I recently came across a triplet apo that displayed astigmatism in the outer 30% or so of the field of view yet stars in the central 70% of the field showed nice tight, round airy disks in focus and concentric circular ring patterns on either side of focus.
I always assumed that astigmatism would manifest itself across the entire field of view. Is it possible that perhaps the lens is not inherently astigmatic at all and that something else could have caused this effect (poor collimation, decentering of lens elements, etc.)?
I always assumed that astigmatism would manifest itself across the entire field of view. Is it possible that perhaps the lens is not inherently astigmatic at all and that something else could have caused this effect (poor collimation, decentering of lens elements, etc.)?