Are Barlow Lenses Good For Deep Sky Observing? 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:


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


Experiences

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).

Conclusion

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.