sci21075 — Announcement

SOAR Calibration Wavefront Sensor Upgrade

July 30, 2021

The recent upgrade to the SOAR telescope Calibration Wavefront Sensor system has extended the lifetime of this critical component. New CCD cameras, optics, and an updated version of the software control system were installed.

A recent successful upgrade to the SOAR telescope Calibration Wavefront Sensor (CWFS) system has extended the lifetime of this critical telescope component. The upgrade, carried out 21-25 June 2021 by a team of NOIRLab scientists, engineers, and technical staff, involved the installation of new CCD cameras, optics, and an updated version of the software control system.

Like most modern major optical telescopes, SOAR has an actively controlled primary mirror support system. As the aperture size of telescopes have increased over time, primary mirror designs have turned to new materials and thin substrates to reduce weight and hasten thermal equalization. In the case of SOAR, its 4.1 m diameter primary mirror is only 10 cm thick. Such large and thin substrates cannot maintain their optical shape to within required tolerances for producing near-diffraction-limited images with only passive mechanical elements. Reaching the maximum optical performance requires implementing Active Optics Systems (AOS).

The SOAR telescope was designed with an AOS that uses a matrix of actuators that control the optical figure of the primary mirror, a secondary mirror that corrects for defocus and aberrations like astigmatism and coma, and a tertiary tip-tilt mirror. In order to evaluate the level of correction of the optical path and configure the AOS elements to deliver optimal image quality, SOAR, like many modern telescopes, relies on a wavefront sensing analyzer. We use a Shack-Hartmann device, which measures local tilts of the wavefront of a star. The telescope pupil is imaged on an array of small lenslets, each of which produces a spot on the wavefront detector. The SOAR CWFS uses two detectors, one for acquisition of the star and the other for wavefront analysis (Figure 2). The CWFS is the heart of SOAR’s optics, and without it the telescope cannot reach its optimal performance.

In 2012 the CWFS acquisition camera failed and was replaced with an available device that happened to be at hand. New cameras were acquired as spares for both the WFS and acquisition sides, in case of further failures. However, because all the components were unique, in particular the microlens array producing the Shack-Hartmann pattern, and since the CWFS had been delivered as a turnkey system, making additional replacements or upgrades at that moment would have resulted in taking the telescope offline for a significant period. Therefore, a decision was made to not intervene in the system at that time, although an upgrade was clearly necessary. Not until 2017 did we find a vendor that could provide the critical custom microlens (the original company was no longer fabricating these specialized optical devices), and at that point the initial plan developed into the full upgrade project.

The recent upgrade not only replaces both cameras with more modern versions and updates the control software, but more importantly, by securing a new, custom-made microlens unit, it provides spare parts for a core telescope component, in practice eliminating a single-point failure mode that has been a threat to the continued operation of the facility for many years. Finally, the upgrade represents an important step forward in efforts at NOIRLab to extend the lifetime of our facilities.

Figure 1. Left: SOAR Calibration Wavefront Sensor assembly. The newly installed cameras (light blue) are connected to glycol cooling lines. The main optical path comes in from the top. Right: SOAR staff, high in the boom lift, working on the open CWFS box located in one of the folded Cassegrain side ports of the telescope.

Figure 2. Layout of the CWFS mechanical model. The back-end of the acquisition and calibration paths were modified to accommodate the new cameras and optics.