Controlling dust spots with flat frame subtraction – for astro cameras

What is a dust “bunny” or dust spot in my image, and why are they called this?

Why dust “bunnies”? They are certainly not cute, but they are fluffy and tend to multiply rapidly. They are often dust particles or, especially in the Spring and Summer months, pollen. If static charges build up, dust bunnies can become attracted to each other, resulting in large “super” dust bunnies. These can collect on the sensor glass of your camera and result in dark blobs when you image. It isn’t possible for dust on the front lens or mirror to show up because it’s at the pupil of the optical system. Filters and reducer lenses often show dust blobs. They are completely inevitable and a normal part of astro imaging. Attempts to clean the front optical window often just make them worse. Even a situation like the one below will produce perfect images.

What can I do to control the effects dust?

You have several options here.

Flat frame subtraction: For the most part, these dust spots can be easily taken care of during post processing of your images by flat “frame subtraction”. That means taking flats with your imaging setup either just before or just after an imaging session. This is a standard part of astro imaging processing, and most image processing software will expect you to have a “master flat” ready, and will prompt you to use that during the processing workflow. A master flat is composed of 10-20 (or more) “flat frames”.

There are many tutorials and guides on YouTube and Google and Astro Photography Forums that will guide you how to take flat frames with your specific camera and setup. You can also ask Altair users for tips on the Altair Astro Facebook group.

Here is an example of a typical flat frame, notice the dark blobs that will be removed from your image when you process it:

How to tell if there’s dust inside the sensor chamber:
If the dust is out of focus, it’s not on the sensor surface, and it’s likely to be either on your reducer, a filter, or the optical window which is the outer glass layer protecting the sensor chamber. By rotating the camera relative to the other glass elements you can determine if the dust is located on the front of the optical window or the back, i.e. on the sensor-side. To check if the dust is on the back of front of the optical window, note the position of the anomaly, clean your sensor class with a dedicated sensor glass cleaning kit, and re-take the flat frame. If the dust particles have disappeared or moved, then the dust was on the exterior. If the dust spots haven’t moved, you’ve narrowed it down to within the sensor chamber.

How to tell if there’s dust on the sensor surface: If you suspect the dust is on the sensor, you can easily check by taking a flat frame. If the dust appears to be in focus or quite sharply defined and not a blob, then it’s usually on the sensor. You can usually see the dust particles as small dark irregular shapes because the closer they are to the sensor, the more the structure of the dust motes will be visible. Usually dust on the sensor cannot be subtracted with a flat frame.

Ambient air cooled GPCAM or fan cooled Hypercam cameras: Although dust on the inside of the sensor glass is unusual, it isn’t unknown for a foreign object or dust to become dislodged inside the camera and show up on the inside of the protective sensor glass. This is more common if you have unscrewed the front of the camera or replaced the optical window glass* on your fan cooled or air cooled camera. This can still be easily dealt with by flat frame subtraction, and in most situations, post processing is the best way to deal with dust, because you can easily introduce more dust by attempting to clean it. However if the dust is on the sensor surface it will look much sharper and in focus instead of soft and fuzzy like the blobs shown above, and it cannot be subtracted by use of flat frames. In that case the sensor can be cleaned just like with a DSLR, and this sometimes needs to be done to maintain your air-cooled or fan-cooled camera over time as the sensor chamber isn’t hermetically sealed.

TEC cooled Hypercam cameras: Because TEC cameras contain a “protective atmosphere” to protect the sensor when cooling it, we don’t want that to escape. Therefore opening the sensor chamber is not possible on Altair TEC Cooled cameras. Breaking these seals will void the warranty and no-frost guarantee. So if you suspect dust INSIDE the sensor chamber of your TEC camera, it’s best to raise a support ticket showing the dust on the image before you attempt to clean anything. NEVER open up the sealed TEC chamber on your camera where the sensor is. You will damage the seals and once dust or pollen is introduced into this sterile, clean area it is very hard to get it removed totally.

Camera cleaning tips: If you have lots of dust or pollen in your area, you could purchase a dedicated sensor glass cleaning kit Click here for an example. This has all you will need, solution, cloths etc, to enable you to clean the protective glass covering your sensor. DSLR sensor cleaning kits are also a good option for cleaning the sensor or any part of the inner chamber of a Fan Cooled Hypercam or GPCAM. But please remember you cannot clean a TEC camera internally yourself. To internally clean a Hypercam Fan Cooled or GPCAM camera, you can unscrew the front black part of the camera anti clockwise and use a normal DSLR sensor cleaner or a quality lens wipe to clean the back of the optical window. When you remove the front black part of the camera containing the optical window, just keep the camera inverted at all times in a relatively dust free environment indoors without strong air currents. Placing the camera face down on a clean surface while you are cleaning the optical window will minimise the chance of any dust getting to the sensor. When complete, just screw it back on, while inverted and you should be good to go.

Finally, if all else fails and flat frame subtraction doesn’t remove the dust visible in the image, you can arrange with your local astro dealer send your camera away to be professionally cleaned in a dust free, clean environment, but this will more than likely be a chargeable option.