The planetary systems instrument: Overview of the instrument architecture and opto-mechanical design of the red arm, AKA scales

The Planetary Systems Instrument (PSI) for the Thirty Meter Telescope (TMT) is comprised of 3 key modules: 1) an Adaptive Optics (AO) bench consisting of a large stroke, high actuator count ‘woofer’ Deformable Mirror (DM) and Wavefront Sensor (WFS) that is common to both the Red and Blue arms; 2) PSI-Red, covering 2-5 microns with a sophisticated suite of modular components based around a lenslet IFS coronagraph; and 3) PSI-Blue, covering 0.6 to 1.8 microns with its own suite of modular components including a ‘tweeter’ DM with its own WFS. An additional module is PSI-10, an 8-13 micron suite of modular components based around a lenslet IFS and is under consideration. The precursor to PSI-Red is currently being developed by UC Santa Cruz as SCALES (Santa Cruz Array of Lenslets for Exoplanet Spectroscopy) and will operate in the mid-IR from 2 to 5 microns at the WM Keck Observatory before being integrated with the remainder of PSI at TMT. Its fully cryogenic optical train uses a custom silicon lenslet array, selectable coronographs, and dispersive prisms to carry out integral field spectroscopy over a 3.6 arcsec field of view with low spectral resolution (50 to 200). A set of insertable mirrors relay light to and from a slicer module sitting behind the lenslet array allows for medium spectral resolution (5000 to 10,000), which has not been available at the diffraction limit with a coronagraphic instrument in the mid-IR before. The opto-mechanical design takes advantage of modern diamond-turning materials and machining techniques with minimal risk and cost while delivering diffraction-limited performance both at Keck and TMT. Unlike previous IFS-based exoplanet instruments, SCALES is capable of characterizing cold exoplanet and brown dwarf atmospheres (<600 K) at bandpasses where these bodies emit most of their radiation while capturing interesting molecular spectral features. We will discuss some of the technical challenges of designing a TMT-ready instrument that will first be deployed at Keck.