Near the bottom of the Inspectors along the right edge of Observatory’s window, is the Channels 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 Channels.
Channels are grayscale images. They are created automatically when you open an image. The image’s color mode determines the number of color channels created. For example, an RGB image has a channel for each color (red, green, and blue), while a grayscale image has only a single channel.
The Channels panel lists all channels in the image. A thumbnail of the channel’s contents appears to the left of the channel name. It is automatically updated as you edit the channel.
You can use the Channels panel to view any combination of channels in the Editor. Click in the eye icon next to the channel to show or hide that channel. Observatory requires you to have at least one visible channel, so you are not able to hide the channel of a grayscale image.
You can select one or more channels in the Channels panel. The names of all selected, or active, channels are highlighted in the panel. Many operations in Observatory are applied to the active channels of the active layer only. For example, by default all channels are selected for RGB images, and the Editor Bar will display the pixel value at the cursor position for each channel, and the centroid position will be an average over the three channels. By selecting only the green channel in the Channels panel, making it the active channel, the Editor Bar displays only the green pixel value, and the computed centroid position is that of the green channel only.
Because it is otherwise easily overlooked, the background color of the histogram in the Histogram panel is a handy indicator of the currently selected active channels.