The CASTOR mission concept has been developed to provide Canadian and international astronomers with state-of-the-art high-resolution and wide-field UV/blue-optical imaging capabilities in the 2020s. Key scientific and technical specifications are summarized in the table below. Some of these parameters may evolve as studies proceed.
CASTOR's wide instantaneous field of view, measuring roughly ~0.25 square degrees, is made possible by its three mirror anastigmat (TMA) design, which delivers a wide field while minimizing spherical aberration, coma and astigmatism. The 1m-diameter mirror primary mirror provides high sensitivity, and the entire optical system is optimized to deliver nearly diffraction limited performance over the blue optical and ultraviolet wavelength range. Dichroics and filters provide simultaneous imaging in three distinct channels spanning the range 150 to 550 nm. Large arrays of CMOS detectors sample each focal plane in 0.1" pixels, with PSF sub-sampling provided by small dithers within survey fields. CASTOR is base-lined to a five-year mission lifetime, divided into legacy survey and Guest Observer components.
In addition to wide-field imaging, CASTOR would be equipped with customized detectors in each channel for high-precision photometric monitoring of bright targets (such as exoplanet hosts) as well as two distinct spectroscopic modes: (1) a low-spectral-resolution grism mode that delivers slit-less spectroscopy over the full imaging field in the UV and u' bands; and (2) configurable DMD mode that provides 2D spectroscopy at intermediate spectral resolution in the UV channel and over a parallel, Hubble-like field of view.
The CASTOR telescope consists of an optical assembly, thermal control system, electronics unit and software. The design is dictated by the optical prescription with the large input aperture and field of view driving the size of the overall optical system. The optical path runs through the three mirror anastigmat (TMA) with its large primary mirror, reflecting off the fine steering mirror (FSM), and passing through a pair of dichroics that feed three distinct focal planes that share a common field of view. Band definition is achieved by a system of dichroics, interference filters, reflective coatings and broadband coatings on the TMA mirrors that also reduce red signal.
Telescope structure showing its two main elements: a baseplate that supports the M1 and M3 mirrors, and a tower that supports the M2 mirror and FSM asssembly.
Optical layout of CASTOR showing its three mirror anastigmat design, fine steering mirror, dichroics, and three distinct focal planes.
The telescope design relies on three powered surfaces with strict constraints on surface form, roughness and stability. The mirrors utilize an open back design with thin ribs and a reduced face skin thickness to give a weight-relieved and optimized approach to reducing overall mass and transmitted loads. The telescope structure relies on an athermal design that provides extreme stiffness while maintaining tolerance against temperature shifts.
Payload, Launch and Orbit
The total volume and mass of the payload are 10 cubic metres and 618 kg, while the mass of the spacecraft itself is 1063 kg. The spacecraft is designed to to be compatible with a variety of launch vehicles (Polar Satellite Launch Vehicle, Falcon 9, etc) and would operate in a sun-synchronous, polar terminator orbit at an altitude ~ 800 km. This is 50% higher than the orbit of the Hubble Space Telescope and slightly higher than that of the GALEX satellite. Such low-earth orbits have the advantage of high observing efficiency with modest communications infrastructure costs. A sun-synchronous low-Earth orbit also provides a stable thermal environment, protection of detectors from solar exposure, and generally stable lighting conditions for astronomy. Given its orbit, CASTOR scientific surveys will be concentrated in the primarily anti-sun direction, with excursions out of the ecliptic plane made possible by solar panels that deploy to provide spacecraft power over a range of ecliptic latitudes. A network of ground stations will provide at least one down-link opportunity per orbit using either high-speed optical down-link technologies.