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Bob,
Interesting results....
You don't mention which barlow you used - Standard long focus, Shorty, triplet etc.
Notwithstanding....
In "Telescopes, Eyepieces Astrographs" by Smith, Ceragioli & Berry, p 374-388 they analyse the design and performance of two barlows and the Powermate.
They say that the typical (good) barlow will control longitudinal and lateral colour, spherical aberration and the amplification factor, but do little for coma, astigmatism and field curvature (p376)
When the barlow is used with slower telescopes >f6 they also say that "slowing the f ratio..... reduces the angular aberrations inherent in the eyepiece" (p388)
My practical experience with barlows has been with spectroscopes (to obtain f ratios >f7) and solar imaging. My preference has been the TMB x1.8 ED barlow or the Powermates.
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