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Hi Truffle,
Geoff is 100% correct there, the Shorty PLUS with 27mm of clear aperture shouldnt present any vignetting issues for visual use. I have used it with my 20mm Pentax XW and the 24mm Panoptic without issue. The Orion Shorty is a cheap barlow and not in the class of the Shorty Plus, its internal field stop is only 22mm which is what causes it to vignette with the 20mm Meade UWA. Most cheap barlows will cause vignetting with long focal length wide field eyepieces. An expensive barlow which also has vignetting issues is the University Optics 2.8X Klee barlow. It only has a 19mm internal field stop and does vignette with "most" long focal length eyepieces (over 15mm FL) , however when used with the appropriate eyepieces, ie short focal length ones, the 2.8X Klee barlow does provide very good on axis performance.
Clear Skies
John Bambury
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