Quality Claims, Wave Ratings and Testing of Mirrors

Why are optical tolerances important for telescope mirrors?

 No manufacturing operation performed by human beings has tolerances as demanding as telescope optics. The quality of a telescope mirror's performance depends on care in its production, including the application of a meaningful tolerance for spherical aberration. By "meaningful" we mean that the tolerances used by the optician must have a direct correspondence to the expected level of performance in the telescope. The process tolerance should be designed to produce the smallest possible star image, also known as a diffraction disk, in addition to minimizing microfinish and large-scale roughness. The diffraction disk is the fundamental unit of resolution, analogous to the pixels that make up a TV image. The smaller it is, the more optimally the mirror will perform. 

Working Tolerance

 Our working tolerance is 1/10 wave (wavefront) peak-to-valley and 1/36 wave RMS (Strehl Ratio = .97) for mirrors of Focal Ratio of greater than F/4.2. Faster mirrors equal or exceed 1/5 wave PV or 1/15 wave RMS wavefront. The software which we use displays identical results to wavefront calculations per Texereau, and program output compares closely to commercial programs designed to calculate wavefront error. Our optics have no more than 1/10 wave indicated error at the wavefront. You will receive a signed Statement of Quality for your serialized mirror that attests to the attainment of these stated tolerances. We maintain records of inspection and test on-site. Additionally, we apply the standard of "diffraction-limited" as defined by the relative transverse aberration (RTA) produced by the mirror, which is more stringent by a factor of 2.5X than the so-called "1/4-wave diffraction-limited tolerance," which is a contradiction in terms. 

Wave Ratings

In addition to Foucault testing we perform a double pass auto-collimation test. This test gives us immediate qualitative results. If the Ronchi lines are straight, then the surface is parabolic. This is the first test we run when we start the refiguring process. We can immediately see common defects such as turned down edge, zone defects, and under or over correction.

Testing and Test Data

 Informed optics purchasers recognize the direct relationship between optical accuracy and performance. Optical accuracy is usually expressed in wave ratings, such as the wavefront peak-to-valley (p-v), RMS or RTA figures we supply. We know that purchasers wish to be assured that their mirror conforms to the stated tolerance, and we provide a Statement of Quality that confirms this, supported by quantitative values. The only meaningful purpose of testing is to determine and apply needed corrections, based on mirror surface conditions revealed by testing, during the figuring process. Test output data must contain sufficient information to confirm achievement of target values and tolerances. Testing is a tool for building in quality, but cannot "add it on" after the fact. We quantitatively test the mirror at each step until figuring is completed. Your mirror will be made as good as it can be made, and will not be released for shipment until it meets the established numeric criteria for wavefront accuracy, as well as smoothness and edge criteria. The data reduction is identical to that described by Texereau in How to Make a Telescope, 2d Edition.