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
 Unique applications
 General advantages
 Limitations
 Future upgrades and instrumentation
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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)
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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 Halpha (656.3nm)
 Imaging mode also possible/used
 Dedicated real-time data analysis pipeline
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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
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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 Bowenblend 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
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ULTRACAM observations of 4U1636-536
 Munoz-Darias et al. (2009),
observed 3 simultaneous Xray/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.
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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)
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ULTRASPEC observations of 4U1636-536
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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
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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)
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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
Mean of 10
frames
Median of 10
frames
Raw ULTRASPEC frames – 10 x 60secs
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Advantages II – Cosmic removal
“Real life” example:
3800 second spectrum of a Quasar taken with UVES
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Advantages IV – Sky subtraction
 Trapping sites limit number of
Simulation of residuals from skysubtraction in ‘nod-and-shuffle’ mode
at different nod-periods shows good
agreement with measured values.
Taken from Glazebrook & BlandHawthorn (2001)
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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
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Advantages III – Fringe removal
Screen-flat from FORS 1 with UVoptimised E2V CCD44-82 detector and
600I grism
 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
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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 10-3 e- pix-1 frame-1 is achievable. In photoncounting 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
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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
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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
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