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Re: Read Carefully

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Posted by Rick Crockett/">Rick Crockett on January 23, 2000 22:25:48 UTC

: : : Check out this link on spherical mirror warping: : : : http://tempest.idle.com/~markv/spherical.html : : : I believe this is what Joe was referring to. : : : : : : This is a good idea in that a relatively easy to make spherical mirror is warped into a paraboloid. However I have wondered if the warping is uneven and causes astigmatism.

: : You want to read that page carefully. The author is using precision ground, polished and figured mirrors that are spheriods already and they are full thickness masking the error. The author states that a little tension pulls the sphere into a paraboloid which is technically not correct. The figure will be a hyperbaloid but by having a fairly small diameter and a fairly thick mirror with a sphere already polished into it, it could look like a paraboloid. The author mentions that with more tweeking it is pulled into hyperbaloid which is exactly what it becam as soon as it was deformed from the sphere. : : The eccentricity of radius for a sphere is zero. There are not degress of spherocity. Same goes a paraboloid with an eccentricity of 1. The hyperbola can be near paraboloid or to an extreme. Extreme is what you will get if you try thin mirrors, large mirrors and non sphere mirrors. You can take a sphere and using vacuum deform it into a parabola but not by using a bolt. I strongly encourage experimentation but you also should be familiar enough with the laws of physics and mathmatics to know what to expect.

: : Enjoy Experimenting : : -Rick

: Hi Rick

: I am interested in this approach because I own a full-thickness, 8", f/8 mirror with a smooth spherical curve. It's rated at about 1/2 to 1/3 wave p-v. I agree that the bolt method won't warp the mirror into a parabola, but would it warp it into a shape that would give better star images (ie 1/4 wave or better)? The author of the article indicates that it would. I am little suspicious though. My guess is that if the bolt, carpet ring, and/or wooden block are not made/configured to very high tolerances, problems with the image would result.

: At this point, I am just curious about this approach. I'll probably parabolize the mirror the old fashioned way with a pitch lap.

Hi Bill:

I think this would be a fun thing to play around with but if I had one good piece of glass, I am sure I would finish it out correctly. I never had a problem parabolizing a mirror. I have always been able to visualize the correction and polish it in. In fact I polish to figure rather than sphere. The important thing is a good work surface and consistent habits. I prefer a pitch on the hard side of medium. I recomend the old ATM books by Ingalls but I use saran wrap to cover the mirror to keep the pitch pour from sticking and I then press with fabric mesh reinforcement about 3/16ths squares. Rub liquid dish soap over everthing before pressing. I don't mess with cutting channels. I will often do a second press with fine Nylon mesh for faster action. If you have a good polish, deepen the center. For a questionable polish push back the edge.

I too think that one would need a consistant edge ring and the blank would have to be very accurate for this to work. On the other hand a series for bolts on the back might tweek out an astigmatic optic.

Good Luck with which ever way you try. -Rick

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