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How do we know how good a telescope mirror is?

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Free Optical testing Service

Iris Nebula: image by Harry Page using optics made by Nichol Optical

Welcome. Nichol Optical is a business dedicated to making high precision astronomical optics.  I have been making astronomical optics for in excess of  30 years and have supplied optics to customers in Germany, France, Belgium, Denmark, Sweden, Netherlands, Ireland, Japan, Slovenia and South Africa as well as the UK.

To ensure my optics are of the highest quality a variety of tests are used. Together with the more traditional Foucault and Ronchi tests I also employ the Ross null test (using a certified lens), and the double pass null test or, autocollimation test as it is also known. These are well established tests and are used widely by professional optical shops the world over. Parabolic mirrors are generally finished using the Ross null test or autocollimation test, these tests are quantitative in nature and do not produce a set of numbers describing the mirrors surface. I do not produce a test certificate giving a detailed numerical analysis of  a mirrors surface.  With a null test a perfect mirror produces straight bands when viewed with a ronchi grating, any curvature of the bands indicates the amount of spherical aberration present. For a given mirror a computer simulation may be generated showing the degree of curvature in the observed ronchi bands for varying degrees of spherical aberration. In order to assess a mirrors surface the observed curvature of the ronchi bands is compared with a computer generated image. With experience this test method is easily capable of revealing wavefront errors of  1/8 wave and smaller. For more information about optical standards please read the notes on my articles page. Perhaps the best recommendation for any optic is confirmation of it’s performance in a telescope, I am pleased to say that I have received much feedback from customers who have been delighted with the performance of their optics, for further information please see the comments page.