Click this button to perform an automatic screen stretchCtrl-click this button to open the STF Auto Stretch dialog. On this dialog, you can modify the two parameters that control calculation of automatic STFs:

  • Shadows clipping
  • This controls the shadows clipping point of the computed STF. This parameter is expressed in sigma units, measured from the median of the target image. Recall that the median identifies the main histogram peak in deep sky images where the sky background strongly dominates their pixel value distributions. Hence, this parameter will normally have negative values, since we want to clip at the left side of the main histogram peak. The default value is −1.25.
  • Target background
  • This is the desired mean background value in the normalized [0,1] range. ScreenTransferFunction will compute the required nonlinear midtones balance transformations to achieve this mean background level in the screen representation of the target image. The default value is 0.25.

Automatic STFs are computed separately for individual RGB channels when the Link RGB channels option is disabled; otherwise a unique STF is applied to the three RGB channels. By default, the Link RGB channels option is enabled. Separate STFs are useful to compensate for color balancing problems in raw data in most cases; however, always take into account that when separate STFs are being applied, you are not seeing, in general, a screen representation of the actual RGB color image that you have.

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