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While there is not much difference between the mounted and unmounted filters fro a size perspective 31.25mm for mounted and 31mm for unmounted the unmounted filters are able to be used down to f2.5 ratio i believe (it is stated on the QSI website). where as the mounted filters only down to f4, i am not 100% sure but i am fairly confident this is because the mounted filters end up slightly further away from the CCD sensor (only a few mm) but this is enough to limit the F range of the Camera before vignetting occurs.
The unmounted filters can be closer to the CCD surface and therefore can be used on faster telescope systems. As to why they are more expensive, i can t say maybe more workmanship in installing them carefully is needed?
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