lens or mirror to the point at which it forms an image of a very distant object. In other words, it’s the distance that light travels within the telescope before getting to the eyepiece. Eyepieces have a focal length as well, expressed in millimeters. This number is important because it is used in some calculations.

Focal Ratio Calculation

The focal ratio is based on focal length and aperture. The smaller the focal ratio, the faster the telescope. Fast telescopes like f/4 or f/6 provide wider fields of view (FOV) making them most suitable for deep space observation. The larger the focal ratio, the slower the telescope. The FOV becomes smaller as the focal ratio gets larger. Telescopes with large focal ratio’s like f/8 or f/10 are most suitable for high power planetary work.

Here’s how you calculate the focal ratio: Take the focal length (in mm) and divide by the aperture (also in mm). Our 8″ (203.2 mm) Dobsonian with a 1200mm focal length therefore has an f/5.9 ratio. (1200 divided by 203.2 = 5.9). A bigger aperture means a smaller focal ration, hence a faster telescope.

Enter Focal Length (mm):
Enter Aperture (mm):

The focal ratio is :


Telescope Magnification Calculation

A lot of people will want to describe the power of a telescope with magnification, and that’s because they don’t know what they’re talking about! Telescope aperture is much more important because it influences the focal ratio explained below. Aperture refers to the size of the opening at the end of the telescope (the end pointing to the sky). The idea is that a bigger aperture allows more light to come in, and with more light we can see more objects, and we can see them more clearly. Magnification does come into play at some point, but it is not the most important aspect.

The magnification factor is based on the telescope’s focal length and the eyepiece focal length. The formula is simple: divide the focal length of the telescope by that of the eyepiece. For example, if I have an 8″ dobsonian with a 1200mm focal length and use a 10mm eyepiece on it, then I am viewing the sky at 120x magnification. If I use the exact same eyepiece on a different telescope, one that has a focal ration of 800mm, then I am viewing the sky at 80x magnification. Therefore magnification is really defined by the eyepiece AND the telescope you use it with. The higher the mm of the eyepiece is, the less magnification you are using.

Telescopes have a range of “useful” magnification. The department store scope that advertises 700x magnification can probably do just that on paper, except the eyepiece is so cheap you won’t be able to see anything at that magnification. Additionally, the maximum magnification is probably exceeded. A telescope’s maximum “useable” magnification is based on the aperture size in mm x 2. Our 8″ telescope translates to 203.2mm. Multiply this by 2 and that gives the maximum magnification of about 406x. Department store telescopes claiming 700x magnifications probably have a 90mm aperture, meaning their maximum usable magnification is actually 180x magnifications. That’s a HUGE difference from what is advertised so beware!

Enter Focal Length (mm):
Enter Eye Piece Focal Length (mm):

The telescope magnification is :


Minimum Magnification Calculation

There’s also a minimum telescope magnification which is based on your telescope’s focal ratio. Now that we know our 8″ Dobsonian is an f/5.9, we can calculate the minimum magnification by multiplying that number by 7. In our case, 5.9 x 7 = 41.3 so this means that the highest mm eyepiece I should use with my telescope is a 41.3mm. Anything beyond that will exceed my telescope capacity. Remember, the higher mm value of the eyepiece, the less magnification you get. In other words, a 10x eyepiece has more magnification than a 32x. Less magnification means a wider field of view.

Enter Focal Ratio :

Max eyepiece size (mm) :


Maximum Telescope Magnification

Telescope marketing is often based on maximum magnification, and this is misleading. While a telescope can theoretically magnify up to a certain range, in practice the maximum specifications found in ads are often beyond the range of a “useful” magnification. Too much magnification means it’s difficult to focus, the viewing quality drops considerably, the clarity is compromised. It’s important to understand how far you can push your telescope because it would be a shame to purchase eyepieces that are too powerful for your telescope. What we recommend as a comfortable guide under normal viewing conditions is 30 x the aperture size (in inches) of the telescope. For example, if using an 8″ dobsonion telescope, we would suggest not to exceed 240x magnification (240 = 8 inches x 30). Of course, you can go beyond this if you choose too, but you’ll find that the viewing experience will diminish the more you magnify.

Enter aperture size (inches):

The maximum magnification is :


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