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I think you already know an extension tube & the 2.5X don't mix, so the extension tube(s) will only work with the 2X.
Got a diagonal? that will turn the 2X into a 3X...Not sure what it would do with a 2.5X.
I use a 75mm extension tube with 2X barlow in crook seeing, 2 / 75mm tubes in the 2X barlow in average seeing & I'll throw the diagonal in with either 1 or 2 tubes, depending on the seeing.
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