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I have an AO7 permanently on my camera. It certainly used up back focus so that I can't use the focal reducer on my VC200l and EQ6. This means I image at 1800mm. Using just my guide scope I get a bit of flexure that becomes apparent with 5min exposures. I can use the AO7 and keep an object on exactly the same pixels. When I use it is for time series photometry. It corrects the guiding much better than the guidescope. This reduces error in the data. It can't always be used but I can usually get a guidestar with 1/4-1 sec exposures.
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