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Mission Specifications

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 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.

Telescope Aperture
1m, unobscured
Telescope Design
Three Mirror Anastigmat
Pixel Size
10 microns
Field of View
0.44 deg x 0.56 deg (channel separation by dichroics) = 0.25 sq. deg.
Image Quality
~0.15" FWHM
UV (150–300 nm), u' (300–400 nm), g (400–550 nm)
Sun-synchronous, low Earth orbit (800 km)
Payload Mass
618 kg
Spacecraft Mass
1063 kg
5 year minimum
Observing Modes
wide-field imaging; grism and 2D spectroscopy; precision photometry
Grism Spectroscopy
R=300–420; slit-less spectra over 0.25 sq. deg. in UV and u' channels
DMD Spectroscopy
R~1500; 2D spectra over 207"x117" parallel field in UV channel

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 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 of ~ 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 high-speed optical down-link technologies.

Cutaway view of CASTOR, showing its compact design and folded light path.

CASTOR light path

Artist’s conception of CASTOR’s polar terminator, dawn-dusk orbit. As Earth moves along its orbit (red arc), CASTOR carries out observations in the anti-Sun direction.

CASTOR orbit

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.

CASTOR structure

Optical layout of CASTOR showing its three mirror anastigmat design, fine steering mirror, dichroics, and three distinct focal planes.

CASTOR optical layout

Telescope Design

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.

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.

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