Dear all,

At the SPIE conference in Copenhagen this June, I met Stefan Schwinde from the Fraunhofer Institute for Applied Optics and Precision Engineering. We discussed the issues we face in high-contrast imaging polarimetry and for PCS with the large retardances of K-mirrors. He then offered to see if they could design a coating that minimizes the retardance over the full wavelength range of PCS, from the optical to the near infrared, while maintaining an acceptable total reflectivity. He and Steffen Wilbrandt (from the same institute) sent their first results today. Please have a look at this Powerpoint presentation where I have plotted their results in a format that is useful for us: https://docs.google.com/presentation/d/1f6BC5ddD1JJNpfPooXJgKw23n-d9M0-R/edi...

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They investigated three coatings:

* Protected silver: a single dielectric layer on top of silver * Enhanced silver: a few dielectric layers on top of silver * A multi-layer design where for each of the three mirror a different stack of dielectric layers is placed on top of silver

As expected, the retardance for the protected and enhanced silver solutions is bad, but for the multi-layer design they managed to keep the retardance within 18 deg away from 0 over the full wavelength range. And at the same time, the total reflectivity is not bad, although there are significant drops in reflectivity at optical wavelengths.

The main question is 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. If necessary, perhaps the coatings can be simplified to limit fringing, while allowing a larger variation in retardance (18 deg may be more strict than necessary). In any case, designing a different coating for each of the three mirrors of the K-mirror seems a promising way forward.

Please let me know your thoughts.

Kind regards,

Rob

----------------------------------------------------------------- Rob G. van Holstein + E-mail: rob.vanholstein@eso.org + E S + European Southern Observatory (ESO) O Alonso de Córdova 3107 + Vitacura, Santiago, Chile -----------------------------------------------------------------