This is something which is bugging me a lot. There is this massive spot on every frames and I do not know what it is
The telescope lens appears to be dust free, the filter too and the CCD is also free of apparent dust bunnies.
Would anyone has a clue?
It's clearly visible in the bottom right corner.
Dust on the lens of the telescope (in the pupil) is not visible in the image plane. It's too large to be on the sensor itself, so either on the camera cover glass or a filter. Try looking in from the lens side of the camera, else you have to closely look at those two (rather three, could be on both sides of a filter) surfaces when camera is removed from the telescope.
The displayed image is stretched, so the dirt might be quite inconspicuous.
try rotating the filter relative to the camera. If the spot moves it's on the filter, if it doesn't it's on the camera. It's also possible to calculate the distance the dust spot is from the sensor from its size and the focal ratio of the scope.