A CubeSat Mission for Exoplanet Transit Detection and Astroseismology Jeremy Bailey (UNSW, Physics) Steve Tsitas (UNSW, ACSER) Daniel Bayliss (RSAA, ANU) Tim Bedding (Univ. Sydney)
ESO Very Large Telescope (4 x 8m) Overwhelmingly Large Telescope (OWL 100m) Giant Magellan Telescope (22m)
Ground-based transit telescopes OGLE (Las Campanas) HAT-South KELT North (Kilodegree Extremely Little Telescope)
Larger telescope sees fainter stars Same Detector Size Smaller telescope has wider field The small telescope can see about the same number of stars as the larger telescope but on average these are brighter stars.
HD 209458 Discovery observations from ground HD 209458 Hubble Space Telescope Observations
Kepler
Estimated magnitude distribution for a 6U CubeSat (60 mm aperture, 676 deg 2 ) Kepler planet candidates magnitude distribution (1m aperture, 115 deg 2 )
Asteroseismology From analysis of the frequencies of the oscillation mode many basic parameters of the star can be derived.
6U CubeSat concept for Exoplanet Detection and Asteroseismology
Proposed Orbit: Dawn-dusk Sun Synchronous (6am ascending node) 98.19 o inclination at 700km Period 99 minutes Sun > 90 o
Imaging System Lens of 60 100 mm focal length f/1.5 or faster feeding a 2K x 2K or 4K x 4K back-illuminated CCD Len could be a commercial camera lens e.g. Nikon 85mm f1.4g e2v CCD 203-82 4K x 4K 12 mm e2v CCD 42-40 2K x 2K 13.5 mm
Low Power CCD System Payload Computer I 2 C FPGA Horizontal clock drivers SPI Vertical clock drivers CCD E2V 42-40 To S-band transmitter 16-bit ADC Video Amp & Processing Clock Generation FPGA Horizontal Clocks (fcv 2 ) * Vertical Clocks (fcv 2 ) On-chip readout amp (30V 6mA) Video processing ADC (AD7622) Power 300 mw 90 mw 5 mw 180 mw 60 mw 100 mw Payload Computer 1500 mw Total 2.3 W *For 1 MHz readout (4 sec full frame).
Lessons from Kepler A shutter is not necessary Better focus gives better photometry Good photometry can be done from saturated images No advantage in defocusing
Data Processing We will not be able to downlink full images On-board processing Exposure times ~30 seconds Data will be accumulated for ~10 min (sufficient for most transit work). Pixels in a software aperture around each star will be downlinked. Selected bright stars can be returned at higher cadence (needed for asteroseismology).
Kepler Software Apertures
CCD Cooling The CCD will need to be cooled to reduce its dark current. Based on the data sheet figures T ~ 30 C should be adequate. This should be achievable with a passive cooling radiator BUT the CCD will degrade in orbit due to radiation and dark current will increase. Additional cooling could help.
Attitude Control System Require pointing to a fraction of a CCD pixel <5 arc sec in pointing axis position <15 arc sec in rotation around pointing axis. Reaction Wheels Sinclair Interplanetary RW-0.03-4 Comtech AeroAstro Miniature Star Tracker Only gives pointing to ~ 1 arc minute
Attitude Control System Fine Guiding Telescope Sense pointing axis position to < 1 arc sec 500 mm focal length telescope (60-80 mm aperture) Standard ½ 1280 x 1024 CMOS sensor (5.2 mm (~ 2 arc sec) pixels) Frame rate ~27 fps (faster with windowing). Use bright star at the centre of each field.
Attitude Control System The third axis (roll around the pointing direction) can be sensed from the science CCD. But only at a slow rate ~30 sec. Combine with a fibre-optic gyro to maintain position between exposures. KVH DSP-1750 Angular random walk < 0.013 o / hr In 30 sec rms error < 4.2 arc sec.
Can we achieve ~arc-second pointing? Was achieved with MOST microsat. Key issue is likely to be the performance of the small reaction wheels available for CubeSats.
Attitude Control System May need better reaction wheels. Or two-stage approach. Fine pointing control is done with piezo stage in focal plane. ExoPlanetSat (MIT)
Communications S-band transmitter for data downlink 1W transmitter from Cape Peninsula Univ. of Technology data rates up to 2 Mbps Matching S-band patch antenna (8dB gain). UHF/VHF transceiver for telemetry and commands.
Communications At 2Mbps can downlink ~600 Mb per 5 minute ground station pass. With 2500 stars, 10 minute cadence and ~40 pixels per star: 8 Mb per orbit 112 Mb per day 50 stars with 800 pixels at 1 minute cadence 32 Mb per orbit 448 Mb per day
All components commercially available except: CCD electronics Optics for science and guide telescope Software Most components have flight heritage Exceptions System computer, S-band transmitter (due to fly on Ukube-1) Hardware cost ~$320K
Component Mass (g) Star Tracker 375 Guide Imager 375 Guide Telescope (est) 500 Fibre Optic Gyro 90 Reaction Wheels 555 S-band Transceiver 80 S-band Antenna 50 System Computer 70 Payload Computer 70 Electrical Power System 133 Battery 256 6U Structure 1160 VHF/UHF Transceiver 85 Science Telescope * 1025 VHF/UHF Antenna 100 CCD Imager and Electronics (est) 600 Deorbit device 80 Solar Panels 340 Fasteners/Harnesses (est) 400 Total 6344 *Canon 85 mm f/1.2 lens Most massive of commercial lenses considered
Power Budget Component Total (W) Observing (W) Downlink (W) System Computer 1.5 1.5 1.5 Reaction Wheels 1.5 0.3 0.3 Star Tracker 2 0 2 Guide Imager 1 1 0 Fibre-Optic Gyro 3 3 0 S-band transmitter 6 0 6 Payload Computer 1.5 1.5 1.5 CCD System 1.5 1.5 1.5 VHF/UHF Transceiver 1.7 0.2 1.7 Total 19.7 9 14.5 6U Solar Panel (BOL) 18.7 18.7 6U Solar Panel (EOL) 16.2 16.2 Margin 7.2 1.7
Conclusions A wide field telescope in a 6U CubeSat could: Detect transitting planets around many bright stars Measure stellar parameters through asteroseismology Key issues include: Performance of the ACS and its effect on photometry. Design of low power CCD electronics. CCD cooling and radiation degradation. Downlink performance.