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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
EMCCDsULTRASPECUnique applicationsGeneral advantagesLimitationsFuture upgrades and instrumentation
14th October 2009 Detectors for Astronomy 2009, ESO Garching 2
EMCCDsHighest gains when low
photon flux high-TIME resolutionhigh-SPECTRAL
resolutionEffectively doubles the
aperture of the telescope (ignoring read-out time)
14th October 2009 Detectors for Astronomy 2009, ESO Garching 3
ES Cet – 10sec exposureTop: avalanche modeBottom: normal read-out
mode (3e- noise)
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
14th October 2009 Detectors for Astronomy 2009, ESO Garching 4
Unique Science - HTRAULTRASPEC 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
14th October 2009 Detectors for Astronomy 2009, ESO Garching 5
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
14th October 2009 Detectors for Astronomy 2009, ESO Garching 6
ULTRACAM observations of 4U1636-536Munoz-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
14th October 2009 Detectors for Astronomy 2009, ESO Garching 7
With fast spectroscopy we can accurately remove the continuum contribution, enabling us to measure the inclination and subsequently the Neutron Star mass.
ULTRASPEC observations of EXO 0748-676
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)
14th October 2009 Detectors for Astronomy 2009, ESO Garching 8
Feb ’08: ULTRASPEC on ESO 3.6m
Commissioning and Science Demonstration run
Problems with CTE and CIC, which have since been solved
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
14th October 2009 Detectors for Astronomy 2009, ESO Garching 10
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)
14th October 2009 Detectors for Astronomy 2009, ESO Garching 11
Advantages II – Cosmic removalCosmic 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 60secs14th October 2009 Detectors for Astronomy 2009, ESO Garching 12
Mean of 10 frames
Median of 10 frames
Advantages II – Cosmic removal
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“Real life” example:3800 second spectrum of a Quasar taken with UVES
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
14th October 2009 Detectors for Astronomy 2009, ESO Garching 14
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
14th October 2009 Detectors for Astronomy 2009, ESO Garching 15
Screen-flat from FORS 1 with UV-optimised E2V CCD44-82 detector and 600I grism
LimitationsNeed 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 problemCIC rate of 10-3 e- 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 problemCTE seen in ULTRASPEC frames but can be mitigated
14th October 2009 Detectors for Astronomy 2009, ESO Garching 16
Instrument upgradesX-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 linesBetter 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
14th October 2009 Detectors for Astronomy 2009, ESO Garching 17
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
14th October 2009 Detectors for Astronomy 2009, ESO Garching 18
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