Color Balance

The sensitivity of most cameras as a function of wavelength (color) is different from the response of the human eye. The filters used for creating color composites also have their own characteristics, as do the telescope optics. Although ”perfect” color rendition is an elusive if not impossible goal (all individuals see colors slightly differently), it is straightforward to get ”good” color balance with simple weightings.

The Color Balance command adjusts the three color planes so that proper white balance can be achieved. The first step is to remove any background cast by clicking the Auto Background button (manual adjustment is also possible). The next step is to set the scaling factors for the red, green, and blue filters. The simplest way to do this is identify a white object in the image, and then click on it.

Usually you will find a number of stars in an image that are ”white enough” to use as an accurate reference for color balance. For better accuracy, it is best to calibrate using a source known to be pure white. Stars vary in color according to their spectral class, but the Sun is considered to be white. Therefore any star of the same G2V spectral class (or close to it) should also be white. The following table (ref. Berry et.al., Sky & Telescope Magazine, December 1998) lists a number of these ”solar analog” stars:

To calibrate your exposures, take an image of one of these stars, taking care to ensure that it is at a high altitude to avoid extinction. You can then use the Color Balance command to set the proper scaling. Use the auto background to remove any sky background color, and then use the Click on White Area to Set Scaling feature. For more control, you can use the Information Window to determine the intensity of the star in red, green, and blue, and calculate the necessary scaling values.

Another approach is to use a photographic grey scale card (available from Kodak) and sunlight. Arrange for your telescope, preferably shielded from sunlight, to image a grey card placed in sunlight. It will probably be necessary to stop down the instrument to reduce the light levels sufficiently to take an exposure without saturating.

The White Balance command is a simplified version of the same command, which allows you to select a region of the image by creating a drag rectangle with the mouse. It is more suited for lab imaging applications, e.g. calibrating microscope imaging, where a large white area can be imaged.

Once you get your color balance parameters, you can enter them into the Stack, Convert Color, or Combine Color command, so they are applied automatically on new images.

You can also adjust color balance using the Color Adjust command. This command allows you to see the image with various adjustments applied, and then pick the image with the best appearance.