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Paul, it’s better to have the filter as close to the sensor _if_ their sizes aren’t too dissimilar. If the filter is much larger than the sensor, then it can be further away before causing an obstruction to light.
FWIW, 1.25” filters usually work out about 28.5mm clear aperture. How noticeable any obstruction is will depend on the focal ratio of your scope/reducer combo.
A 2” filter is substantially larger...
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