Gradients

Zoom out and stretch hard. The ramp has a direction and is not centered on the frame. Its strength changes as the target moves across the sky.

1. 01

   Light pollution

   Skyglow from towns brightens the sky toward them, so the ramp points toward the brightest part of your horizon.

   FixImage targets when they are highest and away from the bright side of the sky, then remove what remains with background extraction in processing. Siril and GraXpert both do it for free.

   Sky brightness method
2. 02

   Moon

   Moonlight brightens the sky toward the Moon, and the ramp points toward it.

   FixCheck where the Moon is when you plan the night. Narrowband tolerates moonlight far better than broadband.

   Session Checklist
3. 03

   Dawn or dusk

   Twilight brightens the sky from one horizon, and subs taken in it carry a ramp toward that side.

   FixGive the sequence a stop time before dawn, and start it after dusk has ended.

   Session Checklist
4. 04

   A neighbor's floodlight

   A nearby light adds its own ramp, from wherever it is.

   FixShield the scope from the light, or image when it is off.

• Vignetting

  Vignetting is round, centered and the same in every frame. A gradient has a direction and changes with the sky.

• Amp glow

  Amp glow sits in the same corner in every frame, darks included. A gradient changes as the sky changes.

• Green cast

  A green cast tints the whole background evenly. A light-pollution gradient ramps across it.

• Over-stretched

  An over-stretched image is gray all over. A gradient left in is brighter on one side.

One frame shows the ramp and its direction. Telling the Moon from a town needs the time of the frame and where the Moon was.

After background extraction, a background that stays even under a hard stretch.

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.