The barlows with screw off lenses are often used with the lens portion threaded into the filter threads of an eyepiece. Not all of them do this. The standard 2-element barlows typically are a little better optically, due to the "less steep" light cone, however many "shorty" barlows incorporate a 3rd lens making them just as good, and some of those thread off as well.
The barlow is a telenegative lens, and these are sometimes built in to eyepiece designs, such as the Televue T6 Naglers.
Most 1.25" barlows will vignette a low power, wide field eyepiece, just slightly.
A barlow is designed to be placed after the diagonal on a refractor or SCT, but if placed before the diagonal, it will have a higher amplification factor. In many modern refractors, there won't be enough "outward focuser" movement to do this without the use of an extension tube.
No barlow is apochromatic. That is simply marketing speak. Most modern barlows are excellent optically, and can reduce eyepiece astigmatism in fast instruments due to the increase of focal length. They cannot reduce chromatic aberration at all.
Edited by SteveG, 22 July 2021 - 03:01 PM.