Sensor and Filter Cleaning

Know your donut

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...

Size matters

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

Prepare for cleaning

Here is a list of the equipment I like to use:

Magnetic screw driver (dropping a screw on the sensor is the last thing you want)
2mm Hex key (ZWO camera and manual rotator)
Bamboo cotton swabs (at least 100x)
Isopropyl alcohol
Baby wet wipes
Dry air spray
Air blower pump
Camera lens blower
Nitrile gloves
White desk pad
Plastic wrap
Zipper storage bag (Ziploc or equivalent)
Desk lamp (at least 30W)
Time. At least 1h30

Bottle spray

Sensor cleaning set

Dry air spray

Air blower

Cleaning procedure

While all parts are still assembled on the telescope, clean them carefully with the baby wet wipes as good as possible (to avoid dust falling during dismantling)
Disassemble the camera and its filter wheel (Unless it has to be cleaned, the coma corrector remains installed)
Protect the focuser and corrector with a plastic wrap held by an elastic band
Transport the camera in a zipper storage bag
Clean the camera cooling fins using a piece of paper and dry air
Clean the cooling fan with dry air
Open the protection window
Remove the desiccant and dry them. ( I recommend 3 mn at medium power in the microwave or 2h at 100°C / 210 °F in the oven)
Spray dry air with the sensor pointing towards the ground to remove larger particles.
Spray isopropyl alcohol on the sensor and wipe it with cotton swab only once, always in the same direction
Repeat this task as long as you see dirt. Usually I repeat this operation 20 to 50 times, using one new cotton swab each time
Once it's clean, small drops usually remains. Spray alcohol one last time, and clean in at once with the specific sensor cleaning swab
Protect the camera by putting it in the zipper bag
Clean the protection glass using the same technique
If needed: clean dissasemble the filter wheel and clean the filter using the same technique
Re-assemble the camera and the filter wheel

Learn from my mistakes !

Here is a short list of tips and tricks:

Do not spray dry air at the end of the cleaning process. Dust located around the electronics will accumulate on the sensor!
Never even use a bottle spray used for an usage utilization, especially soap. Traces will appear.
Do not clean a dirty item above a clean item... yes... I know...
Do not try to save on cotton swabs! Double the number you think would be useful :-)
Do not tell your children: "Dad (or mum) will be back in 10 minutes after the sensor is clean..." :-)

FAQ

"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.

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