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Perplexing Astigmatism in a Triplet

Started by ryderc1, 09/25/2007 10:34AM
Posted 09/25/2007 10:34AM | Edited 09/25/2007 10:36AM Opening Post
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.)?

Posted 09/25/2007 01:00PM #1
>>poor collimation, decentering of lens elements, etc.)? >>

Astigmatism and perhaps some coma is always present in any lens as you go away from the center of the field. Eyepieces also add astigmatism as you go off-axis. It is not a collimation or decentering probem of the objective. If the astigmatism is not uniform around the edge of the field, you may have a tilted focuser.

A miscollimated objective lens will cause the "sweet spot" of zero astigmatism to shift from the center of the field toward the edge. It is a simple matter of tilting the lens cell back to square to bring this sweet spot back to the middle of the field. Gross misalignment of the lens or lens cell causes the sweet spot to go completely off the field of view (or focal plane). Decentered lens elements always will cause severe coma on axis as well as off-axis.

Rolando