From the menu Tools → Image statistics

Show statistics about the pixel values in the current image.
When the button Select area is checked you can select an area of the image to get the statistics from.

This show the following:

• Pixel count: the number of pixel used to compute the statistics.
• Min: The minimal value of a pixel.
• Max: The maximal value of a pixel.
• Mode: The mode of the image, i.e. the most frequent value. This is a good estimate of the sky background.
• Median The median value of the image.
• Mean: The mean value of all the pixel.
• Std.Dev: The standard deviation from the mean value.
• Mean2: The sigma clipped mean value.
• Std.Dev2: The sigma clipped standard deviation.

If the image is plate solved the following values are added:

• Center RA2000: The J2000 right ascension of the image center.
• Center Dec2000: The J2000 declination of the image center.
• FOV: The image size width x height in minutes of arc.
• Image scale: The image scale in second of arc per pixel.
• Actual focal length: The real focal length of the instrument, can be used to update the value in the Preferences.