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just adding to what Mike mentions below that to increase image scale with a barlow you have to add the extension between barlow and ccd - in effect if you simply shift your diagonal to that position it will function like an extension tube and give you an increased amplification factor from your barlow - there's a limit to how much you can push a barlow though, not sure of the maths or the effects but doubt you'd want to go much beyond say 3x from a 2x barlow
hope that helps,
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