Author Topic: How difficult is Ross Null Test?  (Read 504 times)

Harry Smull

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Re: How difficult is Ross Null Test?
« Reply #15 on: January 31, 2018, 12:16:59 PM »
I guess then if you perform a Foucault test with the Ross null lens, a perfect parabola will null like a perfect sphere without the Ross Null lens?

erbarmauhump

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Re: How difficult is Ross Null Test?
« Reply #16 on: February 08, 2018, 05:29:49 PM »
Yes

And ceravolo states in his write up, if the outer zones null 1st correction is ---- and if the ctr nulls first correction is -----.

If I remember, outer nulls 1st its overcorrected. But don't quote me

Kareem Gillespie

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Re: How difficult is Ross Null Test?
« Reply #17 on: February 09, 2018, 10:56:45 AM »
Quote
I guess then if you perform a Foucault test with the Ross null lens, a perfect parabola will null like a perfect sphere without the Ross Null lens?

Correct. When the lens is placed at the correct distance from the mirror it adds spherical aberration that is opposite that of the mirror. The result is when the mirror is figured to the conic shape you want, that it will look like a sphere being tested at the sphere's radius of curvature. It will show a null like a sphere ie grays out evenly and shows dead straight Ronchi lines. The errors that you see are read the same way that you read zones on a sphere ie a hole is a hole, a hill is a hill and a turned edge is a turned edge, etc. A sphere will test as very much under corrected and look like an oblated spheroid. If the figure is smoothly over corrected it will look like a parabola. The difference is that these errors are showing you how the shape of mirror departs from a parabola or any other conic you want. That is what is so  powerful about null type tests. You >see how the figure is departing from the shape you want. No measuring and analyzing data. You have the mirror on and off the test stand in mirrors in minutes and get back to figuring. When you start getting close you can then slow down and carefully check everything. The result is that you can quickly home in on the figure you want and then refine it. This is one of the reason we can make 15 to 20 mirrors at the class I help teach in 3 days or less that are typically in the size of 8" to 12" in diameter and we having made a number of larger ones as well. A fair number of the "students" are first time mirror makers and they leave the class with excellent mirrors that continued to be independently tested to show this.

        - Dave