As users einarin and nimitz69 have already indicated, the dark splotches are shadows due to dust in the optical train.
They do not seem to be on the sensor. My best guess due to their size is that they are more likely to be on the camera window.
I calculate that they are 5.02 mm from the sensor. That number comes from:
- I loaded your image into PixInsight and scaled it back up to full ASI294MC-Pro size
- Plate Solving the image gives a scale of 2.585 "/px and 369.5 mm focal length
- I think you are using a 100 mm telescope operating at F/3.7
- The dark rings on the image span an average of 293 pixels each
- Plugging the numbers into a dust mote calculator shows the distance of 5.019 mm from the sensor to their location.
I have to guess the diameter scope you are using in order to get a focal ratio but did notice from your posts that you have a SW Esprit 100. I also took a guess that the image had not been significantly cropped before being resized for posting here. If either of those guesses are wrong, my numbers will be suspect. I also had to assume that you are using some sort of focal reducer / corrector on your scope.
If my guesses were close, the distance of the dust from the sensor's surface is consistent with it residing on the camera's window. It is most likely on the outside, so a gentle cleaning should take care of it.
As others has also said, a good set of Flat Frames will remove the artifact in the future.
Edited by jdupton, 22 September 2019 - 08:27 AM.