I'm using a 750mm (focal length) by 150mm (aperture) newtonian reflector and I've been curious about using Barlow lenses on deep sky objects.

Some of the astronomers at my local observatory say that if you get the lens selection correct so you don't lose any light through the eyepiece, you don't need a Barlow lens. While others say that the Barlow lens doubling the focal length of the telescope allows me to see deep sky objects better than without it.

So, is a Barlow lens an effective tool for deep sky observing? If not, what should I use it for?

Note: I already have a 2x Barlow lens.


The Failures of High Magnification

Higher magnification doesn't help you observe deep sky objects better. Deep sky objects unlike stars are extended objects. They subtend a finite solid angle on you. This ensures that the surface brightness(brightness per unit solid angle) of extended objects remains constant. Hence, a higher magnification would not make it brighter for you to see.

It is worse for another reason. After a certain magnification, the apparent angular size of the extended object becomes comparable to the field of view, or sometimes even greater. Your eye won't be able to distinguish the object against the background since most of the background is the object itself.

Good Uses of Barlow

Some of the uses that you can put your Barlow:

  1. Observe the Moon, Solar System objecs
  2. Some of the clusters which seem like point objects really "explode" when using a Barlow. The best example is Omega Centauri.


I have a 8" Dobsonian Newtonian Reflector. The viewfinder is 20x80. I really get a better picture of deep sky objects like Ring Nebula, which have a small angular size, when using Barlow. But for cases like Lagoon and Trifid Nebula; they are visible to me from my viewfinder but not through the eyepiece(even in the lowest magnification).


Barlow really helps if you have a deep sky object with small angular size. For larger objects, a binocular may do better, especially for city conditions.

  • $\begingroup$ While all this is correct, I think it may be a bit more discouraging than it should be. I think a Barlow lens is a great tool to have in the collection. It essentially doubles the selection of power/angular sizes available to you (with the price of a small amount of additional abberation). $\endgroup$ – Brian Knoblauch Oct 9 '13 at 17:48
  • $\begingroup$ @BrianKnoblauch Of course it is! I insisted upon buying one while buying my eyepieces, and it comes under use on many occasions! $\endgroup$ – Cheeku Oct 9 '13 at 22:30

It depends. The Barlow is usually a cheap shortcut for avoiding to buy an expensive short focal length eyepiece.

Also you need Barlows and high magnification usually only for small objects. For example the ring nebula (M57) is pretty tiny, and might benefit. Your telescope is still rather wide field, so getting high magnifications might be neccessary to view M57. The Barlow may help.

Disadvantages of Barlow lenses:

  • Additional optical surfaces: Every glass surface produces light loss through reflection and also produces abberrations. Good Barlows will use multiple elements to reduce abberration.

  • Instabilities: Barlows are quite long. Putting this in the focuser, plus an eyepiece, and maybe attaching a camera to the eyepiece will be very fragile.

If you cann afford it, buy a specialized eyepiece instead. A high quality 3.5mm or 5mm eyepiece will be much better with your Newtonian.


You can get a simple formula for making an aperture mask (just a circle with the diameter of your tube, but with a smaller circle cut into it--templates are available). This creates an "off axis" instrument that obviates the use of a Barlow. This is not a big deal. I have made one of heavy poster board. This changes the focal length of your telescope (the focal change, for example, will convert a 4.5 ratio (fast)to an 11.8). It's like having another telescope. The mask goes on for planets and their moons, etc. The faster ratio is for deep sky objects. If you make the mask out of heavy poster board, you simply masking-tape it on to the end of your tube. It works very well. All this being said, I am speaking about Newtonian scopes, specifically Dobsonians, the best, imo, for the budget and for the seeing.


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