By Alexander Merz · Updated on · Details checked on 20 September 2026
With the telescope calculator, you can quickly determine the key figures of your telescope:
⚡ The most important formulas at a glance: Magnification = telescope focal length ÷ eyepiece focal length. Focal ratio = focal length ÷ aperture. Useful maximum magnification ≈ 2 × aperture in mm (a 130 mm telescope therefore usefully reaches about 260x). With the calculator below you get all values automatically.
- What is the focal ratio of my telescope?
- Magnifications? What is the maximum and the useful magnification of my telescope?
- What magnification do I achieve with which eyepiece?
If you have further wishes about what the calculator should be able to compute, feel free to write me an e-mail at info@sterngucker.de!
If you want to buy a telescope, you can quickly enter the values in advance and see what can be displayed with the telescope.
Telescope calculator
Output
Focal ratio:
Magnification:
Maximum useful magnification:
Minimum useful magnification:
Optimal magnification:
What will I see with this telescope?
Object
- Magnification
- –
- True field of view
- –
- Exit pupil
- –
- Limiting magnitude
- –
- Resolution (Dawes)
- –
Drawn to scale: the circle is the eyepiece view, the objects appear at the size that follows from magnification and field of view. Colours and brightness are simplified – the nebula stays grey in the eyepiece, the galaxy a faint glow.
The recommended telescopes compared
| Telescope | Magnification | True field of view | Exit pupil | Limiting magnitude | Resolution (Dawes) |
|---|---|---|---|---|---|
| Skywatcher Heritage 130/650 130/650 mm | 26× (25 mm) 65× (10 mm) | 2.00° 0.80° | 5.0 mm 2.0 mm | ≈ 13.3 mag | 0.9″ |
| Skywatcher Skyliner 200/1200 Dobson 200/1200 mm | 48× (25 mm) 120× (10 mm) | 1.08° 0.43° | 4.2 mm 1.7 mm | ≈ 14.2 mag | 0.6″ |
| Omegon AC 70/700 AZ-2 70/700 mm | 28× (25 mm) 70× (10 mm) | 1.86° 0.74° | 2.5 mm 1.0 mm | ≈ 11.9 mag | 1.7″ |
| Skywatcher Skymax 127/1500 Maksutov 127/1500 mm | 60× (25 mm) 150× (10 mm) | 0.87° 0.35° | 2.1 mm 0.8 mm | ≈ 13.2 mag | 0.9″ |
| ZWO Seestar S50 Pro (smart telescope) | – | 1.29° × 0.73° (Sensor field of view) | – | ≈ 11.2 mag | 2.3″ |
| Binoculars 7x50 (Celestron Cometron) 50/175 mm | 7× | 6.86° | 7.1 mm | ≈ 11.2 mag | 2.3″ |
A telescope is a fascinating instrument that allows us to observe the universe and explore its mysteries. To get the most out of a telescope, it is important to understand the various key figures that result from the aperture, the focal length of the telescope and the focal length of the eyepiece used.
In this text we will take a closer look at some of these key figures: magnification, light-gathering power, resolving power and field of view.
Magnification
The magnification indicates how much larger an object appears through the telescope compared to the naked eye. It is one of the most important key figures and depends on the focal length of the telescope (F_T) and the focal length of the eyepiece used (F_O). The magnification (V) is calculated as follows:
V = F_T / F_O
A telescope with a focal length of 1000 mm and an eyepiece with a focal length of 10 mm produces, for example, a magnification of 100x.
What magnification makes sense and how eyepieces work together is explained in our eyepiece guide.
The calculator tells you what an instrument can do – not which one fits you.
Magnification and aperture are only half the answer: your sky, your space and your budget decide the rest. Six short questions, an honest answer by email – no sales pressure.
Light-gathering power
The light-gathering power of a telescope describes its ability to collect light from celestial bodies and thereby increase image brightness. It depends on the aperture of the telescope (D) and is often given in relation to the light-gathering power of the human eye. The light-gathering power (L) is calculated as:
L = (D / D_eye)^2
where D_eye is the diameter of the pupil of the human eye (about 7 mm). A telescope with an aperture of 200 mm has, for example, a light-gathering power of about 820, which means it collects 820 times more light than the human eye.
Resolving power
The resolving power of a telescope is its ability to display objects that lie close together as separate. The better the resolving power, the more details can be recognized in an image. The resolving power (A) depends on the aperture of the telescope and is measured in arcseconds (arcsec). It can be calculated with Dawes’ formula:
A = 116 / D
where D is given in millimeters. A telescope with an aperture of 200 mm has, for example, a resolving power of about 0.58 arcseconds.
Field of view
The field of view (FOV) is the area of the sky that can be viewed through the telescope. It depends on the focal length of the telescope and the focal length of the eyepiece used, as well as on the so-called “apparent
field of view” (AFOV) of the eyepiece. The field of view is important for knowing how much sky area can be observed at once and can help to select suitable eyepieces for specific observations. The true field of view (TFOV) is measured in degrees and can be calculated as follows:
TFOV = AFOV / V
where AFOV is the apparent field of view of the eyepiece (in degrees) and V is the magnification. If, for example, an eyepiece with an apparent field of view of 50 degrees is used and the magnification of the telescope is 100x, the true field of view is 0.5 degrees.
Summary
The various key figures that result from the aperture, the focal length of the telescope and the focal length of the eyepiece used play an important role in fully exploiting the potential of a telescope.
By understanding magnification, light-gathering power, resolving power and field of view, observers can choose the best possible combination of telescope and eyepiece to view celestial bodies and phenomena in the best possible quality.
The choice of the right eyepiece, depending on the intended observations and the properties of the telescope, is crucial for achieving the best possible results. Knowing the key figures mentioned above and how to calculate them helps to make informed decisions and ultimately to gain a deeper understanding of the universe in which we live.
Want to buy your first telescope? Here’s a quick check.