8 Tolerancing - Tolerancing Optical Systems Why are...

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Introductory Optomechanics J. H. Burge 1 Tolerancing Optical Systems • Why are tolerances important? – Somebody is going to make it (hopefully) – It must meet some performance requirement – Cost (and schedule) are always important • Why is it difficult? – Involves complex relationships across disciplines • System engineering • Optical design and analysis • Optical fabrication • Opto-mechanical design • Mechanical fabrication • If you can tolerance effectively, then you can be a good designer, otherwise you are not.
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Introductory Optomechanics J. H. Burge 2 Process of optical system tolerancing 1. Define quantitative figures of merit for requirements 2. Estimate component tolerances 3. Define assembly/alignment procedure and estimate tolerances 4. Calculate sensitivities 5. Estimate performance 6. Adjust tolerances, balance cost and schedule with performance 7. Iterate with system engineer, fabricators, management
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Introductory Optomechanics J. H. Burge 3 System Figure of Merit • Keep this as simple as possible • Must propagate all performance specs through assembly • Typical requirements RMSWE (root mean square wavefront error) MTF at particular spatial frequencies Distortion Fractional encircled energy Beam divergence Geometric RMS image size Dimensional limits Boresight
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Introductory Optomechanics J. H. Burge 4 Parameters to tolerance Allowable errors are called tolerances. What needs to be toleranced? • General parts (usually machined metal) • Physical dimensions of optical elements • Optical surfaces • Material imperfections for optics • Optical assembly
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Introductory Optomechanics J. H. Burge 5 Estimate system performance For a merit function that uses RSS to combine independent contributions: Φ 0 is from design residual – simulation of system with no manufacturing errors ΔΦ i is effect from a single parameter having an error equal to its tolerance () ( ) ... 2 2 2 1 2 0 + ΔΦ + ΔΦ + Φ = Φ
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Introductory Optomechanics J. H. Burge 6 Calculate sensitivities Define merit function Φ Make list of parameters to tolerance, x 1 , x 2 , x 3 , … all of the things that will go wrong. Use simulation to calculate the effect of each of these on the system performance. –For each x i , use perturbation to find sensitivity So the contribution from a tolerance Δ x i on parameter x i is ii x x ∂Φ ΔΦ ∂Δ i x x ∂Φ ΔΦ = Δ = (change in merit function) / (change in parameter) = (sensitivity) * (tolerance)
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Introductory Optomechanics J. H. Burge 7 If the nominal merit function Φ 0 is small (residual aberrations) calculate sensitivity directly Δ x i is perturbation (by the expected tolerance) Φ ( x i + Δ x i ) is the system merit function of the perturbed system To include the nominal merit function Φ 0 This can be tricky Evaluate Φ 0 and Φ ( x i + Δ x i ) – If Φ 0 << Φ ( x i + Δ x i ) then ΔΦ = Φ ( x i + Δ x i ) as above – If Φ 0 is correlated with Φ ( x i + Δ x i ) then ΔΦ = Φ ( x i + Δ x i ) - Φ 0 –E lse Φ 0 and ΔΦ combine in RSS, so Sensitivity calculation () 2
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8 Tolerancing - Tolerancing Optical Systems Why are...

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