Mechanical Vignetting

Mechanical vignetting comes from physical objects that actually block light, such as a dewshield on a telescope that is too long, or lens shade that is too small. Using a camera adapter that is physically too small can also cause vignetting, such as a DSLR in a 1.25 inch focuser. Improperly spaced telecompressors, particularly on Schmidt-Cassegrains can cause vignetting. Undersized secondary mirrors in Newtonians can also cause vignetting.

The mechanical vignetting seen here is caused by a lens shade that was too small for the 18mm wide-angle lens.

• Solutions:
  • Use the correct size lens shade or dewshield.

  • Use a 2 inch adapter and 2 inch focuser.

  • Properly space your telecompressor.

  • Use a larger secondary mirror.

By far and away, the best way to correct this kind of vignetting is to fix the problem at the source. This kind of vignetting can produce images with literally no signal at all in the corners, so flat-field calibration frames and image processing in software have no data in the vignetted areas to work with.

Optical vignetting is caused by the complex arrangement of optical elements in a camera lens where front elements can block part of the off-axis light reaching rear elements.

This type of vignetting in common in camera lenses when they are used wide open.

Mouse-Over

Hold your mouse cursor over the image to see an example of optical vignetting in an image shot wide open with a 50mm f/1.4 lens, and the vignetting corrected by stopping down to f/4. Spherical aberration is also present in the image at f/1.4 and is greatly corrected at f/4.

• Solution:
  • Stop the lens down 2 or 3 stops.

  • Use a flat-field calibration frame.

  • Correct during image processing with a program like Background Subtraction Toolkit.

Natural vignetting is light falloff in the corners of the frame due to geometrical reasons and the angle at which light reaches the sensor. It can not be fixed by stopping down the lens.

The natural vignetting seen in this image is caused by geometric light falloff.

• Solution:
  • Use a flat-field calibration frame.

  • Correct during image processing with a program like Background Subtraction Toolkit.

Whereas vignetting is related to the optical system, gradients are usually caused by real brightness variations in the sky. They are frequently caused by light pollution. They can also be caused by natural atmospheric extinction near the horizon.

The brightness and color gradient seen here is caused by light pollution.

• Solution:
  • For light pollution gradients, shoot at a true-dark sky site.

  • For atmospheric extinction, wait until the object is higher in the sky, if possible.

  • Correct during image processing with a program like Background Subtraction Toolkit.

Flat-field calibration frames will not fix in-sky gradients.

Problem: Vignetting and Gradients - The Bottom Line Vignetting is an in-scope problem. It can be mechanical or optical in nature. It is best to correct mechanical vignetting at the source. Flat-field frames can also help if the problem is not too bad. Gradients are usually present in the sky from things such as atmospheric extinction and light pollution. Gradients must be corrected during image processing.