I took a sharpie to my cheap 2x achromat barlow. Much improved contrast!

[realflow100]

before the edge of the lens looked a shiny matt kind of glossy color. and bright too!

now its more than 10 times darker on the edges of the lens and can hardly even see it in bright lighting. it gives me far better images now in use than before.
the contrast pops out more even in the daytime its much clearer and sharper image. no haze anymore. good clear images and more magnification for my short focal length F4 reflector telescope.

I also have a 5x barlow but it doesnt seem to have this problem even though the edges are not blackened. I guess the edges are well hidden by the fact the lens elements are quite thin and held apart by spacers just right to not have the edges be visible.

[UlteriorModem]

Good job, yea I would bet that the 5x either has the edges blackened or they are 'set' into slight recesses.

Cheap barlows dont have these features as you have found.

[Voyageur]

Good idea! Did you take photos of the process? if not, be sure to do so next time you make any adjustments like this. It could be really helpful to other people.

[UlteriorModem]

Ive done it to various optics, focal reducers, filters, etc. It's quite easy really.

The most difficult but most important aspect is to observe the lens orientations and placement.

Lay out a large microfiber cloth, undo the first retainer ring, and carefully extract the lenses and spacers and place them in order with a particular side up, so it can be re-assembled in the proper order and orientation.

Whatever you do, do NOT drop the lens

A pair of micro fiber gloves help too.



Some folks go as far as to do this procedure in a bathroom after some water has been run in the shower to knock down the dust levels.

[realflow100]

yeah my 2x barlow just has one lens in it. with the flat side facing the sky and the concave/curved side facing the eyepiece/camera so it was easy to orient it.
but might be harder if the curve is more subtle on a different brand or with more lens elements