Near the bottom of the Inspectors along the right edge of Observatory’s window, is the Histogram panel. It has a disclosure button at its left which, when clicked, reveals and hides the panel. The click area actually covers the whole panel’s title bar, so you don’t have to be too accurate when clicking. Alternatively you can choose View ▸ Inspectors ▸ Show/Hide Histogram.
The histogram is a graph which shows how many pixels are in the image in different brightness ranges. The darkest pixel represented is at the left side, and the brightest pixel is at the right side. The highest point on the graph corresponds to the most common brightness level in the image. The vertical scale usually is logarithmic, but it can be changed to linear using the histogram action menu.
The histogram allows you to conveniently adjust the brightness and contrast of the displayed image without affecting the contents of the image. The red caret controls the black level. Moving it leftwards makes the background of the image lighter. The green caret controls the white level. Moving it leftward increases the image contrast, but also tends to saturate bright parts of the image. The contrast is highest when the two carets are close together.
The carets are affected by the two fields near the bottom of the panel. They can also be dragged with the mouse. In addition they can be moved left or right in steps by clicking in the area left or right of them.
Instead of manually adjusting the black and white points, you can also use one of the percentile presets in the histogram action menu. Even in its collapsed state you can quickly adjust the black and white points this way. The available settings are:
Low – low contrast setting with a dark background.
Medium – medium contrast setting with a dark background.
High – high contrast setting with a dark background.
All – adjust so that the brightest pixel is set to the maximum brightness, and the zero level is set to black.
For RGB images, the histogram panel displays the histogram for each selected (active) channel. By default all the channels are selected, but you can select one channel, for example Green, using the Channels panel. The background color of the histogram is a handy indicator of which channels are active for a given image.