spherical aberration, and coma? Sorry if this is a dumb question. Can you give me an example of what each aberration will do to an image?
What's the difference between astigmatism...
Started by explore, 01/27/2003 10:05PM
Posted 01/27/2003 10:05PM
Opening Post
Posted 01/27/2003 10:32PM
#1
Not dumb at all.
Spherical aberration implies an inability to focus light properly. Most commonly, rays handled in outer zones of the optics focus at a different place than rays handled in the inner zones. It means you can't find a pinpoint focus.
Coma refers to a similar inability to focus light properly, but it is progressive, from no aberration on-axis, i.e. in the middle of the view increasing as you look farther and farther off-axis. The name is reminiscent of the shape of the blur one gets from a point source imaged off-axis. It looks like a little comet with the head pointed towards the center of the view.
Astigmatism can occur anywhere in the view and is due to asymmetry in the way light is refracted or reflected. For example, a spoon-shaped cornea causes astigmatism in your eye and is corrected with a lens with an asymmetry to cancel. Astigmatism often occurs with constrictive primary lens cells in Newtonians during cooldown (e.g. when the mirror is hard glued in as is too often done in inexpensive imports) and with thin mirrors that are not properly supported. In eyepieces it may occur anywhere in the view (the result is a lack of sharpness), but most often it is not present on-axis but increases off-axis, somewhat like coma but growing faster generally. People often mistake off-axis astigmatism in eyepieces for coma. There's a nice interactive tutorial on this at
http://www.microscopyu.com/tutorials/java/aberrations/astigmatism/index.html
Some of the other demos/tutorials on this site are quite good. Look for others on
http://www.microscopyu.com/tutorials/java/index.html .
--
Mike
Spherical aberration implies an inability to focus light properly. Most commonly, rays handled in outer zones of the optics focus at a different place than rays handled in the inner zones. It means you can't find a pinpoint focus.
Coma refers to a similar inability to focus light properly, but it is progressive, from no aberration on-axis, i.e. in the middle of the view increasing as you look farther and farther off-axis. The name is reminiscent of the shape of the blur one gets from a point source imaged off-axis. It looks like a little comet with the head pointed towards the center of the view.
Astigmatism can occur anywhere in the view and is due to asymmetry in the way light is refracted or reflected. For example, a spoon-shaped cornea causes astigmatism in your eye and is corrected with a lens with an asymmetry to cancel. Astigmatism often occurs with constrictive primary lens cells in Newtonians during cooldown (e.g. when the mirror is hard glued in as is too often done in inexpensive imports) and with thin mirrors that are not properly supported. In eyepieces it may occur anywhere in the view (the result is a lack of sharpness), but most often it is not present on-axis but increases off-axis, somewhat like coma but growing faster generally. People often mistake off-axis astigmatism in eyepieces for coma. There's a nice interactive tutorial on this at
http://www.microscopyu.com/tutorials/java/aberrations/astigmatism/index.html
Some of the other demos/tutorials on this site are quite good. Look for others on
http://www.microscopyu.com/tutorials/java/index.html .
--
Mike
Funding Member
