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Opinions on Ronchi images of 12.5" .200+

Started by Preston E, 08/09/2003 03:34PM
Posted 08/09/2003 03:34PM Opening Post
.200 plus
Thanks for any comments or corrective advice.
Preston

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Preston E's attachment for post 8228
Posted 08/09/2003 06:03PM #1
Looks like hyperboloid (outer ~50%) that contains a sphere
(inner ~50%).
Posted 08/10/2003 09:11PM #2
On a closer look, the mirror is probably ellipsoid up to ~60% radius, transforming into hyperbola up to ~90% radius, after which goes into TDE. I don't quite get the inside focus pattern; looks like the screen hasn't been centered, as it is with the out-of-focus shot.

If I got it right, defocus between marginal and paraxial rays with the source at the r.o.c. should be 10-11mm for this mirror. With the screen ~5mm out of the marginal focus (which is longer than the paraxial for ellipse/parabola/hyperbola), it is about three times closer to the screen than the paraxial focus. This would make top of the central line appear about three times thicker than its middle. It is about there at the 90% radius, but the line is not thickening at a "proper" rate: deformation is supposed to go in proportion to the square of off-center distance. So if it is 3+ times thicker (corresponds to
150%+ deformation) at the 90%, it should be about half that much at the 60% (about 80% deformation), and it is somewhat less. This indicates that defocus diminishes faster, probably as a resolt of midely hyperbolical ~60%-90% zone.

On the other hand, bellow ~60% zone, change in the line width is barely noticeable, indicating that defocus within this zone diminishes somewhat slower than it should for a parabola. Since some mild curves are noticeable on both central and side lines, it is probably an ellipsoid.

Assuming it is so, profile-wise, the mirror seems deeper than needed up to about 60% zone, then turns down with a stronger slope than a parabola would have. Seems logical to try to flatten ~30%-80% zone, in order to bring it closer to a parabolic surface. I'm sure some other people here know better abot the way to accomplish that (including TDE and not nearly as important central depresion).