Testing and Certification

General Characteristics

The prototype is a 12.5 inch, f/6, Pyrex, parabolic mirror. The substrate was manufactured by Newport Industrial Glass. Newport manufactures a wide range of optical glass products that form the basis of many of the optics produced by R.F. Royce - POC.

Testing during Manufacturing

During the manufacturing process the optic is tested using both the Ronchi and Foucault tests in autocollimation. This produces a null wavefront in double pass showing all errors at twice the magnitude of that seen in actual use. Below are Ronchigrams taken of the prototype mirror. These Ronchigrams were made with a 133 line grating at various positions inside of focus. Most of the mottling seen is due to atmospheric turbulence.

Interferometric Analysis

Once the optic was completed to satisfaction it was subjected to a full interferometric analysis in order to quantify its wavefront characteristics. Contra-rotation of interferograms at 90 degree positions was not done for this test inasmuch as the mirror is mounted exactly as it would be when in use and therefore the astigmatism is real for a telescope pointed at the horizon, the maximum astigmatism position likely to be encountered. Astigmatism will diminish considerably as the telescope is pointed upward. 

Analysis of Zero Degree Interferograms

A total of eight interferograms were taken at the zero degree position. These were averaged in order to determine the wavefront produced as impacted by astigmatism due to mounting and aiming horizontally. The results of this analysis is given below:

Analysis of a Single Interferogram

As a aid to visualizing exactly what this process involves I have included a single interferogram below and its interferogram and graphical analysis. This interferogram was selected as being highly representative of the final 0 degree interferogram wavefront conclusion, though it differs slightly because it is only a single interferogram. In analyzing any optical surface for high accuracy results one must take many interferograms and average their individual results into a collective whole. Variations from one interferogram to another are mostly the result of atmospheric turbulence which tends to inject false astigmatism into the interferogram and the final analysis. This single interferogram analysis be shown below. 

Single interferogram

The plus signs in the centers of the individual fringes are the data points utilized by the Quick Fringe computer analysis program to analyze the position of the fringes in relation to each other and thereby calculate the various parameters. These data points are initially automatically placed by the Quick Fringe program and then individually manipulated where necessary. The general procedure is to leave the data points mostly where they fall automatically adjusting only those few which stray grossly from the fringe centers. When averaging many interferograms this yields a very consistent, accurate and honest interpretation of the actual wavefront.

Isometric Contour Plot

Topographic Contour Plot

The pictorial data shown above is a graphical representation of the surface of the prototype mirror in terms of the wavefront produced. The surface of the mirror is actually one-half the error amounts shown on the graphs since the wavefront represents twice the error of a reflected surface.

Test results of the single interferogram are as follows:

Once again, the reader should understand that the analysis data given in this presentation is not of an exceptional mirror but of a typical mirror.