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RE: Understanding The Ronchi Pattern

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Posted by Thomas Picciani on January 2, 2003 23:56:34 UTC

Be careful about the ronchi pattern. Check it inside and outside of ROC! The Ronchi pattern should look almost identical inside and outside of ROC!

I just went through a really ugly discovery that caused me several hours of corrective action!

When you are outside ROC and you have 3 straight lines in the center, are all the lines straight or do you have ovals on each end? Do the straight lines hook at the edges at all? If they do you probably have not polished out to the edge and have a tde. If the ovals look like flanking D's, you probably have a really severe TDE!

I just went through this with a 10 inch f/4.4 mirror. Discovered this pattern and now I know why my edges looked so strange in the couder mask's edge zones. All my lines are now straight. There are no curved ronchi lines anywhere, either inside or outside of focus. That is what a sphere looks like!

The really sad part is that I have 2 other mirrors, each with a $75 new coating on them, that have the same defect! ARRRRGGGGHHHH!

I'm gonna have one mirror that's right!

Tom P.

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