Clipped blacks

The histogram's left side is cut off at zero instead of rising into a hump. Faint areas next to the nebula are flat black.

1. 01

   Black point pushed too far during the stretch

   Setting the black point past the foot of the histogram throws away everything below it, faint nebulosity included.

   FixUndo the stretch, and set the black point just left of where the histogram begins to rise.

2. 02

   Aggressive background extraction

   Extraction that treats faint nebulosity as sky removes it along with the gradient.

   FixKeep the sample points off the nebula, or use a gentler model.

• Too little integration

  Too little integration leaves a noisy, grainy background. A clipped background is flat black.

The finished image shows the clipping. The linear stack shows whether the faint signal was there to lose.

A histogram whose whole hump sits clear of the left edge, with the faint outer nebulosity visible.

No outside source is cited here. The signature, the causes and their order are the atlas’s own, from imaging experience. The order is a judgment of likelihood; no measured frequency is behind it.