Ceravolo Ross Null Lens for Mirror Testing
The COS Ross null lens.
Using your Ronchi or Foucault tester, the Ross null lens allows you to test your parabolic, elliptical or hyperbolic telescope primary mirror at the center of curvature as though it were a sphere. There is no need to interpret difficult shadow measurements because figure irregularities like bumps, holes and zones are easy to see and interpret. That's half the battle when producing a truly fine primary mirror. The Ross null test has been used to make very smooth mirrors with a 1/20th wave peak to valley surface quality. The result: high contrast images of the planets and deep sky objects.
The Ross null lens is placed ahead of the mirror's center of curvature.
The Ceravolo Ross null lens is recommended for mirrors up to 20” diameter. Up to this size an f/4 mirror can be tested to 1/20th wave surface quality. Larger mirrors can be tested as long as they are slower than f/4, for example a 30” f/5 Newtonian primary.
Determining the required null lens - mirror spacings for your set up has been made easy by the efforts of Douglas George of Diffraction Limited. Over ten years ago Doug wrote a small program that calculated the spacings required for the Ross null test. Recently, James Lerch updated the old DOS software so it will run reliably in the Windows XP Operating System.
Download the free Ross null test software. Many thanks to Doug and James for providing this service to the community!
In order to test telescope primary mirrors at the center of curvature with an interferometer, a null test must be used. The Ross null lens is ideal for the application as it avoids the expense of a large optical flat. To learn how to perform mirror testing with the Ross null lens, download the PDF document, "Using the Ross Null Test".
Ross null lens specifications
Diameter: 80 mm
R1: 206mm convex
R2: flat
Thickness: 15 mm
Substrate: BK7
Surface quality 1/10th wave or better on both sides. A Zygo interferometer printout is supplied with each serialized Ross null lens.
Price USD $575.00 + shipping and handling.
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