Reflective optics

Mirror Polishing, Figuring & Optical Testing

A bright coating is not the same thing as an accurate mirror. We separate surface condition, reflective coating, mirror figure, and whole-telescope performance.

Written by Lone Star Observatories · Reviewed by the telescope.repair workshop · Updated

Grinding establishes curvature. Polishing removes the fine ground surface. Figuring then changes the surface by extremely small, controlled amounts until the returning wavefront approaches the intended shape. Each correction must be followed by another measurement; polishing without metrology is guesswork.

Evidence checkpoint. NASA technical literature describes quantitative interpretation of Foucault knife-edge patterns, while NASA’s Webb program documents polishing followed by metrology and cryogenic verification. [1] [2]

Mirror-test geometry and interpretation panelsFoucault geometry at left and qualitative examples of Foucault shadows, Ronchi bands, and interferometric fringes at right.source / knife edgemirrorFoucault shadowRonchi bandsinterferometric fringes
Figure 1. Foucault shadows and Ronchi-band curvature reveal slope and zonal structure; interferometric fringes support wavefront reconstruction. All three sketches are qualitative interpretation cues—not test results or pass/fail templates. [1] [3]
1

Inspect

We record coating condition, sleeks, scratches, edge damage, strain clues, support points, and the condition of the mirror cell before interpreting an optical test.

2

Establish the test geometry

A radius-of-curvature test, autocollimation setup, interferometric arrangement, or star test is only meaningful when alignment, distance, source size, and support are controlled.

3

Measure the figure

Knife-edge or Ronchi patterns reveal slope and zonal structure; interferometry maps wavefront error. We look for under- or over-correction, zones, turned edge, roughness, and astigmatism.

4

Separate causes

Thermal gradients, pinched clips, poor edge support, tube currents, and miscollimation can imitate a bad mirror. We remove mechanical and environmental causes before condemning glass.

5

Verify in the assembled telescope

Bench results are followed by an in-focus and near-focus star evaluation when conditions allow. The delivered instrument—not an isolated number—is the final system.

What we separate and measure

Reflectivity

How much useful light survives the coating; this does not describe surface shape.

Surface figure

How closely the mirror sends an incoming wavefront toward the intended focus.

Smoothness and zones

Small-scale roughness and annular errors redistribute light away from the central image.

Support and thermal state

A good mirror can test poorly when it is stressed, cooling unevenly, or supported incorrectly.

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Sources

Primary agency, manufacturer technical, and original optical-testing references ground the principles described above. Schematics are explanatory and not customer measurement data.

  1. NASA: quantitative Foucault-test wavefront evaluation
  2. NASA: Webb mirror polishing and cryogenic verification
  3. NASA: interferometric testing of Webb optics