Wednesday, August 20, 2008

Optical aberrations

To those who like optics, this is probably one of the simplest summary of optical aberrations that I came across in a thread from Cloudy Nights forum (: Something good to refer to (:

WHAT DO ABERRATIONS LOOK LIKE

ON_AXIS
Generally, not all aberrations are seen on-axis. The on-axis image created by aberrations is as follows:

curvature not seen on-axis.

coma not seen on-axis.

If the stars seem never to focus to a fine point, but at best remain slightly bloated and are still circular, and it cannot be focused out it is probably spherical aberration.

If the star seems to not come into sharp focus it may be astigmatism. If astigmatism is severe, then on-axis focused images will show a very small cross, never a fine pinpoint. Passing in and out of focus will show the out-of-focus image to flip orientation by 90°.

Longitudinal CA will produce softness of image focus of perfectly on-axis objects because all colors in the spectrum will not come to focus at the same point. Sometimes seen as a small circular band of color arounnd a finely focused star point.

OFF_AXIS
As for how you notice stars in the outer fov, the image created by aberrations is as follows:

If the star seems slightly bloated, but it can be refocused down to a finer point, then it is curvature.

If the stars seem flared as wedges with the point towards the middle and the flared wedge toward the outer edge, then it is coma. It cannot be focused out.

If the stars seem slightly bloated but are still circular, and it cannot be focused out it is spherical aberration.

If the star seems slightly bloated and slightly elongated than it is astigmatism. Astigmatism will show elongated images off axis. If astigmatism is severe, then on-axis focused images will show a very small cross, never a fine pinpoint.

If you have both astigmatism and coma, the off axis images will look like comets with an oblong axis, or like seagulls.

Longitudinal CA will produce softness of image focus of perfectly on-axis objects because all colors in the spectrum will not come to focus at the same point.

Lateral CA will produce color fringes around all bright objects when viewed slightly off axis, but you should see a different color towards the lens center than you see towards the lens edge.

Clear skies,
Anne (: