This control is used to do a linear stretch of an image. There are two markers, one green, and the other blue, which can be dragged to adjust the high and low points of an image. The Histogram Stretch is used by PEMPro in the Calibration Wizard, Polar Alignment Wizard, Star Finder, and Backlash Exerciser.

 

 

Moving the green marker to the right will darken the image. Any pixel in the image with a value lower than the green marker's value will appear black. Every value between the green and blue markers will get stretched in brightness/contrast.

 

Moving the blue marker to the left will brighten the image. Any pixel in the image with a value higher than the blue marker's value will appear white. Every value between the green and blue markers will get stretched in brightness/contrast.

 

Here is an example image un-stretched, with the green marker to right, and with the blue marker to the left: