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Hi,
Andrew Murrell, Gary Kopff and I actually used the 18" ultracompact SDM prototype at Coonabarabran in February of this year. This was an F4.5 scope. Peter has based his design on Dave Kriege's with some of his own modifications. Peter's design isn't quite as light and portable as Dave Kriege's but it is stronger and more stable. Peter's design uses a short upper cage assembly and 8x32mm poles, which is very stable. The Obsession design by comparison uses 6 poles which are about 25mm diameter or a little less and a ring as the virtual UCA. The Obsession design is lighter and more compact, but not as stable.
I liked Peter's 18" Ultracompact a lot. In use it feels very stable and just like using a classic style Obsession scope. It depends what is important to you in this regard, stability or weight and transportability.
Cheers,
John B
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