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Is it possible to design a low-retardance coating for the K-mirror of ELT/PCS covering the full wavelength range?

Rob van Holstein

2026-10-21

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Total K-mirror retardance

Coating designs and simulations by Steffen Wilbrandt and Stefan Schwinde

from Fraunhofer Institute for Applied Optics and Precision Engineering

(abridged) “The multi-layer coating has individually customized designs for the three mirrors, consisting of silver with multiple dielectric coatings containing silica, titania and alumina.

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A purely dielectric solution does not seem promising (>250 layer with total physical thickness >40 µm).”

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Total K-mirror reflectivity

Coating designs and simulations by Steffen Wilbrandt and Stefan Schwinde

from Fraunhofer Institute for Applied Optics and Precision Engineering

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Total K-mirror diattenuation

Coating designs and simulations by Steffen Wilbrandt and Stefan Schwinde

from Fraunhofer Institute for Applied Optics and Precision Engineering

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Conclusions and open questions

  • A low-retardance coating for the K-mirror of PCS covering the full wavelength range is possible, at the cost of a few percent-point decrease in total reflectively

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  • The biggest question is probably whether this multi-layer design causes significant fringing or whether it can be kept at an acceptable level for PCS with regard to the AO, coronagraphy, and contrast

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  • The reflectivity can probably be enhanced and the fringing reduced by allowing the retardance to vary more strongly away from zero.

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