Optical design of SPIDERS, a Subaru Pathfinder Instrument for Detecting Exoplanets & Retrieving Spectra

NRC’s NEW-EARTH Lab has demonstrated in the laboratory a Self-Coherent Camera (SCC) concept combined with a Tilt-Gaussian-Vortex focal plane mask (FPM). This speckle suppression technique, a.k.a. Fast Atmospheric SCC Technique (FAST), can enhance the contrast up to 100 times. Based on this success, NRC is now building SPIDERS, a visitor instrument for Subaru telescope to be installed on the infrared Nasmyth platform behind AO188 and the new Subaru Beam Switcher. The beam can be either shared between SPIDERS and SCExAO for simultaneous observations or sent entirely to only one instrument. SPIDERS should also benefit from the upcoming AO188 deformable mirror (DM) upgrade (64x64 actuators) turning A188 to AO3k. The key-components of SPIDERS are an ALPAO DM468, used as a second-stage AO correction, a pupil apodizer mask, a Tilt-Gaussian FPM, a Lyot stop, a beam-splitter feeding (i), a C-RED2 camera imaging a 5” FoV in narrow bands and (ii), an imaging Fourier-Transform Spectrograph and a SAPHIRA camera for spectroscopy up to R~20,000 over a 3.3” FoV. SPIDERS optical design is fully reflective up to the FPM to avoid chromatic aberrations and reduce the number of surfaces. Two off-axis ellipsoid mirrors are enough to form the pupil planes required on the DM and the apodizer mask, and the f/64 focus on the FPM. Only lenses are used from the FPM up to the C-RED2 camera to mitigate the sensitivity of the SCC to vibrations. The Lyot stop reflects the blocked light to a camera acting as a Low-Order Wavefront Sensor complementing the SCC focal plane wavefront sensing.