One-Shot Color Processing

Most astronomical cameras use monochrome sensors, and require individual shots taken through filters to be combined to produce color images. DSLR cameras were designed for one-shot color photography. The individual pixels in the sensor are covered with an array of alternating red, green, and blue color filters known as the Bayer matrix.

Typical Bayer Matrix

The Bayer matrix allows the camera to instantly create a color image, but at some loss of resolution. The camera sacrifices some spatial resolution in exchange for the ability to capture color in one exposure.

The raw data coming from the chip looks like a black and white picture with a speckle pattern. The pattern is due to the different color filters; each pixel is seeing only a particular color. To convert this to a color image, each pixel must have a red, green, and blue value. This means that software must interpolate the missing color values. This process, called "debayer" or "color conversion" can be done by MaxIm DL, or in some cases it can be done inside the camera itself. This conversion is performed by looking at adjacent pixels, and estimating what the two missing colors should be. There are many different algorithms for doing this conversion, but all are compromises. For some camera models, MaxIm DL's Convert Color command offers a choice of conversion algorithms (usually Fast or High Quality).

Generally you should capture images in RAW mode, not color. DSLR cameras in particular produce compressed, lower bit-depth images when operating in JPEG color mode. Even if that is not an issue, as for one-shot color CCD cameras, the interpolation during the debayer function will spread the effects of any hot pixels to adjacent pixels. By subtracting a dark frame first, before debayer, you get a much cleaner, higher-quality final image. This requires the debayer to be done in software.

When working with DSLRs, you can capture RAW frames through MaxIm DL, in which case you get FITS images with RAW data in them, or you can capture RAW files to the compact flash card on the camera. Prior to DSLRs, "raw" format usually meant an array of numbers with no header. DSLR cameras actually produce a "raw" image that is in a proprietary image format; these formats vary from model to model, not just brand to brand. MaxIm DL can read several hundred variants of RAW file formats. Please note that new camera models may require an update to MaxIm DL before they can be read.

Similarly, MaxIm DL knows how to perform the debayer algorithm for many different camera models. Most popular models have preset conversion settings; simply dial your camera model and the default settings will work. In some cases the settings will not be known for your camera, either because it is a new model, or in some cases the settings may have changed due to a software or firmware update by the vendor.

Many cameras produce images that are offset slightly from the nominal starting position. This results in a misalignment of the Bayer pattern, which in turn produces grossly incorrect colors. This can be corrected in the Convert Color command by adjusting the Offset X and Offset Y values; simply dial in different values until you get reasonable looking color. More often you simply need to tweak the color balance; this can be done in the Convert Color command as well.

It should also be noted that flat-fielding one-shot color images can result in the removal of the color! To prevent this, the Set Calibration command has a Boxcar Filter option; this should be turned on for the Flat Field calibration groups when using one-shot color cameras.

Here is a set of recommended processing steps for all one-shot color cameras: