You're ready to start looking for a new telescope, but you're coming across a number of technical terms that are unfamiliar to you.
"Why is this so complicated?", you may ask yourself, "Can't I simply look through the eyepiece and see all the stars and planets?".
Well, sort of, but the specifications of the telescope you choose can make a significant difference on your ability to see the various celestial objects in the night sky.
Some telescope specifications can be difficult to intuitively grasp at first glance, so in this post we will provide you with a clear overview of the primary technical terms you will see.
Although there are additional model-specific specifications sometimes listed under product details, we will be focusing on the most commonly recognizable terms you should absolutely know.
5 primary telescope specifications to know
Aperture - determines the amount of light gathered
The telescope's aperture is the diameter of the main lens or mirror that determines how much light can be gathered from a distant object being imaged. The larger the aperture, the more light can be gathered. This is important for clarity, sharpness and overall detail that can be observed in distant objects.
Aperture is usually measured in millimeters, however, you will sometimes see aperture listed in inches as well.
As a quick reference on aperture size, our human eyes have an aperture of 7mm. In comparison, a good entry-level telescope has an aperture in the 70mm - 100mm range.
Focal Length - determines field of view
Focal length is the distance the light travels within a telescope tube. The entry point is the aperture and the exit point is the focuser.
In many cases, the longer the telescope tube, the longer the focal length. However, this is not always true since certain types of telescopes incorporate additional mirrors, which allow the light to bounce around multiple times, thereby increasing the focal length without actually increasing the size of the optical tube.
Entry-level telescopes have a focal length in the 600mm range and higher-end models extend towards 2,000mm focal lengths.
A longer focal length provides the ability for a more narrow field of view, which is great for observing the planets and the moon. On the other hand, a shorter focal length widens the field, which is more ideal for observing galaxies and the milky way.
Focal Ratio - determines speed and brightness
The focal ratio is often referred to as the f-number. It is calculated by dividing the telescope's focal length by the aperture. This measurement is important for collecting more or less light per pixel. Essentially, it is a measure for image brightness.
Often times focal ratio will be described in speed metrics as faster or slower. This is in relation to photography. Faster means less exposure time needed, slower means more exposure time needed.
The faster focal ratio (ex. f/3) delivers more light per pixel and the slower focal ratio (ex.f/9) delivers less light per pixel. For reference, the f-number of the human eye ranges from f/8 to f/2.1.
Another way to think about this concept is that the larger the focal ratio, the higher the magnification and the narrower the field of view. Alternatively, the smaller the focal ratio, the lower the magnification and the wider the field of view.
When observing the moon and planets, a focal ratio of f/10 or higher is ideal. If galaxies and the milky way are being observed, a focal ratio of f/7 or less will be more useful.
Magnification - determines the enlargement of objects
Magnification is sometimes referred to as the telescope's power. Basically, it provides the ability to increase the size of an image while limiting the field of view.
A telescope's magnification is calculated by dividing the telescopes's focal length by the focal length of the eyepiece.
For example, if your telescope has a focal length of 1200mm and your eyepiece has a focal length of 30mm, than 1200 / 30 = 40. Therefore your image is magnified 40x. If you are looking to adjust the power of your telescope, you will need to exchange the eyepiece for a different one.
Typically, the maximum usable power is about 50 times the telescope's aperture in inches or 2 times the aperture in millimeters.
However, it is important to note that as power increases the sharpness of the image decreases. In other words, if the telescope is not collecting enough light, when the image is enlarged it becomes too faint.
Additionally, at very high power, the image is more easily blurred by atmospheric turbulence.
In theory, below are the commonly quoted specifications regarding maximum usable power and telescope aperture size:
| Aperture (inches) |
Aperture (mm) |
Maximum Usable Power |
| 2.4 | 60 | 120x |
| 3.1 | 80 | 160x |
| 4 | 100 | 200x |
| 5 | 125 | 250x |
| 6 | 150 | 300x |
| 8 | 200 | 400x |
| 10 | 250 | 500x |
| 12.5 | 320 | 600x |
| 14 | 355 | 600x |
| 16 | 400 | 600x |
| 17.5 | 445 | 600x |
| 20 | 500 | 600x |
Resolution - determines the observable detail
Telescope resolution determines the amount of detail that can be resolved in an image. This becomes important when you are trying to distinguish between two closely aligned stars or viewing the details of the moon's surface.
Resolution is measured in arcseconds (1/3600 of a degree) and is calculated using either Rayleigh criterion or Dawes limit.
Rayleigh resolution is calculated in inches as 5.45 / aperture or in millimeters as 138 / aperture. Dawes resolution is calculated in inches as 4.56 / aperture or in millimeters as 116 / aperture.
As a rule of thumb, the larger the aperture, the better the angular resolution.
Next steps
With your new understanding of telescope specifications, you are ready to confidently browse the various telescope types on the market.
At AstroTelescopium, we carry a curated selection of telescopes from industry-leading brands at value prices.
Feel free to browse our telescope collection.
If you're looking for more information, be sure to read how to choose the right telescope or simply connect directly with a member of our team.
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