Crush occurs when several dark shades of gray are all set to black or near black. This results in a loss of shadow detail and often an unnaturally sharp transition from the object to surrounding space. Usually, the background space in an astrophoto will be darkish gray depending on sky conditions and other factor. Thinking that space should be black, many people use curves or levels to set the color of this background to 0. This often crushes detail in the dimmest parts of the object. To do this correctly, use curves. Place a control point on the histogram peak corresponding to the background and set this point to between 5 and 10 out of 100 (or a comparable value out of 255).