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Hi John.
The extension needs to go between the barlow and the CCD, not before the barlow.
Eyepiece projection will give you the biggest image scale, but if you're still working at prime focus then yes you can use an extension for your barlow, but only if the barlow supports it.
That is, not all barlows increase the focal length (image scale) by moving the CCD plane further out. In some, you'll simply have to rack further in to focus.
But in my 5x powermate, I can pull the Toucam out of the powermate as far as it will go to gain a little extra image scale. I will also get an extension tube to do it even more.
On your refractor i'm not sure how far you'll be able to push it, depends on your focal length and local seeing of course.
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