October 2026 - Pcspolarimetry - mailing lists STRW

Low-retardance coating for PCS K-mirror
by Rob van Holstein 21 Oct '26

21 Oct '26

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/ed… (attaching the file to the e-mail required approval of the mailing list owner) 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(a)eso.org + E S + European Southern Observatory (ESO) O Alonso de Córdova 3107 + Vitacura, Santiago, Chile -----------------------------------------------------------------

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Meeting 19th | Summary Email with Action Items
by dsmith 19 Oct '26

19 Oct '26

Dear PCS Polarimetry Team, Thank you to those who attended the meeting this week. Below is a summary of the key action items and open questions we identified: AC1 (Daniel): Document what the current science models/requirements say regarding Polarimetry. AC2 (Daniel): Request and obtain the official optical drawing of the PCS from WP6. AC3 (Daniel): Get in touch with WP6 to discuss options for scaling: - Option A: Reduce the beam size for the K-mirror to fit the Half-Wave Plate (HWP) around it. - Option B: Conduct a coating study. AC4 (Daniel): Generate polarimetric analysis plots: Zenith angles from 0 to 90 at a single wavelength (to be repeated across multiple wavelengths) and do this while replicating the 3 plots shown in the meeting (cumulative throughput, cumulative IP, and cumulative linear -> circular): Show the optical effects due to the K-mirror (specifically retardance and diattenuation as a function of wavelength). AC5 (Team): Please put in your availabilities for the next meeting in November here: https://www.when2meet.com/?39353244-iNAna Open Questions / Key Points for Follow-Up: How accurate is the modelled retardance? Evaluating performance with an unpolarised source -> diattenuation -> overall accuracy of the model coatings. Please let me know if I missed any key points or if any adjustments are needed. Kind regards, Daniel Smith

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[PCSPolarimetry] Meeting Reminder: 2pm Monday OCT 19 | CEST
by dsmith 17 Oct '26

17 Oct '26

Dear PCSpolarimetry Team, Just a reminder for the upcoming meeting on Monday the 19th of October at 2pm. Please find below the details for the meeting: Here is the meeting link: https://universiteitleiden.zoom.us/j/7426684228?pwd=1aFgtwiWZB598vTHa1MlbQr… Meeting ID: 742 668 4228 Passcode: .jKdVx4= We will discuss ELT+PCS simulations, coatings, the design from WP6, and where to place HWP/QWP. Kind regards, Daniel Smith

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