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Ok, so no takers on this topic? Perhaps let me phrase the problem in a different way.
If the flattener produced f/6 and the flattener/reducer produced f/4 I think I'd be inclined to just use the flattener on the grounds that f/6 is still fast and it would maximise the image size.
The actual scenario though is a difference between f/8 and f/6 and 1200mm vs 900mm focal length. F/8 seems just very slow and I'm wondering if I would regret that decision.
Is there anyone imaging at speeds as slow as f/8 for deep sky subjects and if so, is it a real problem?
Chris
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