Color Balance with eXcalibrator

To accurately balance color CCD images, there are a number of techniques and tools to get you in the ballpark. I use eXcalibrator to balance all my color-filtered images. The program analyzes your red, green and blue FITS files in search of white stars, then automatically computes RGB ratios based on the stars’ relative brightness.

Although the author of the program promises to fully incorporate the star data in an upcoming version, currently users are required to first plate solve images in another program. This can be accomplished in a number of programs, though I’m most familiar with the PinPoint Astrometry function in MaxIm DL. This requires that you first download and install the Guide Star Catalog plug-in. Once completed, open one of your images in MaxIm. Choose Analyze/PinPoint Astrometry, and click the Process button. In a moment, your image should be plate solved; save the resulting file in a convenient location and close MaxIm.

Next, launch a program that is included in the eXcalibrator distribution called Aladin and drag your plate solved file into it. Load a catalog by choosing File/Load Catalog/Surveys in VizieR/Nomad.If no calibration stars are found, you can switch to the SDSS catalog, which is considered more accurate but covers only a limited portion of the sky. Aladin will take a few minutes while it downloads catalog data for your image.

Once completed, load the Nomad filter (also provided with eXcalibrator). Choose File/Open Local File, and select eXcalibrator_Nomad_Filter.ajs. (or the equivalent file if the SDSS was used). This filter will find all the white stars in the field of view of your image. Press Ctrl and A keys, and a table of values will appear at the bottom of the window. Right-click in the table and choose Copy/Copy all measurements (for Excel). Open a new text file in Notepad and press Ctrl-V to paste the values. Save the file and close Aladin.

Open eXcalibrator, identify your Red, Green and Blue FITS files, set the Data File to be the text file you just created, and set the WCS File to be the plate solved FITS file. If you used the Nomad catalog, select “Use (b-v), (g-r)”. If you used SDSS, select “Use (u-g), (g-r)” and click Calibrate Image. Your RGB ratios will be displayed at the bottom of the screen. Make a note of them and simply input these values when combining your color files in your CCD processing program.

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