So, my grandfather so graciously gave me his TASCO 49060700 60x700mm Spacestation Refractor Telescope, which is extremely wondrous. Any way, I found this answer to my 3x barlow lens, which I believe is saying multiply the magnification on the lens to the barlow. I've got three separate lenses; 25mm, 10mm, and a 4mm. My question is, do I multiply 25 by three(25 being the 25mm) and I get 75x magnification?


1 Answer 1


A telescope's magnification is its objective focal length divided by the eyepiece focal length: $$ \frac{700~\text{mm}}{25~\text{mm}} = 28 $$ A Barlow lens multiplies the effective focal length of the objective: $$ \frac{3 \times 700~\text{mm}}{25~\text{mm}} = 84 $$ The 10 mm eyepiece gives 70x magnification without the Barlow. Other configurations exceed the maximum useful magnification of 2x per mm of objective diameter (60 mm → 120x).

  • $\begingroup$ So, are you saying that using the Barlow lens with my 4mm is not recommended because it exceeds the Telescope's capabilities? $\endgroup$ Oct 25, 2020 at 1:31
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    $\begingroup$ @JohnathonPerdicaris No harm in trying. You won't see more detail on a planet that way. On a bright star you might see the Airy disk. $\endgroup$
    – Mike G
    Oct 25, 2020 at 1:55
  • $\begingroup$ @JohnathonPerdicaris some people call it "empty magnification" because while things look bigger, the blurriness is bigger by the same factor. Go ahead and experiment. You can try it on something bright and easy to find like the Moon and see what happens, or even a distant object on the horizon during the day, but don't point towards the sky because the Sun will damage your eyes and the telescope! $\endgroup$
    – uhoh
    Oct 27, 2020 at 23:00
  • $\begingroup$ So, you said that I could see the Airy Disk, what is that exactly? Is it possible to even clearly see the surface of another star, like betelgeuse or polaris? $\endgroup$ Oct 29, 2020 at 13:57
  • $\begingroup$ @JohnathonPerdicaris It's a diffraction artifact as shown in the link from my earlier comment. Other stars are indistinguishable from point sources in conventional telescopes. $\endgroup$
    – Mike G
    Oct 29, 2020 at 15:17

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