Is a focault tester really accurate enough to guarantee a supurb figure on a fast mirror? What do Zambuto and other well known mirror makers use to asses mirror quality?
Is a focault tester enough....
Started by ddrake, 08/10/2003 07:38PM
Posted 08/10/2003 07:38PM
Opening Post
Posted 08/11/2003 03:18AM
#1
Darren,
A foucault tester in conjunction with the star test is capable of producing a very good figure. Superb? Depends on your definition of the word. Many variables exist in Foucault such as number of zones tested, ones ability to interpret the shadows, quality of tester etc.. Many large manufacturers use laser interferometry or double pass collimators and other equipment we ATMs can't afford.
I once had a reputable optics shop (E & W in Minneapolis) 'look' at a 6" I ground before they coated it. When I told them I used Foucault and star testing they told me I couldn't get it any better using those methods. I was happy hearing that, but, in reference to your question, TO ME, that means the mirror could have been improved upon if I had other means to test it. In my mind, superb means no visible room for improvement.
If you are going to make several mirrors or have a spare grand or so, Ceravolo (.com) is again offering reference lenses for a Ross null tester and an ATM laser interferomter.
Check out his site. This is the route I would take if I were to start pushing glass again.
Herbert and others on this forum can speek much better about the accuracy of Foucault testing than I can. All I know is 'figure till the numbers are dead on'.
( and that may not be 'superb' enough for some.
Hope this helps, Bob
A foucault tester in conjunction with the star test is capable of producing a very good figure. Superb? Depends on your definition of the word. Many variables exist in Foucault such as number of zones tested, ones ability to interpret the shadows, quality of tester etc.. Many large manufacturers use laser interferometry or double pass collimators and other equipment we ATMs can't afford.
I once had a reputable optics shop (E & W in Minneapolis) 'look' at a 6" I ground before they coated it. When I told them I used Foucault and star testing they told me I couldn't get it any better using those methods. I was happy hearing that, but, in reference to your question, TO ME, that means the mirror could have been improved upon if I had other means to test it. In my mind, superb means no visible room for improvement.
If you are going to make several mirrors or have a spare grand or so, Ceravolo (.com) is again offering reference lenses for a Ross null tester and an ATM laser interferomter.
Check out his site. This is the route I would take if I were to start pushing glass again.
Herbert and others on this forum can speek much better about the accuracy of Foucault testing than I can. All I know is 'figure till the numbers are dead on'.
( and that may not be 'superb' enough for some.
Hope this helps, Bob
Fujinon 10x70 FMT-SX binocs
Tak FS-78 ƒ/8.1 - 630mm fl.
6" ƒ/6.5 homeground newt. - 981mm fl.
10" ƒ/4.85 homeground dob. - 1231mm fl.
Posted 08/11/2003 03:23AM
#2
RF Royce uses two pass optical flats, Foucault knife, Ronchi and interferometry (i.e. Zygo). He web site stresses Strehl over RMS or P-V.
http://www.rfroyce.com/products_ast.htm
You can make a decent mirror using only the knife edge (with all the zonal masks) and Ronchi test, but don't expect to compete with Zambuto. Remember, it's about smoothness too.
http://www.rfroyce.com/products_ast.htm
You can make a decent mirror using only the knife edge (with all the zonal masks) and Ronchi test, but don't expect to compete with Zambuto. Remember, it's about smoothness too.
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