Histogram Clipping
This section is disabled by default. When it is enabled, the parameters in this section allow you to clip the specified percentages of the total pixels in the shadows and/or the highlights of the target image. Since the noise is higher in dark areas, a relatively small amount of pixels can usually be clipped in the shadows without destroying significant data. For this procedure to work correctly, make sure to crop any black borders caused by partial coverage during image registration, or similar border artifacts.
In general, for histogram manipulation of astronomical images we recommend using other tools that provide more accurate control, such as HistogramTransformation. However, when used carefully this automatic clipping feature can yield very good results in combination with the different automatic stretch functions available (see below).
Target Median Values
Use these parameters to specify the desired median value(s) in the target image. These parameters can be used to alter both the global illumination and the color balance. When the process is applied to a color image, different target median values for individual RGB channels modify color balance. When the same values are specified for all channels, a neutral background will be achieved, assuming that the sky background is the dominant feature in the image.
For images not dominated by the background sky—for example, when large DSOs cover the field of view—a useful alternative is to extract the lightness (CIE L* component) as a grayscale image, process it with AutoHistogram, and then reinsert the modified lightness in the color image. See the ColorSpaces category for ways to perform this task.
Click the Set As Active Image button to copy target median values from the active image. If the active image is in the grayscale color space, the joint RGB/K channels option will be enabled automatically. If the active image is a color image, the individual RGB/K channels option will be enabled.
Enable the capture readouts option to read target median values from image readouts when this tool is visible. Readouts are generated when you click on an image with the mouse or other pointing device. You can click and drag to generate readouts as you move the cursor over the image.
Stretch Method
The selected algorithm isn't critical to the result, but it affects the global contrast of the transformed image and can make easier or harder further fine adjustments (with CurvesTransformation or HDRMultiscaleTransform tools, for example). In general, a gamma transformation generates less contrasted images, while the MTF (midtones transfer function) tends to yield more contrasted images. The logarithmic transformation provides intermediate contrast, but the achieved median values may be somewhat less accurate. We encourage you to test the different methods available and select the result that best suits your needs in each case.
