JOST

Full testbed name

James Webb Space Telescope Optical Simulation Testbed (JOST)

Managing institution

Space Telescope Science Institute (STScI)

Main scientific focus

The JWST Optical Simulation Testbed (JOST) is a tabletop experiment to simulate the main aspects of wavefront sensing and control (WFS&C) for a segmented space telescope, including both commissioning and maintenance. JOST has an optical design using three aspheric lenses that reproduces the physics of JWST's three-mirror anastigmat. A segmented deformable mirror stands in for the segmented primary. The optical system provides equivalent sampling and image quality as a JWST NIRCam module, but at HeNe wavelength. With 59 degrees of freedom, we can implement many commissioning activities such as phase retrieval algorithm validation studies with a hexagonally segmented DM, test validation of pupil imaging lens concepts and designs, investigate field-dependence (multiple field point sensing and control), test wavefront control software, and train and develop staff expertise.

Environment of the testbed

JOST is located on a floating table in a class 10000 clean room, in a dark optical enclosure.

Key hardware items

• Fiber source as star simulator, monochromatic laser at 638 nm.

• Steering mirror (tip/tilt) in order to explore a wide field of view.

• JWST-like pupil mask including the central obscuration and three struts.

• Three custom lenses in form of a Cooke triplet, i. e. a refracting anastigmat.

• IrisAO segmented mirror with hexagonal elements each 1.4 mm in diameter (total mirror: 7 mm), in a JWST-like configuration.

• Pupil imaging lens in order to obtain pupil images of the setup.

• ZWO ASI1600MM monochrome CMOS camera on a translation rail to allow for focus diversity in the images.

Current status

All hardware is integrated and the overall optical alignment, including the three lenses and the IrisAO segmented mirror, have an rms wavefront error of under 40 nm rms and can be corrected down to 15 nm rms when the LAPD (Linearized Analytic Phase Diversity) algorithm is working in closed loop. Current goals are to test new phase retrieval algorithms for segmented apertures (Hybrid Diversity Algorithm - HDA, Geometric pHase Retrieval - GPR, and others) and upgrade the software to be compatible with the HiCAT code package.