will be best to push the mirror up the tube somewhat, I would think atleast 5cm, otherwise youll have either too much mag. or the b/viewer will be stuck way out. you can use lnger bolts/heavier springs while youre at it -just the same as if you're moddding for photo d-slr to reach focus,
if you put ext between barlow & unit you will multiply the barlow effect, instead of 2x it may turn out to be close to 3x magnificartion
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