Vignetting with Kowa TE-11wz + Panasonic 20mm f1.7 (1 Viewer)

Hi All,
I recently got a Kowa 883 with a TE-11wz along with a Panasonic 20mm f1.7 attached with a DA-10 adapter for digiscoping.
However, I find that it vignettes rather badly at 25mm. This is contrary to what is suggested on this page on Kowa's site (The Kowa DA10 rises again…) http://www.kowaproducts.com/DigiscopeBlog/
Photos by Paul Hackett using the same scope, zoom eyepiece, adapter and lens combination do not indicate any vignetting.
What am I missing? Is there something more to consider? Such as shooting mode? Camera (Panasonic vs Olympus)? Cropping in to remove the vignetting?
Given the popularity of this lens for digiscoping, could someone please advise?
Cheers!
Pavan

Having read through the forums more closely, I found the answer I was looking for in the discussion below: http://www.birdforum.net/showthread.php?t=325889&highlight=sigma+30mm

The sigma 30mm f2.8 should not vignette, but is longer and less bright. Have ordered one from Sigma's site and will report back.