Astronomy webcam RAW mode... what's all the fuss about?
Most of the images on my pages taken using webcams utilising colour RAW mode. However, some confusion exists as to exactly what this means.
How a colour CCD works.
Most CCD sensors found in webcams, be they colour or monochrome, are an array of identical light sensitive pixels. As photons (light energy) falls on these pixels, they build up a charge. After the duration of the exposure, the charge on each pixel is read out and converted from an analogue voltage to a digital signal. Expensive astronomical CCD cameras do this with a 16 bit converter. Our webcams use an 8 bit converter.
The signals from each pixel are then put together into a picture inside our computers.
So where does the colour come from? The above system works fine for monochrome images, but how does our webcam give colour? Over the top of the pixel array is whats called a Bayer Matrix. This is a grid of very small colour filters built onto the chip. Some are red, some are green, and some are blue. This means that some pixels are more sensitve to some colours than others. To get a colour value for each pixel, we average the colour information from the nearby pixels. Some of the nearby pixels give blue data, some green, and others red.
This process is know as debayering.
The disadvange is a loss of detail.. each pixel is interpolated from the surrounding pixels... hence a monochrome camera with a proper seperate colour (RGB, or something more fancy) filter set will always give a better image.
So whats RAW mode then?
Raw mode means that the image we get from the ccd is the raw greyscale frame, before the camera debayered it into a colour image.
We then debayer this image in our own software, such as AVIRAW to get a colour picture. This gives more control and a better result than doing it in the camera. Our computers are more powerful than the webcam!
For example, the image on the left is the raw frame recorded the the AVI File during capture, and the right hand image is the same frame after de-bayering.
Is that it?Not quite. The webcam is desgined to give pretty pictures of Aunt Maude beamed over the internet from Australia. Its not designed to take pictures of dim and distant DSOs.
So to get a nice image of Auntie, the webcam does a certain amount of image sharpening, nosie reduction etc etc.
This is, of course, a bad thing for astronomical work. We don't want the camera playing about with the image and ruining it. We are quite capable of doing that ourselves during post-processing.
So Raw mode switches off all the settings to give the untouched output of the ccd.. no sharpening, no colour, no noise reduction.
The main effect of this is to remove ears. When sharpening a non-raw image, you'll notice that sometimes the stars get ears. Raw solves this problem.
How do I use it?
There is a little macro program out there that sets your camera into RAW mode. This only needs doing once. You can put the camera back to the original state with the same program.
How do I process it?
You must debayer your AVI files first using something like AVIRaw and then process in the normal way.
To take this futher and get the required software, visit here. You will find lots of details on raw mode, and colour optimised raw mode.
Update Aug 2005
Page last updated 2005-11-16