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Secondary Size.

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Posted by Robert May on May 28, 2003 21:01:05 UTC

The size of the secnodary isn't as critical as you seem to think. On-axis the focal plane should be seeing all of the primary mirror. At some point (the 100% ring) is the point at which the edge of the primary touches the edge of the secondary and that is the limit of the full illumination of the focal plane. After that, the primary is gradually obscured by the edge of the secondary and that will slowly take some illumination out of the image. The 75% ring in Newt is where 25% (about 1/2 mag.) of the light from the primary doesn't make it to the focal plane.
The various propounders of wisdom often have a personal limit of where the 100% ring happens and you have just read about one where he thinks that that ring should be larger than what you have. I'll also note that if you are looking with your eyeball at some point further from the primary than the focus is, your black ring around the outside of the primary, indicating excess secondary size for the on-axis image, will appear smaller than reality.

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