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The Clairaut cemented aplanatic doublet as manufactured and sold by Clave as a 2x Barlow with -113.4mm focal length is coma correcting to f5 according to Texereau in his book "How to Make a Telescope", Willmann-Bell.
This is a long Barlow with a large clear aperture (available in 1.25" as well as the slightly smaller French standard size). I have found it to be essentially invisible with my Clave Plossl eyepieces and my 15cm f5.5 Newtonian. However I didn't realise it was coma correcting until recently and never really looked for it as I mainly observed planets on axis with RA tracking.
There is some debate as to whether the Klee 2.8x Barlow is coma correcting and I don't have one but did try one many years ago with a Tele Vue 24mm Widefield eyepiece and I found it to vignette. The same eyepiece in my Clave Barlow did not vignette. I always thought it was the smaller clear aperture of the Klee but I think it had more to do with the length of the Barlow with the Clave almost twice the length of the Klee.
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