I just bought a Skywatcher telescope with a diameter of 60 mm and a focal length of 900 mm. I have a 2x Barlow lens and two eyepieces with focal lengths of 10 mm and 20 mm.

How can I calculate the magnification of my telescope, for example using the 2x Barlow and the 20mm eyepiece?

I'm fascinated by watching the sky and I can't wait to try it... Unfortunately, it's currently cloudy.

One more question; what I can see to be fascinated with this telescope?

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    $\begingroup$ I'm pretty sure you mean this and English is not your first language, so I've made some adjustments. Have a look and see if this is what you mean. Thanks, and Welcome to Stack Exchange! By the way if you can mention which model it is from one of these websites for example, it will be very helpful! skywatcher.com/category/telescopes and opticalvision.co.uk/astronomical_telescopes-sky-watcher.html $\endgroup$
    – uhoh
    Commented Jun 11, 2020 at 15:02
  • $\begingroup$ The Pleiades are a good target. Easy to find, and you'll see a lot more stars there with your scope than are visible with the naked eye. $\endgroup$
    – PM 2Ring
    Commented Jun 12, 2020 at 7:07
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    $\begingroup$ @PM 2Ring Pleiades is a great suggestion, but not in mid June, as it is too close to the sun. $\endgroup$
    – Dr Chuck
    Commented Jun 12, 2020 at 11:11

1 Answer 1


Magnification of your telescope depends on the ratio of the focal length of your primary optics and the focal length of your objective. We can represent this with this simple formula: $$P=\frac{f_{objective}}{f_{eyepiece}}$$ where $P$ is magnification (power) and $f$ is focal length.

Your telescope has 900 mm focal length, so the magnification is $P_1=\frac{900\text{ } mm}{10\text{ }mm}= 90\text{x}$ (10 mm ocular) and $P_2=\frac{900\text{ }mm}{20\text{ }mm}= 45\text{x}$ (20 mm ocular).

A Barlow lens is used to increase the focal length by the given factor (thus 2 in this case), so directly impacts the magnification in the same way, increasing it to 180x and 90x - but gives you in return an equally smaller field-of-view and makes area-objects fainter as the same light is projected to a larger area. Note that there is a maximum useful magnification which is roughly twice the aperture (measured in mm), which is 120x in your case, so the Barlow lens will not be very helpful as it magnifies beyond reasonable due to inevitable diffraction on the clear aperture.

One usually compares them by their focal length and by their aperture - the ratio defines how bright the objects appear and thus how faint objects you can see. With an aperture of 60 mm, it has an opening ratio of f/15 which is only a moderate one.

Such telescope is particularly useful for observations of the planets in our solar system as well as star clusters and the brighter deep-sky objects. Now in summer would be an excellent time to hunt for the globular clusters like the one in Hercules (M13). Also, you can download a useful program Stellarium to find more interesting features.

  • $\begingroup$ f/25 is good or bad?20mm have better zoom of 10mm?Sorry,but this is the first time works with telescope. $\endgroup$
    – astrokid
    Commented Jun 11, 2020 at 12:36
  • $\begingroup$ The smaller the focal length of your ocular, the larger is your magnification. An aperture of f/15 is just describes how bright things appear... the wider your telescope is, the more light it gathers, the brighter things appear... $\endgroup$ Commented Jun 11, 2020 at 12:41
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    $\begingroup$ yes indeed. I added a small paragraph about that. $\endgroup$ Commented Jun 11, 2020 at 15:03
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    $\begingroup$ Thanks a lot guys. $\endgroup$
    – astrokid
    Commented Jun 11, 2020 at 20:47
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    $\begingroup$ "A Barlow lens is used to reduce the focal length..." No, just the opposite! Barlow lenses increase the focal length & the magnification. $\endgroup$
    – D. Halsey
    Commented Jun 12, 2020 at 23:43

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