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EMCCDs for Optical Spectroscopy: Everyone’s a Winner?

EMCCDs for Optical Spectroscopy: Everyone’s a Winner?. Kieran O’Brien ESO, Paranal Science Operations. (UCSB, Santa Barbara, California). Vik Dhillon (Sheffield, UK), Tom Marsh (Warwick, UK), Derek Ives (UKATC, ESO), Naidu Bezawada (UKATC). Outline of talk. EMCCDs ULTRASPEC

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EMCCDs for Optical Spectroscopy: Everyone’s a Winner?

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  1. EMCCDs for Optical Spectroscopy: Everyone’s a Winner? Kieran O’Brien ESO, Paranal Science Operations (UCSB, Santa Barbara, California) Vik Dhillon (Sheffield, UK), Tom Marsh (Warwick, UK), Derek Ives (UKATC, ESO), Naidu Bezawada (UKATC)

  2. Outline of talk • EMCCDs • ULTRASPEC • Unique applications • General advantages • Limitations • Future upgrades and instrumentation Detectors for Astronomy 2009, ESO Garching

  3. EMCCDs • Highest gains when low photon flux • high-TIME resolution • high-SPECTRAL resolution • Effectively doubles the aperture of the telescope (ignoring read-out time) ES Cet – 10sec exposure Top: avalanche mode Bottom: normal read-out mode (3e- noise) Detectors for Astronomy 2009, ESO Garching

  4. ULTRASPEC • E2V CCD201-20 1024 x 1024 pixel, frame transfer device • 536-stage electron multiplying register • SDSU controller with custom high-voltage clock board • Mounted on EFOSC2 on ESO 3.6m and NTT • Range of grisms available, including VPH around HeII (468.6nm) and H-alpha (656.3nm) • Imaging mode also possible/used • Dedicated real-time data analysis pipeline Detectors for Astronomy 2009, ESO Garching

  5. Unique Science - HTRA ULTRASPEC on ESO 3.6m and ESO NTT • Bowen-blend echo-tomography of Scorpius X-1 and 4U 1636-536 using ULTRASPEC • Revealing the population of Ultra-Compact X-ray Binaries • Studying brown dwarf weather with ULTRASPEC • Understanding dwarf nova oscillations • High-speed phase resolved spectroscopy of 4U 1822-371 • A search for optical pulsations in the Anomalous X-ray Pulsar XTE J1810-197 • Probing rapid multi-wavelength accretion-driven variability in the X-ray binaries GX 339-4, 4U 1957+11 and GX 9+9 • High time-resolution imaging and spectroscopy time series of early GRB afterglows with ULTRASPEC on NTT. • Bowen-blend echo-tomography of EXO 0748-676 using ULTRASPEC • High-speed optical spectroscopy of the Vela pulsar • The rotation rates of white dwarfs in binaries • Spectral eclipse mapping of accretion discs in Cataclysmic Variables … also QUCAM on WHT • High-time resolution spectroscopy of the eclipsing double degenerate SDSS J0926+3624 • Bowen blend echo-tomography of Sco X-1 using ISIS+L3CCD Detectors for Astronomy 2009, ESO Garching

  6. Bowen-blend echo-tomography In Low-Mass X-ray Binaries, some of the optical emission is the result of reprocessing of X-rays into lower energy optical/IR emission Phase-resolved optical spectroscopy reveals that the Bowen-blend fluorescence lines (~464nm) are a centred on the irradiated face of the companion star Correlated X-ray and Bowen emission will tell us the offset (in light seconds) between the X-ray source and the companion star. This is a function of the binary phase and will enable us to determine the inclination of the system From O’Brien, et al. 2002 Detectors for Astronomy 2009, ESO Garching

  7. ULTRACAM observations of 4U1636-536 • Munoz-Darias et al. (2009), observed 3 simultaneous X-ray/optical bursts with the triple-beam imager ULTRACAM, mounted at the VLT, equipped with a special NB filter • Continuum contribution subtracted from the flux in the red-channel • Uncertainty in the continuum subtraction leads to uncertainty in the delay With fast spectroscopy we can accurately remove the continuum contribution, enabling us to measure the inclination and subsequently the Neutron Star mass. Detectors for Astronomy 2009, ESO Garching

  8. ULTRASPEC observations of EXO 0748-676 • Feb ’08: ULTRASPEC on ESO 3.6m • Commissioning and Science Demonstration run • Problems with CTE and CIC, which have since been solved • Lightcurves show 4 simultaneous bursts (one ‘random’ example above) • Cross-correlation analysis shows different delays between continuum (right, top) and Bowen (right, middle). • However, statistics are not good enough for the continuum subtracted case (right, bottom) Detectors for Astronomy 2009, ESO Garching

  9. ULTRASPEC observations of 4U1636-536 Detectors for Astronomy 2009, ESO Garching

  10. General applications Warning: spoiler!!! • You cannot lose…. • Always have to option of using a ‘standard’ (2-3 e-) readout port … unless you need large mosaics Detectors for Astronomy 2009, ESO Garching

  11. Advantages I – duty cycle Simulation based on 42-m ELT with an EMCCD with RON= 0e- (cf. 3.6e-), R=5000, T=11,000K, dark sky (Courtesy: Tom Marsh) Detectors for Astronomy 2009, ESO Garching

  12. Advantages II – Cosmic removal Cosmic rays only affect an individual frame (100-102s) rather than the entire exposure, so can be removed ‘cheaply’ in terms of S/N Raw ULTRASPEC frames – 10 x 60secs Mean of 10 frames Median of 10 frames Detectors for Astronomy 2009, ESO Garching

  13. Advantages II – Cosmic removal “Real life” example: 3800 second spectrum of a Quasar taken with UVES Detectors for Astronomy 2009, ESO Garching

  14. Advantages IV – Sky subtraction Simulation of residuals from sky-subtraction in ‘nod-and-shuffle’ mode at different nod-periods shows good agreement with measured values. Taken from Glazebrook & Bland-Hawthorn (2001) • Trapping sites limit number of shuffles (~100) and hence minimum dwell time • Significant power remains at high frequencies • Typical timescale for PCM with UVES would be ~10secs (less with improvements in CIC) • Simple beam-switching would enable extremely accurate sky removal Detectors for Astronomy 2009, ESO Garching

  15. Advantages III – Fringe removal • Variations of interpolated sky due to fringing are removed due to common path of sky and object • Uncertainties due to instrument flexure is reduced (removed?) as sample rate is increased Screen-flat from FORS 1 with UV-optimised E2V CCD44-82 detector and 600I grism Detectors for Astronomy 2009, ESO Garching

  16. Limitations • Need a larger format!!! • Currently only available in 1k x 1k format, which is not useful in majority of cases. • Need (at least?) 2k x 4k buttable devices • Multiple (8) read-out ports (10Mhz) to avoid the need to window, especially desirable for cross-dispersed instruments • CIC a problem • CIC rate of 10-3e- pix-1 frame-1 is achievable. In photon-counting mode this leads to effective RON of 0.1 e- pix-1 for a typical exposure • This should be reduced even further as devices are better understood. • CTE a problem • CTE seen in ULTRASPEC frames but can be mitigated Detectors for Astronomy 2009, ESO Garching

  17. Instrument upgrades • X-Shooter: An EMCCD would allow telescope nodding to improve the sky subtraction on the IR-arm without the additional read-out noise on the UV and VIS-arms. • High resolution spectrographs (UVES, FLAMES, HIRES…) • RON limited between sky lines • Better sky subtraction and cosmic removal allow longer integration times • Low resolution spectrographs (FORS, VIMOS, LRIS, GMOS, DEIMOS…) • >32 2k x 4k devices would benefit from EMCCD upgrade Detectors for Astronomy 2009, ESO Garching

  18. Future Instruments… double white-dwarf system, 322second orbital period, V=21.1, 8hrs of observations with 30-sec integrations a) FORS2 with conventional CCD on 8-m, b) with an EMCCD on 8-m, c) conventional CCD on a 42-m, d) frame-transfer CCD on a 42-m and e) an EMCCD on 42-m Detectors for Astronomy 2009, ESO Garching

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