hard to remove. You must keep your sensor clean for focus
stacking.
Sensor Cleaning
This is the ugliest part of digital camera work but you have to do it.
There are different levels of cleaning the sensor. On my Nikon
cameras I have to lock the mirror up, take off the lens, and look
inside. Behind where the mirror was (before it was locked up) is the
sensor which is actually covered by an AA (Anti-aliasing) Lithium
Niobate filter that is pretty tough and does not scratch easily. Still
doing anything with the sensor area requires care and can be really
nerve wracking.
For beginners (and occasionally for any of us) cleaning the sensor
is not only difficult but often fraught with worry about damaging the
camera’s sensor. It is no fun at all. The single most-important tool
for cleaning the sensor is some way to know if you have it clean.
The traditional way is to go outside, point the camera/ lens at the
sky and take a photo. Then get the photo image off the card, put it
in Photoshop (or somewhere), expand the photo, and minutely
inspect it for dust, what are called “dust bunnies.”