Vignetting

Zoom out and stretch hard. The darkening is round, centered and about the same in each corner. It is in every frame and does not move.

1. 01

   No flats

   Flats calibrate the vignetting of one optical setup. Without them, whatever darkening the optics make stays in the image.

   FixShoot flats at the end of each night, before focus, rotation or anything in the train changes.

   Session Checklist
2. 02

   Undersized image circle

   The corrector or the scope lights a circle smaller than the sensor's diagonal, so the corners get less light.

   FixCheck the corrector's stated image circle against your sensor's diagonal. Flats correct mild falloff; corners that get almost no light need cropping or a different corrector.

   Backfocus Solver
3. 03

   Filter drawer or adapter too narrow

   A drawer or adapter with too small an opening clips the edge of the light cone before it reaches the sensor.

   FixFind the narrowest part of the train and replace it with one that has a wider opening.

• Gradients

  A gradient runs in one direction across the frame. Vignetting is round and centered.

One frame shows the darkening. A flat frame shows how deep it goes.

After calibration with flats, corners that match the center 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.