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Microscopic dust particles on the imager surface/low pass filter will show up on the images. Dust can be quite intrusive.
Dust spots
Can the filer/low pass filter surface be cleaned?
Yes, we clean out the mirror box area using compressed air, then we wet swab the filter clean.
Some dust may remain if the dust is under the filter surface. This can be costly to clean as the sensor needs to be removed and the filter removed to clean underneath. Most dust marks will disappear in the image if the image contains a lot of contrast.
Under the filter clean
Refer to link: CCD cleaning