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Barlow lens (1 Viewer)


New member
Hi everyone. Hope you are all well.
Okay. I’m gonna stick my newbie neck out and ask a newbie scoping question.
Can you use a Barlow lens with a spotting scope?
I have a Celestron Ultima 100 20-60x. The other night I did what a lot of spotting scope owners do and pointed mine to the night sky. The view of the moon was spectacular. And I was thrilled to see Saturn with its rings. It was small but you could see the rings quite clearly. And Neptune and four moons were visible. It was very cool.
But I couldn’t help thinking that it would be nice to somehow boost the magnification without investing in a telescope. The 100mm objective lens should allow enough light to maybe double the magnification? So would a 2x Barlow lens work with this scope? Is there one that would even fit this scope? Has anyone tried this? From the little I’ve read there might be an issue that you could never reach focus.
Probably wishful thinking but thought I would ask.


Well-known member
Welcome Swanny.

You must have pretty good eyes to see Neptune's moons with a 100mm scope.

Yes, a 2x Barlow should work depending on whether you can reach into the scope.

Many spotting scopes have extenders typically 1.6x.

120x should be well within a good 100mm scope's abilities.

Titan, Saturn's brightest moon should be easily visible and possibly Iapetus, Rhea, Tethys and Dione although Saturn is low nowadays. Nearer the equator is better.



Well-known member
The Celestron 100 may take 1.25 inch fit eyepieces.
For astro use a 4mm eyepiece would be simpler.



Well-known member

first of all, welcome to birdforum!

With a 4 inch scope you might have seen Neptune and maybe barely Triton (magnitude 13.5), but certainly not any of its smaller moons (all beyond magnitude 19 and needing a few meters of aperture).

While 120x on a 100mm scope is certainly ok for planetary and people often go higher, it is not so much a good idea on a fast f5.4 achromatic doublet. I would recommend to not buy a barlow unless you plan to use it on an astro scope too...
As for whether you will be able to get to focus - in general a barlow moves the focal plane outward, which will will help (as getting your EP further out is quite easy with an extension but further in is usually a no-go unless you want to modify the instrument), but I have doubts that you will able to get a good focus point at 120x on a fast 100mm achro - at best a wide area of least fuzziness.

A cheaper way would be to find an astro club in your area and join them on some observation night (whenever that will be possible again) and maybe be able to try a borrowed shorter EP or barlow...

Or get a used 6 or 8" dobsonian and you will see a lot more in the skies...



Well-known member
Hello SwannyBC and welcome to Birdforum,

Your Celestron Ultima 100 model uses screw-on eyepieces, so unfortunately, an astronomical 1.25" slip in Barlow will not fit. You MAY be able to make an adapter to do that, but I do not have an Ultima, so cannot offer any confirmations on that. The Celestron Regal 100 model DOES use slip-fit 1.25" barrel diameter eyepieces, so an astronomical Barlow will fit and MAY come to focus there.
I have Pentax PF model scopes and was able to use a low cost astronomical 2x Barlow in them with only very minor modification. See this old 2013 Birdforum thread for details: https://www.birdforum.net/showthread.php?t=249576

I have no experience with the Ultima OR Regal models, but have modified the Pentax models to use a Barlow and even to hold some adapted 2" barrel diameter eyepieces to provide a wider field of view. Those larger eyepieces must still be limited to those with a field lens of 1.25" or less, since that is the eyepiece port diameter. Recently I've had some success adapting a Masuyama 20mm 85˚ AFOV 2" eyepiece I have for astronomical use to the Pentax PF100 and PF80 to provide a little higher power wide field view than my original 26mm 70˚ AFOV SWA 2" eyepieces.

BTW, I suspect the planet around which you saw 4 moons was Jupiter, rather than Neptune. You are correct, though, that it IS a very cool view. You should have been able to see some bands visible on the planet's surface, too. I still recall the first time I viewed Jupiter through a friend's homemade scope. I was "hooked" for life!

It is nice that some scopes used for birding can also provide acceptable nighttime sky views, though Joachim is certainly correct that a larger astronomical scope will give much better views. - Bill


New member
Thanks for all the info everyone. It was a Jupiter and four moons, btw, not Neptune (that’s what happens when you stay up late and post comments :). Think I might just have to find some money (and room) for a telescope.

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