CASTOR: A Wide-Field, UV/Optical, Imaging Space Telescope!Daryl Haggard (Department of Physics, McGill University, Montreal, QC) on behalf the CASTOR mission team

Summary

The Cosmological Advanced Survey Telescope for Optical and UV Research (CASTOR) is a proposed CSA mission that would provide a unique capability for panoramic, high-resolution imaging at UV/optical (150–550 nm) wavelengths. A consortium of scientist and engineers from industry, universities and government (NRC and CSA) are developing the concept. In addition to providing UV access, CASTOR would surpass any ground-based optical telescope in terms of angular resolution, and provide ultra-deep imaging in three broad pass-bands (UV, u and g) that supplement longer-wavelength data from planned international imaging telescopes (most notably, LSST, Euclid and WFIRST). Combining one of the largest focal plane ever flown in space with an innovative optical design that delivers HST-quality images over a field 80 times larger than Hubble, CASTOR would be able to image, during an 18-month primary survey, an area of 7000 deg2 to a (u-band) point-source depth ~1.3 mag fainter than will be possible with LSST even after a decade of operations. The remainder of its 5-year mission lifetime would be devoted to legacy and Guest Observer programs.

Figure 5. A comparison of CASTOR’s discovery efficiency to that of instruments on the Hubble Space Telescope. Discovery efficiency is defined as the product of total system throughput and telescope field of view. At UV and blue-optical wavelengths, CASTOR would be the world’s pre-eminent imaging telescope in the 2020s.

Figure 4. With its dawn/dusk, polar terminator orbit, CASTOR’s observations would be concentrated in the anti-sun direction. Solar panels would provide spacecraft power and permit slews to high ecliptic latitudes. The continuous viewing zone is located close to the anti-Sun direction for much of the year.

Key Features and Capabilities!

§  A 1m-diameter, un-obscured Three Mirror Anastigmat telescope provides Hubble-like image quality of FWHM ≈ 0.15″ over a panoramic 0.25 deg2 of view.

§  A gigantic 1 Gigapixel camera with wavelength coverage from 150-550 nm, allowing access to wavelengths not visible or easily observable from the ground.

§  High observing efficiency would allow a survey area one sixth of the sky in less than 18 months.

§  A u-band sensitivity of 27.4 AB mag for WFIRST, Euclid and LSST optimized surveys (see Figures 7).

§  Continued and unique access to the critical ultraviolet spectral region during 2020s.

Figure 3. CASTOR uses beam-splitters and a novel three-mirror anastigmat (TMA) design to image a panoramic, 0.25 deg2 field of view, in three pass-bands (UV, u and g) simultaneously. Each CASTOR image covers an area 80 times larger than that available from the Hubble Space Telescope.

Gigapixels per exposure 0.972 Exposures/image 4 Time per exposure 10 min Bits per pixel 16 Exposure time per image 40 min Data generation rate 2.4 MB/sec Operation duty cycle (target) 80% Orbit period 100 min Data per orbit 16 GB Data per day 232 GB

Figure 6. (Left) Depth of upcoming wide-field imaging surveys as a function of wavelength. Results are shown for CASTOR, LSST, Euclid and WFIRST. For CASTOR, we show its 7000 deg2 primary science survey which covers the overlap region between LSST and Euclid. The labels under each filter indicate the image quality (EE50 radius) for each survey. (Right) Comparison of g-band images for a typical low-mass galaxy in the Virgo cluster. From top to bottom, these panels show an actual image from the SDSS, and typical images expected from LSST and CASTOR. Despite its modest 1m aperture, CASTOR would provide deeper and far sharper images than is possible with even the largest ground-based telescopes.

Figure 2. CASTOR’s optical design features an off-axis primary mirror that offers a wide field with excellent quality. A shutter is located at an intermediate image plane, and a three-axis fine-steering mechanism (FSM) sits at a pupil: this is either a plane mirror for imaging or a grating for slit-less spectroscopy.

Figure 1. Visualization of CASTOR in low-Earth orbit.

Table 2. Data Volume Table 1. Detector Performance CMOS sensors SRI Mk x Nk (12 in total) Photometric accuracy <1% Random noise <2.5 e/600 sec UV-, u- and g-band sensitivity (AB mag @ 1 e/s)

24.23, 24.71, 24.78

10 micron pixel pitch 0.1” (dithered to 0.05”) Dark current <0.01 e/p/s (end of life) Low power dissipation <50 mW on focal plane Sub-windowed fine guiding 7.5 Hz

Figure 7. CASTOR would enable transformational research across a wide range of fields. Eight Science Working Groups have developed a comprehensive observing plan for the mission. Here, we show how CASTOR’s panoramic field of view, excellent images quality and high sensitivity would enable searches for ultra-faint Kuiper Belt Objects moving in the outer fringes of the solar system.

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