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:

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

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:

RA

Dec

Mag

Class

Name

00h 18m  40s

-08d 03m  04s

6.467

G3

SAO128690

00h 22m  52s

-12d 12m  34s

6.39

G2.5

9 Cet (SAO147237)

01h 41m  47s

+42d 36m  48s

4.961

G1.5

SAO37434

01h 53m  18s

+00d 22m  25s

9.734

G5

SAO110202

03h 19m  02s

-02d 50m  36s

7.052

G1.5

SAO130415

04h 26m  40s

+16d 44m  49s

8.10

G2

Hyades vB 64 (SAO93936)

06h 24m  44s

-28d 46m  48s

6.374

G2

SAO171711

08h 54m  18s

-05d 26m  04s

6.008

G2

SAO136389

10h 01m  01s

+31d 55m  25s

5.374

G3

20 LMi (SAO61808)

11h 18m  11s

+31d 31m  45s

4.85

G2

Xi UMa B (SAO62484)

13h 38m  42s

-01d 14m  14s

9.975

G5

105-56 (SAO139464)

15h 37m  18s

-00d 09m  50s

8.433

G3

107-684 (SAO121093)

15h 44m  02s

+02d 30m  54s

5.868

G2.5

23 psi Ser (SAO121152)

15h 53m  12s

+13d 11m  48s

6.084

G1

39 Ser (SAO101792)

16h 07m  04s

-14d 04m  16s

6.314

G2

SAO159706

16h 15m  37s

-08d 22m  10s

5.494

G2

18 Sco (SAO141066)

19h 41m  49s

+50d 31m  31s

5.976

G1.5

16 Cyg A (SAO31898)

19h 41m  52s

+50d 31m  03s

6.237

G3

16 Cyg B (SAO31899)

20h 43m  12s

+00d 26m  15s

9.977

G2

SAO126133

21h 42m  27s

+00d 26m  20s

9.074

G5

SAO127005

23h 12m  39s

+02d 41m  10s

7.708

G1

HD219018 (SAO128034)

 

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.