Here you can calculate the minimum filter (size) clear aperture needed to fully illuminate the sensor of your camera.

If a camera sensor in not fully illuminated the images captured with this configuration will exhibit vignetting, ranging from strong black corners to lightly dimmed ones, depending on the light falloff percentage.

The formula used on the calculator is :

MFS = CCDdiag*(FL-DC)/FL + (DC/f)

Where :

MFS = Minimum Filter Size
FL is focal length
CCDdiag is the size of the chip diagonal
DC is the distance between the chip and the filter
f is the focal ratio of your scope or lens