Wavefront error measurements and alignment of CLASP2 telescope with a dual-band pass cold mirror coated primary mirror

Yoshida, Masaki; Song, Donguk; Ishikawa, Ryoko; Kano, Ryouhei; Katsukawa, Yukio; Suematsu, Yoshinori; Narukage, Noriyuki; Kubo, Masahito; Shinoda, Kazuya; Okamoto, Takenori J. et al.
Bibliographical reference

Proceedings of the SPIE, Volume 10699, id. 1069930 13 pp. (2018).

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"Chromospheric LAyer Spectro-Polarimeter (CLASP2)" is the next sounding rocket experiment of the "Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP)" that succeeded in observing for the first time the linear polarization spectra in the hydrogen Lyman-α line (121.6 nm) and is scheduled to be launched in 2019. In CLASP2, we will carry out full Stokes-vector spectropolarimetric observations in the Mg ii h and k lines near 280 nm with the spectro-polarimeter (SP), while imaging observations in the Lyman-α line will be conducted with the slitjaw optics (SJ). For the wavelength selection of CLASP2, the primary mirror of the telescope uses a new dual-band pass cold mirror coating targeting both at 121.6 nm and 280 nm. Therefore, we have to perform again the alignment of the telescope after the installation of the recoated primary mirror. Before unmounting the primary mirror from the telescope structure, we measured the wave-front error (WFE) of the telescope. The measured WFE map was consistent with what we had before the CLASP flight, clearly indicating that the telescope alignment has been maintained even after the flight. After the re-coated primary mirror was installed the WFE was measured, and coma aberration was found to be larger. Finally, the secondary mirror shim adjustments were carried out based on the WFE measurements. In CLASP2 telescope, we improved a fitting method of WFE map (applying 8th terms circular Zernike polynomial fitting instead of 37th terms circular Zernike fitting) and the improved method enables to achieve better performance than CLASP telescope. Indeed, WFE map obtained after the final shim adjustment indicated that the required specification (< 5.5 μm RMS spot radius) that is more stringent than CLASP telescope was met.
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