A&A 641, A27 (2020) Astronomy https://doi.org/10.1051/0004-6361/202038208 & c ESO 2020 Astrophysics GREGOR: Optics redesign and updates from 2018–2020 Lucia Kleint, Thomas Berkefeld, Miguel Esteves, Thomas Sonner, Reiner Volkmer, Karin Gerber, Felix Krämer, Olivier Grassin, and Svetlana Berdyugina Leibniz-Institut für Sonnenphysik (KIS), Schöneckstrasse 6, 79104 Freiburg, Germany e-mail: [email protected] Received 20 April 2020 / Accepted 13 June 2020 ABSTRACT The GREGOR telescope was inaugurated in 2012. In 2018, we began a complete upgrade, involving optics, alignment, instrumen- tation, mechanical upgrades for vibration reduction, updated control systems, and building enhancements, and in addition, adapted management and policies. This paper describes all major updates performed during this time. Since 2012, all powered mirrors except for M1 were exchanged. Since March 2020, GREGOR observes with diffraction-limited performance and a new optics and instrument layout. Key words. telescopes – Sun: general 1. Introduction GREGOR was that its image quality did not reach the theoretical limit, partly because a risk was taken with untested technologies, Solar telescopes have always striven to evolve in diameter such as silicon carbide mirrors, which could not be polished well and thus spatial resolution (e.g., review by Kleint & Gandorfer enough, and partly because of design problems. These difficul- 2017). Before 2000, their diameters remained smaller than 1 m, ties have recently been solved by replacing all silicon carbide with the exception of the Mc Math Pierce telescope at Kitt mirrors with mirrors made of Zerodur, which can be polished Peak, which mostly observed in the infrared, however (Penn to the required quality, and by redesigning the AO relay optics. 2014). A new generation of telescopes started in the twenty- GREGOR now operates at its diffraction limit. The goal of this first century with the Swedish Solar Telescope (SST) in 2002. paper is to summarize the recent upgrades and enhancements The SST has a clear aperture of 0.98 m and was optimized for a that were carried out from 2018 to 2020. We only briefly sum- very high image quality. It routinely delivers impressive high- marize the general properties of GREGOR and we refer to the resolution solar images, especially also at wavelengths in the article series from 2012 for more details. blue (Scharmer et al. 2003, 2013). It was followed in 2009 by GREGOR obtained its name by being a Gregory system the Goode Solar Telescope (GST, Goode et al. 2010), which has with three imaging mirrors (M1, M2, and M3) whose proper- a 1.6 m clear aperture. The GST, for example, has obtained flare ties are summarized in Table1. More than 99% of the sunlight is images with the highest resolution to date (Jing et al. 2016). reflected away at the cooled primary field stop F1. Only a beam GREGOR is Europe’s largest solar telescope. It became with circular diameter of 15000 passes through its central hole operational a few years after the GST. Its 1.5 m diameter with to M2. The F1 field stop is recoated yearly, currently with an an optical footprint of 1.44 m allows us to resolve structures on aluminum layer on top of a nickel layer. The mirrors M4–M11 the Sun as small as 50 km at 400 nm. The GREGOR project are flat mirrors with the purpose of directing the beam into the started with its proposal in 2000 (von der Lühe et al. 2001) and optics laboratory one story below the telescope level. M8-M10 carried out a science-verification phase from 2012 to 2013. The can be rotated about the optical axis, thus acting as a derotator, state of GREGOR at that time was published in a series of arti- which compensates for the solar image rotation induced by the cles in Astronomische Nachrichten Vol. 333, No. 9, in particu- alt-az mount of the telescope. A schematic drawing of GREGOR lar, the GREGOR overview by Schmidt et al.(2012). GREGOR is shown in Fig.1. was designed to explore solar features at smaller scales than other telescopes at that time. Its theoretical spatial resolution sur- passes that of the SST and is similar to that of the GST, and all 2. Optics three telescopes have significantly improved their image qual- 2.1. Redesign of the optics laboratory ity with advanced adaptive optics (AO) systems (Schmidt et al. 2016; Berkefeld et al. 2018; Scharmer et al. 2019). Their designs The original optics laboratory layout was devised during the differ, however; GREGOR focuses on high-precision polarime- GREGOR design phase before 2008. It focused on the first- try, which has enabled investigations of polarization signals as light instruments GRIS, a spectropolarimeter based on a grat- −4 small as 10 Ic to detect spatial variations of turbulent magnetic ing spectrograph (Collados et al. 2012), GFPI, a dual-etalon fields (Bianda et al. 2018; Dhara et al. 2019). Another advantage spectroscopic imager (Puschmann et al. 2012), plus an asso- of GREGOR is its potential for polarimetric night observations, ciated broadband imager, and BBI as a stand-alone imager which has been used to study the polarization of planets and (von der Lühe et al. 2012). Guest instruments, such as ZIMPOL thus their atmospheres (Gisler et al. 2016). A past drawback of for high-precision spectropolarimetry (Gandorfer & Povel 1997) Article published by EDP Sciences A27, page 1 of 10 A&A 641, A27 (2020) Table 1. Mirror properties for M1–M3. M1 M2 M3 Focal length [mm] 2506.35 519.40 1398.5 Optical footprint for 15000[cm] 144 38.1 27.5 f# 1.7 1.4 5.1 Curvature radius [mm] 5012.70 1039.79 2797.00 Conic constant −1 (parabolic) −0.306 (elliptic) −0.538 (elliptic) or the GREGOR planet polarimeter (GPP, Gisler et al. 2016), were also regularly operated. The AO was mounted on a ver- tical bench, which saves space, but may be disadvantageous in terms of vibrations and alignment. The beam is collimated for the AO after F3 via M12 and then reimaged via M15. The AO itself consists of a tip-tilt (M13) and a deformable mirror (DM, M14). The setup is shown in the top panel of Fig.2. During the past two years, we realized that there were diffi- culties with beam stability, alignment, and the optical quality induced by the two biconic mirrors M12 and M15. These mir- rors originally featured biconic shapes to minimize the astigma- tism that arises due to an oblique incidence on a focusing mirror. However, we realized that they created field-dependent aberra- tions (mostly astigmatism and coma) that are prominent at the design angles. Unfortunately, M12 and M15 could not be aligned arbitrarily to minimize the aberrations, which never fully van- ished, however, because of a fixed focal plane and vignetting at other optical elements. Their alignment tolerances were also too strict. Additionally, there was a lack of space in the optics labo- ratory to develop and install new instrumentation. Therefore we completely redesigned the optics after M11, including new off- axis parabolic mirrors to replace M12 and M15 and an improved instrument layout. The new layout was devised based on the following criteria: – There shall be more space for the science instruments and a future multi-conjugate adaptive optics (MCAO) via a different beam distribution while keeping all current capabilities. – The setup of the AO relay optics shall be horizontal (for Fig. 1. GREGOR optical layout until F3. vibration reduction, stability, and easier alignment). – The (total) angle at M12/M15 shall be =8◦ (the two mirrors compensate for each other). Angles <8◦ are not possible because The original and the new design are shown in Fig.2. The of the BS plate between M15 and M16. new off-axis parabolic (OAP) mirrors M12 and M15 are identical – There shall be 1:1 imaging of the AO relay optics between and compensate for each other’s aberrations. Their paraxial focal F3 and F4. The exit pupil shall be telecentric (at infinity). length is 1978 mm and the off-axis distance is 277 mm. They – The image quality in F4 shall be perfect over a radius of are made from Zerodur with a diameter of 125 mm, thickness 6000(=2 arcmin image diameter). of 20 mm, with a silver coating. Their measured surface error – F3 shall be located at least 1000 mm after M11. RMS is 6.9 and 5.2 nm for the two mirrors, and this quality was – The pupil size is limited by the DM size. achieved with two ion beam figuring (IBF) polishing runs per – A visible/infrared (VIS/IR) BS between M15 and M16 mirror. reflects the VIS, only IR passes through toward GRIS. This The original and new MTFs and spot diagrams are shown in improves the antireflection coatings for the VIS and the IR Fig.3. The MTF for di fferent field angles improves with increas- elements. This BS is mounted on a rotating table and can be ing wavelength, but the spatial resolution obviously decreases at exchanged for instance by a 50/50 BS for special setups that higher wavelength. We therefore show the MTF at 500 nm to require other wavelength distributions. illustrate the problems with loss of contrast of the original setup – The angles at mirrors shall be small for polarimetry. and the much better performance of OAPs. All plots assume a – DM-M15 shall be parallel to M11-M12 (this is not strictly maximum field of view (FOV) of 12000. required, but it simplifies the alignment). Figure4 shows the new instrument layout and a photo of the – F4 IR shall remain unchanged (1420 mm from M11).