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For a good go to, the base must be perfectly level or it won't be very accurate.
With the Barlow - the 10mm with a x2 will be the equivalent of a 5mm lens - presuming you have 1200 focal length then 240x is your magnification - which should be reasonable in a 250mm mirror.
It may be that your Barlow is not too good or that the atmosphere is pretty turbulent - either will give you a blurry image - but I reckon the atmosphere is more likely
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