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Just a couple of things here. 3x barlow will produce nearly 4x the focal length once the filter wheel is put in place. This spacing has a multiplying effect from the rear of the barlow glass to the sensor, so you will need to consider this if using a filter wheel. I use a 2x and it gives me about 3x on a 3910mm focal length scope. So I image near 12m.
You should think about 8m as your maximum focal length for your size scope.
A 2.5x powermate does not have the multiplying effect a barlow does. It in fact has the reverse effect. The further away the sensor is from the rear of the glass the less magnification is applied. So be wary of buying this powermate. The 5x does not have this problem.
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