Hi Daniel, I in the past used a 3x Barlow with reasonable image quality, it may be possible to improve on this but with a Power Mate I don't really know
EG:
DSLR with 3 x Barlow -
http://waelect.com.au/mswhin63/Jupit...2010_11_23.png
Webcam with 3 x Barlow -
http://waelect.com.au/mswhin63/Jupit...tml#2010_10_03 Seq2.png
The Barlow I use is
http://www.cameraconcepts.com/store2...cat=357&page=2