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The request was for a good 2X budget Barlow, so I mentioned the 2-element GSO, which is the same as the Celestron Omni.
There is no reason to make a 3 element Barlow unless the purpose is to make the Barlow higher power, or shorter in length without necessitating extreme curves on the lenses.
It really depends on the eyepiece focal lengths whether a 2x or 2.5x will be more useful.
I don't have any objections to the 2.5X GSO if the magnifications resulting are not too high for the seeing conditions.
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