ASTROPHOTOGRAPHY

Zbynek Vrastil



Assisted Color Calibration

White Balance

Three white balance coefficients, one for each channel. Each channel in the image will be multipled by corresponding coefficient. The value of 1.0 means that channel won't be affected at all. Lower values weaken corresponding color in the image. Higher values strengthen the color. Reset button located to the right of the Blue coefficient slider resets all three coefficients to 1.0.

These three coefficients are the only parameters applied to regular images. Other parameters are only applied to previews.

Note: In theory it is better to have all three coefficients less or equal to 1.0. This prevents any clipping and thus loosing any information in the image. However, for me it is easier to tune up the coefficients by strengthening color I'm missing rather than by weakening other two colors. In any case when you found correct coefficients you can compute their < 1.0 equivalents by dividing all coefficients by maximum of their actual values. This affects brightness of your image but not the color calibration.


Preview Parameters

Parameters in this section do the "basic post-processing" of the calibrated preview. Their purpose is to give you a chance to evaluate an effect of the white balance coefficients. Minimal post-processing should involve at least the following steps:

  • Subtracting the background to remove sky glow, light pollution etc.
  • Nonlinear histogram stretch to reveal faint details
  • Color saturation enhancement to reveal image colors

Of course post-processing usually involves much more sophisticated methods for noise reduction, mutliscale or HDR processing, using masks etc. Still, three essential steps noted above should give you decent approximation of what you get in the end; at least good enough to evaluate actual white balance coefficients.

Important: These parameters are ONLY applied to previews. If you apply the process to regular image, none of the parameters in this section is taken into account. The typical process is to tune the white balance coefficients on preview and then apply them to original image and continue with post-processing.

Background Correction

Here you can (and should) select a view containing valid background reference of your image. This means that the view should be strongly dominated by sky background pixels. There should not be any large nebulosity or galaxy. Typically, you'll define an extra preview on your image to serve as background reference. If possible, this background reference preview should be close to the preview on which you do the color calibration. This will elliminate effect of possible background gradient (often comes from light pollution).

Background reference is computed as median of each channel. When the process is applied to the preview this reference is also calibrated with the white balance coefficients and subtracted from the processed image. After these two operations (applying white balance and subtracting background reference) you should get neutral background color.

Histogram Transformation

Applying histogram transformation to stretch the histogram is essential to reveal faint parts of your image and evaluate actual white balance coefficients. There are three parameters known from standard PixInsight Histogram Transformation process:

  • Shadows Clipping - the black point, position of the left-most triangular handle on the histogram scale. Pixels with equal or lower intensity in the original image are black after transformation
  • Highlights Clipping - the white point, position of the right-most triangular handle on the histogram scale. Pixels with equal or higher intensity in the original image are white after transformation
  • Midtones Balance - the nonlinear stretch of histogram - value below 0.5 brightens the image. Value above 0.5 darkens the image.

You can either set those parameters manually using three numeric editors or by dragging three triangular handles on the scale below the black MTF (midtones transfer function) plot. The plot shows actual shape of the MTF curve. The histogram itself is not rendered here. There is also Reset button in the bottom-right corner. This button resets actual histogram transformation parameters.

Saturation Enhancement

Saturation of astronomical images is typically very low (with the exception of emission nebulae). In order to evaluate white balance coefficients precisely, it is necessary to boost color saturation of the preview. The color saturation is multiplied by the coefficient that is specified by using Saturation slider.

Feel free to boost saturation by the factor of 3 or 4. Oversaturated image helps you verify current white balance coefficients very precisely.


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