G'day David,
The specs of my scope, f/5.5 506mm FL sit right in the middle of the advertised range that the flattener (not reducer) are said to work for - 400-650mm FL and f/5 to f/7.5
http://www.bintel.com.au/Astrophotog...oductview.aspx
That's what's deceptive and frustrating. You'd think, reading the ad, that it should work at 55mm backfocus.