One-Shot Color Processing

Most astronomical cameras use monochrome sensors, and require individual shots taken through filters to be combined to produce color images. Some are known as "one-shot" cameras, that are designed to produce a color image from a single image.  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).

If a camera can generate color converted images on the fly, it is generally preferred to use the RAW monochrome mode.  That is because 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.

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:

• Calibrate (with Boxcar Filter for flats)

• Remove Bad Pixel command (be careful not to get too aggressive and remove the Bayer matrix)

• Convert Color

• Stack

• Other image processing such as filters, stretching, etc.

MaxIm DL does not have Bayer offset information for every model of camera available.  Sometimes this information is provided by the driver, but in many cases it is not.  In that situation you will want to follow this procedure for determining the correct settings:

Step 1: Basic Settings

• Take a picture of something colorful and that you know what it is supposed to look like.  It can be a terrestrial or deep space image.
• Open the image in MaxIm DL and run Color Ribbon Convert Color
• For most models set Camera to Generic RGB. Some cameras will require Generic Sparse Color or specific model settings.
• Under Scaling % click the Reset Scaling button.
• For terrestrial images click Background Level - Reset. For deep space images click Background Level - Auto.
• Turn on Auto Full Screen Preview
• If the color looks very weird / completely wrong, adjust the Offset X and Offset Y until you see approximately correct color. For RGB matrix cameras there are four possible settings; Sparse Color and CMYK sensors will have additional offset options.
• Once the color looks approximately correct, click OK.

Step 2: Find an Accurate Color Balance

The sun is a G2V spectral class star.  Stars that are very similar to the sun's spectral characteristics can be used as a white reference.  The following table (ref. Berry et.al., Sky & Telescope Magazine, December 1998) lists a number of these ”solar analog” stars:

• Take a well-exposed image (lots of signal but not saturated!) of a G2V star that is high in the sky. These have a very similar spectrum to the sun and are therefore great celestial "white cards".
• Open Color Ribbon Convert Color with the exact same settings as determined above.
• Under Scaling % click the Reset Scaling button.
• Click Background Level - Auto.
• Click OK
• Run Color Ribbon Color Balance
• Under Background Level turn on Auto
• Under Scaling % turn on Mouse
• Turn on Auto Full Screen Preview
• Point the mouse at the G2V star and click on it
• The image should now have correct color balance. Click OK

You should now have a very accurately color balanced image.

Step 3:  Set Up Convert Color for Perfect Color Balance

Your last step is to copy these settings back over to the Convert Color command, so that this color balance is automatically applied every time you convert an image.

• Reload the RAW (unconverted) image
• Run Color Ribbon Convert Color
• Under Scaling % click the CB button. This copies the settings over from Color Balance!
• Click OK
• With these settings, Color Balance should now reliably produce good color conversion every time it is used.