Candidate detector assessment for the CASTOR mission
ABSTRACT
The Cosmological Advanced Survey Telescope for Optical and UV Research (CASTOR) is a proposed Canadian Space Agency (CSA) mission that would provide panoramic, high-resolution imaging of 1/8th of the sky in the UV/optical (150-550 nm) spectral region. This small-satellite class mission would provide high angular resolution ultra-deep imaging in three broad filters to supplement data from planned international dark energy missions (Euclid, WFIRST) as well as from the Large Synoptic Survey Telescope (LSST). One of the leading technical risks on this mission is the UV sensitivity required to approach 26th magnitude in the near UV band. We are planning to characterize a selected candidate technology down to 150 nm. We will review the main scientific and technical drivers for the mission and show how they constrain the available detector options. We will compare the sensitivity and general applicability of CCD, EMCCD, hybridized and monolithic CMOS FPA options.
1. INTRODUCTION
COM DEV is currently working with the CSA on a study to review available Focal Plane Array (FPA) technology and its application to the CASTOR mission. In support of this work the CASTOR science team are reviewing the science case and providing support for the requirements flow-down and detector selection trades. The goal of this project is to provide the Canadian Space Agency with a broad assessment of available UV-sensitive detector technologies for future astronomical space missions. This work is aimed to develop a scientifically relevant and technically achievable FPA specification addressing vacuum ultraviolet (NUV) sensitivity risks identified on the earlier CASTOR concept study. The team are evaluating state-of-the-art technology, selecting a supplier, and plan to leverage their existing James Webb Space Telescope (JWST) cryogenic detector test hardware to cost-effectively characterize the performance of the selected FPA.
This project is intended to provide an experimental demonstration of a high-speed, low-noise silicon array sensor with low power dissipation—critical for the operation of the huge focal plane needed for the CASTOR mission. The detector will be ‘back-illuminated’ so no gate structures will be present on the input surface to absorb signal photons. COM DEV is hoping to work with manufacturers to incorporate optimized surface treatments of the detector for improved quantum efficiency in the NUV spectral region, while investigating the trade space surrounding anti-reflection coatings in this region.
1.1 Mission Overview
The CASTOR mission is a proposed Canadian-led 1-m class, unobscured off-axis free-flying observatory targeted at a polar low earth orbit (LEO)1 . CASTOR would make a unique, powerful, and lasting contribution to astrophysics by providing panoramic, high-resolution imaging of 1/8th of the sky in the UV/optical (0.15-0.55 μm) spectral region over its first 5 years. This versatile small satellite-class mission would far surpass any ground-based optical telescope in terms of angular resolution, and would provide ultra-deep imaging in three broad filters to supplement longer-wavelength data from planned international dark energy missions (Euclid, WFIRST) as well as from the ground-based LSST.
Imaging in the blue and NUV over a fully paved field of ~1 x 0.5 degrees, CASTOR will be diffraction limited at 300 nm at ~0.1” pixel scale (10 μm). This will require an array of 9 x 5 detectors, each with 16 Megapixels, or similar total size in alternative format. The wide field imager will take ~3-10 minute exposures sampling up the slope in 3 colours: u, g, and NUV. A fine pointing mirror will use 10 Hz guiding data from a few bright stars on the main array to stabilize the image to <0.05”, and a series of 4 dithers will be used at half integral offsets to fill in the gaps in the image array. One of the requirements is to do photometry on stars to levels of <1% accuracy. This impacts requirements on gain linearity knowledge and allowable levels of inter-pixel capacitance (IPC). A secondary goal of the mission will be to provide high speed photometry on a set of ~300 bright stars at a refresh rate of > 20 Hz. This corresponds to at least 10 bright stars per array.
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