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Originally Posted by chunkylad
Hi Vespine and welcome
You tend to get little 'seagulls' where there should be pinpoints of light.  This occurs "off axis", meaning the centre 50-60% of your field of view is in focus, and the remainder - the 'edge' of the field of view - is out of focus. The is due mainly to a condition called coma, inherent in fast designs.
Dave W |
Dave,
I have quoted a little from your post. I just wanted to clarify something because what you have said is not quite correct, it is very close.
The little seagulls you get with cheap widefield eyepieces in conjunction with a fast telescope are more attributable to astigmatism and field curvature eminating from the eyepiece itself than from coma eminating from the primary mirror. The cheaper eyepieces are unable to eliminate those aberrations internally, when combined with a "steep angle of incidence" of the light cone. Those aberrations effectively "mask" the coma coming from the primary and overpower it so you dont see the coma itself. This is not always the case but true in 95% of cases. Premium eyepieces like Naglers, Panoptics and Pentax XW's are free of astigmatism and go a long way towards correcting for the coma eminating from the primary mirror, hence they can show pinpoint stars to the EOF even though there is coma inherent in the primary optic at EOF. Its all in here
https://www.willbell.com/tm/tm6.htm
This book is a great reference for anything "telescope and eyepiece optics". It is a timeless classic work, if your interested in what makes it all happen. Although it's getting a little dated, being wrtitten in the 1980's prior to the release of the more modern of the Nagler and Pentax designs, the physics doesn't change.
CS-John B