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CCD dust removal
Dust specs on the CCD chip is a problem for anyone using webcams or digital SLR cameras for astro imaging.
Unlike traditional 35mm SLR cameras where every image uses a new "chip" (i.e. new area on film),
in digital imaging we are using the same physical chip over and over again. If the surface is damaged or
dirty, all following images will suffer. On this page I describe how I detected a dust speck on my CCD
and how I successfully removed it.
WARNING: Please understand that removing dust from a CCD is risky. If you do it wrong, you can
cause more damage than the original problem. You might even destroy it completely. Be very careful, do not touch the CCD, do not blow directly
on the CCD. Please also understand that I will not be held responsible for any damage or problem
you might get from follwing the procedure described here. Any such activity is entirely at your own risk , I
just offer to share my personal experience.
Problem detection
I first detected a problem while trying to capture the reflection nebula around Merope in the
Pleiades cluster (M45). I was using my Vesta 675K SC3 long exposure modified webcam. There was a strange
and ugly dark spot near the center about 25% from the bottom. It was located
in the same position in all frames, even if the stars moved from frame to frame, so clearly this was not a phenomenon in the
sky, but rather a problem with my equipment. Below you can see one of the raw images of Merope showing the
effect. Move the mouse in and out of the image to highlight the problem I saw.
Cause of problem
The spot had to be caused either by my telescope optics, or by something on the CCD. In order to find
out for sure, I captured an indoor image of a piece of cardboard with some text. I used a 55mm SLR
lens for this purpose, stopped down to about f11. If the problem could be detected here, the telescope
optics could not be blamed for causing the problem. Move the mouse in and out of the image below.
The same problem can be seen, and in the same location as in the Merope image. The size of the spot
is slightly different, since different optics is used. Clearly, the problem had to be a dust spot on the CCD.
Verification
Using a magnifying glass, I could see a small dust spot in the location indicated below. Again, move the mouse in and out of the image.
Dust removal tool
I have mixed experience with "canned air". Some of the cans contain gas/perfume or whatever that you definitely don't want
on the CCD or any optical surface. At one point in time I created a big blob on a focal reducer when trying to remove some dust. Something
came out of the can and got stuck on the glass. No fun at all. That can was not used anymore.
Still, I was advised to try "Kenair" air duster, specifically made for removing dust from photographic equipment. It has a
metal nozzle allowing precise pointing. When using the Kenair, it is important not to shake it and to use it upright. That way you are minimising the
chance that something else than clean air coming out.
Dust removal
Using the Kenair, I followed all instructions. I pushed the trigger while pointing to my hand first, making sure
only air came out before I tried the Kenair on the camera.
When blowing in the camera, I took great care in pointing the nozzle to the wall on the side of
the CCD. That way, the "wind" would blow across the CCD surface at a shallow angle and stand a
greater chance of taking dust with it. Also, any moisture would
not hit the CCD directly.
Success, dust gone!
This method proved very successful. After one quick blow, I shot another image with the SLR lens, and the black blob was gone.
Again, move the mouse in and out of the image below to see the difference.
