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Message: <blockquote data-quote="jring" data-source="post: 3356726" data-attributes="member: 133897">Hi,I cannot really comment on the big Kowa as I haven't used it yet, let alone with extender, but I did a quick test ind not too bright light under overcast sky with my 80mm f7.5 ED refractor this morning with the Baader Hyperion Zoom and corresponding 2.25x Barlow. As expected the 12mm setting for a magnification of 600mm/12mm*2.25 = 113x was naturally darker than with lower magnifications but still very usable for a detailed view.  When zooming in to the highest magnification possible with this combo (600mm/8mm*2-25 = 169x) the image went even darker but still usable from a brightness point of view but detail was visibly lacking.This is not really comparable to the Kowa as I was not using an image erecting prism but a star diagonal (nothing really fancy - was measured to be a tad better than a standard 91% transmission example). On the other hand the Kowa has 8mm more aperture or 21% more objective area or light. As these effects counteract each other the results for the Kowa should be similar or better.Joachim</blockquote>