This tool is to control the aspect of the preview image. None of the setting here have any influence to the image saved as a FITS file. They are only to help you to view the most of detail on the single raw images.
It is separated in two part, an histogram of the image and the control buttons.
On the top left, two rows of buttons are to control the range of histogram drawing, using 90%, 95%, 96%, 98% or the full histogram range. Reducing the histogram range help to see faint nebulae in the image.
Bellow the numeric value let you adjust the Gamma for the display. This also help to better see the faint part of the image but without saturation of the bright area.
On the bottom left you can mirror the image horizontally and vertically, or show a negative image that can help to see faint details.
The four right button are preselected zoom level for the image.
You can also get other zoom level using the mouse wheel rotation
When the zoomed image is bigger than the screen you can move it with the left mouse button.
On the histogram you can move the high (green) and low (red) clipping bar with the mouse.
You can also directly edit the low and high threshold by moving the mouse cursor to the top left or right to display the edit box.
The bull eye button show the mark on the image. If the image is plate solved it also display an arrow with the North and East direction and print the image rotation angle.
A focus star can be automatically selected in the square that circumscribe the external circle.
The button let you check for saturation in the image.
The overflow in the highlight area is marked in pink.
The clipped shadows area is marked in yellow.
You can define the clipping level for your camera in ADU in the preferences, the default are 0 and 65535.