GREGOR Optics Redesign 2020

New layout of the optics lab.
Close-up view of the center of the optics lab

General Info

The GREGOR telescope was inaugurated in 2012. In 2018, we started a complete upgrade, involving optics, alignment, instrumentation, management and policies, mechanical upgrades for vibration reduction, updated control systems, and building enhancements. Compared to 2012, all powered mirrors except for M1 have been exchanged by 2020. Starting from 2020, GREGOR will observe with diffraction-limited performance and a new optics and instrument layout.

 

Redesign

The original optics lab layout was devised during GREGOR's design phase before 2008. It focused on the first light instruments GRIS, GFPI plus an associated imager, and BBI as a standalone imager. The AO was mounted on a vertical bench, which saves space, but is 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). During the past two years, we realized that there were issues with beam stability, alignment, and the optical quality induced by the two biconic mirrors M12 and M15. The most noticeable issues were field-dependent aberrations (mostly astigmatism and coma), which depended on the beam angles at M12 and M15 and were prominent at the design angles. Unfortunately, M12 and M15 could not be aligned arbitrarily to minimize the aberrations, which however never fully vanished, because of a fixed focal plane and vignetting at other optical elements, plus their alignment tolerances were too strict. Additionally, there was a lack of space in the optics lab 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:

  • More space for the science instruments and a future MCAO via a different beam distribution
  • Horizontal setup of the AO relay optics (for vibration reduction, stability, and easier alignment)
  • Wavefront sensor is located after M15, thus correcting the aberrations of all powered mirrors
  • Pupil size 49 mm on the DM (given by the DM size)
  • 1:1 imaging of the AO relay optics between F3 and F4
  • perfect image quality in F4 over a radius of 60" (=2 arcmin image diameter)
  • A VIS/IR beamsplitter between M15 and M16 reflects the VIS, only IR passes through towards GRIS. This improves the AR coatings both for the VIS and the IR elements.
  • Small angles at mirrors for polarimetry
  • F4 (GRIS) located at the same position as now (1420 mm from M11).
  • M16-F4 (GRIS) shall be on the same line as M11-F4 (GRIS) to keep the beam angles.
  • Exit pupil telecentric (at infinity)

 

The images on the right show the new layout. GRIS and the slitjaw system remain. A new FPI will be in the straight beam and has minimal reflections. Parallel to it is its broadband channel with phase diversity and a fast imager in the blue. An insertable mirror, which can be rotated sends the light either to a visitor table, or to GFPI/HIFI. One table is saved for a future upstairs spectrograph. The new design has many advantages: By having a constant beam height above the optical tables, all alignment is simplified. The off-axis parabolas have much better alignment tolerances than the current biconic mirrors. The PSF remains near diffraction-limited in all field points, even for small deviations of properties (e.g. conic constant difference of 2%, decenter of 5 mm, 1% deviation in paraxial radius). The MTF remains nearly constant across the whole FOV, compared to a 50% drop in the original setup. There is space for a future MCAO and a future upstairs spectrograph, while all current instrument capabilities remain. Additionally, all optical elements are accessible. The wavefront sensor can be located after M15, thus able to correct for any static aberrations induced by the relay optics. The wavelength splitting is performed using a dichroic beamsplitter plate and no longer a pentaprism, which is an advantage for polarimetry (smaller angles). By using different beamsplitter plates, it will be possible to change the wavelength cutoff for the visible beam, either to 650 nm or 900 nm.

 

The new M12 and M15 were installed on Mar 13, 2020. The optics alignment took place in the subsequent week. The image quality has improved significantly, which can be seen in the relevant technical note. We expect to start observations in June/July.