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Yep, as suspected your Barlows are giving you a higher magnification than what's written on the box. I myself use SkySafari Plus to show me what's happening in the sky, it also gives the size of the planets in arcsec which is what I used to estimate your effective f/num of around f/19 from the equation above. All you need to do is measure the number of pixels across the equator (and yes, just use a photo editor to count them) and apply the formula.
I'm surprised you got anything with the 2 Barlows in series like that, working with an 8m telescope would not have been easy!
Glad you were able to work it all out, at least now you have confidence in your equipment.
Andrew
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