I am new to using a telescope and am having a hard time using a Barlow lens. I was able to find Jupiter multiple times using a 6mm lens with a telescope that has a D = 60mm and FL = 700mm. I am very close to seeing some detail (making out some color/features) but it is still a little too blurry. Whenever I equip the Barlow lens (x2) the object is nowhere to be seen (even when focusing and moving around). Any tips to using a Barlow? Is 6mm too small to use with a Barlow?

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    $\begingroup$ Not an answer, but while the Barlow will increase your magnification, it won't make the image any less blurry. At 700/6 = 117x magnification you may have reached the limit for your telescope and local observing conditions. Further increases are probably going to be what's called "empty magnification". There are some good questions and answers here in this site about the limits of useful magnification. Astronomical seeing is mostly what you are up against. It can vary a lot from night to night and gets better later into the night. $\endgroup$
    – uhoh
    Commented Oct 2, 2021 at 3:24
  • $\begingroup$ It's much more difficult to find objects with high magnification. Have you tried finding the object with a low power eyepiece then changing to the Barlow, being careful not to move the scope. $\endgroup$
    – D. Halsey
    Commented Oct 2, 2021 at 19:24
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    $\begingroup$ Or another approach: Use the Barlow and a longer eyepiece, e.g. 15 mm, to find Jupiter. Once you have it lined up, carefully swap eyepieces. But as @uhoh pointed out, the telescope you have is unlikely to give clearer images at that magnification - just bigger. I found this clears things up nicely: supercooper.jimdofree.com/magnification-guide . Quoting: "Generally you will find that you will use magnifications in the region of 50x - 150x most of the time regardless of the size of your telescope!" $\endgroup$
    – Jason
    Commented Oct 4, 2021 at 22:44

1 Answer 1


There are some rules of thumb when it comes to telescopes and usable magnification.

One is that your maximum usable magnification is two times the diameter of your telescope's aperture in millimetres (or 50 times the diameter in inches).

Another is that the best views will be provided by an eyepiece which has a focal length (in millimetres) equal to the focal ratio of your telescope.
(This one is basically saying "Use an eyepiece which gives an exit pupil of 1mm")

NOTE, however, that these are guidelines and not hard rules. Depending on the seeing where you live and the quality of the optics of your telescope and eyepiece, you can go above these 'limits'.
Think of them as hints to help people avoid spending money on eyepieces that they'll rarely be able to use :-)

Let's take your 60mm / 700mm telescope:
According to the first rule, 60 x 2 = 120x magnification is your theoretical maximum, and your 6mm eyepiece gives about that.
On good nights with exceptional seeing, you might be able to push the magnification higher, but probably not very often.

And the second, 700 / 60 = 11.6mm.
An eyepiece of this focal length will give an exit pupil of 1mm. An eyepiece of, say, 12mm in your telescope will show you all the detail you'll be able to discern. Going lower than this will give you a larger image, which can be useful, but you won't see any more detail. Most of the time you'll only just increase the amount of blur.

To answer your questions: "Any tips to using a Barlow? Is 6mm too small to use with a Barlow?"

Use a Barlow to give you focal lengths which you don't already have in an eyepiece.
Use a Barlow for those rare nights of good seeing, to push the magnification as high as possible.

Yes, 6mm + a 2x Barlow in your telescope results in too low a focal length and too high a magnification for all but the rarest occasions.
Eyepieces with focal lengths that low are for fast F-ratios - around F/4 and below - where they'll give usable exit pupils (and maybe usable magnification, too, given good seeing)
In your telescope, the 6mm eyepiece gives an exit pupil of half a millimetre and probably the highest magnification you'll ever be able to use.

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    $\begingroup$ Thank you for the detailed response. It really had me to understand the topic and telescopes :). Right now I have a 6 and a 25 with a x2 Barlow. So that means that with the Barlow I technically have a 12.5 and a 3mm as well. The 3mm being too small and that is the issue I am facing. :) Would 9mm with a Barlow still be usable? Since it is 4.5 and pushing the maximum? I really appreciate you posting an answer! $\endgroup$
    – Luke
    Commented Oct 2, 2021 at 18:22
  • $\begingroup$ You're welcome, @Luke , and I'm glad you found it useful. You could get an eyepiece that's between 8mm - 10mm, and then use your Barlow with it on a good night to get 4mm - 5mm. The problem, though, is going to be the exit pupil. The exit pupil is the beam of light which comes out of your eyepiece. As magnification increases the width of the exit pupil decreases, and when it gets below 0.5mm it gets difficult: the image is very dim, and it's hard to keep your eye positioned right. I wrote another answer about it which you might find useful. $\endgroup$
    – Aaron F
    Commented Oct 3, 2021 at 0:45
  • $\begingroup$ 700 mm / 60 mm is not 11.6 mm. mm/mm is unitless. Also, where does "An eyepiece of this focal length will give an exit pupil of 1 mm" come from? I don't understand the optics of that at all, and currently don't believe that the exit pupil defines how clear Jupiter will look. Please add a supporting link to an explanation. Thanks! $\endgroup$
    – uhoh
    Commented Oct 3, 2021 at 23:28
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    $\begingroup$ It's early where I am and I'm pre-coffee so "nobody is clear" to me yet :-) Thanks for the link! I'll take a look now. $\endgroup$
    – uhoh
    Commented Oct 4, 2021 at 0:02
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    $\begingroup$ @uhoh ooh nice one! I'd like to try and have a go at answering that after work today :-) (btw your link there is to your meta question, but it's only one more click after that to get to the main question) $\endgroup$
    – Aaron F
    Commented Oct 5, 2021 at 8:35

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