D'oh!
What are those strange donuts on your flat fields? Where do they suddenly come from? And how to get rid of them?
What astronomers sometimes call dust bunnies are actually small particles of dust or pollen, located somewhere in the optical train.
Let's have a close loop on this picture: Most of the donuts have exactly the same size but if you look carefully, one of them is much smaller. Does this mean it's a smaller particle of dust?
We'll see...
Spoiler alert: The shadow size of each donut depends on its distance to the sensor. If you like mathematical formulas, here is it:
Distance (mm) = Pixel Size (mm) X Focal Ratio X Diameter of Shadow / Reflection (pixels)
And because life is short, just select your telescope and your camera in this calculator:
https://astronomy.tools/calculators/dust_reflection_calculator
With our Skywatcher Quattro 200mm and its ZWO ASI1600, a quick calculation shows that a donut size of 580 pixels corresponds to a distance of approximately 8.6 mm from the sensor.
This distance corresponds to the position of the protection glas of the ZWO camera.
Knowing the distance of each glass element from the sensor, we can now calculate where each donut is located. In our case, there are three types of surfaces on which dust can accumulate :
60 pixels: ~1mm : Sensor protective glass
580 pixels: ~8,6mm : Camera protective glass
800 pixels: ~12mm: Filters
On the image above, we can clearly identify where the grains of dust are located: most of them on the camera protective glass, and unfortunately, one of them on the sensor protective glass.
Let's clean them now!
Sensor protective glass
Camera protection
Filters
Here is a list of the equipment I like to use:
Bottle spray
Sensor cleaning set
Dry air spray
Air blower
Here is a short list of tips and tricks:
"Isopropyl alcohol? Are you crazy?"
I use isopropyl alcohol for years now and I have never damaged any optics. Nevertheless, I try to avoid it on filters, and any other item with specific coating.
"Why a magnetic screw driver?"
Because dropping a screw on the sensor is the last thing you want to do.
"Bamboo cotton swab? Is it a superstition or an unfounded belief?"
I observed that this type of cotton swab loses less fibers and and seems to create fewer marks with each pass. The most important is to swipe it just once, always in the same direction, and then throw it away.
"Gloves? What for?"
This is an individual choice. Restorers of old books prefer not to use any gloves, to feel the material better and avoid damages. During filter cleaning, I am always afraid of leaving my fingerprints and I just feel more confident with nitrile gloves. If you are a professional watchmaker and you don't shake at all, probably you don't need them.
Lens cleaning swab
"Specific lens cleaning swabs: does it it worth the price"
I recently purchased specific lens cleaning swabs for CMOS sensors recently and tried them. Their width is very well adapter to the sensor size, but it doesn't make any miracle. They are quite expensive, and I recommend using them only once, at the very end of the cleaning process, just to erase the last traces.
"Dry air spray or air pump?"
I'd say: both. Usually I turn the item upside down and start with the air pump. If I see that dust is still sticking to the surface, I use the spray, but only if I am sure not to damage electronic parts around it.
"Tea of coffee?"
None of them during sensor and filter cleaning! Being absolutely calm is the key. Gentle music can help to breathe slowly. I know it sounds exaggerated but nothing is more annoying than having to repeat the entire cleaning procedure just because of a drop of breathing condensation.