This 1.25" Celestron 2x Ultima Barlow does what Barlows do and does it well. Specifically, it halves the effective focal length of your ocular. To do so, any Barlow lens reduces the amount of light reaching your eye to only 25%--this is a mathmatical equation and there is no way around it.
Celestron's Ultima Barlow is a good one, fully multi-coated and apochromatic. It uses an air-spaced, 3-element design and features a 27mm clear aperture.
It is compact (2.75" long) and weighs only 5 ounces, so it should not be an excessive burden for your focuser.
I tested the Ultima Barlow using my Celestron 102mm ED refractor (900mm focal length) with a number of oculars, including a 25mm Celestron X-Cel, 17mm Celestron Plossl, 14mm TeleVue Radian, 12mm Celestron Ortho and 9mm Burgess Planetary. It worked satisfactorily with all of these, especially the 12mm ortho and the Plossls.
In each case it degraded the image slightly, but was never intrusive. One advantage to a good Barlow lens is that you are getting the effective magnification of an ocular of only half the focal length, while viewing with a longer focal length ocular that is easier to look through. It is, for example, easier to look through a 12mm ortho and a Barlow than a 6mm ortho, even though the view is slightly degraded. To me, that is the main benefit to a good Barlow lens.
If you need a Barlow lens, this is a good one to buy.
Pros: A high quality Barlow that minimized image degradation
Cons: Any 2x Barlow l...