High Aperture implies lot of photons High SN high precision Open this new parameter space LEGACY Precision Fidelity Doppler V elocity Wavelength Accuracy Stellar atmospheres finest abundances ID: 513959
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The quest for precision
High Aperture implies lot of photons: High S/N , high precisionOpen this new parameter spaceLEGACYPrecision: Fidelity, Doppler Velocity, Wavelength Accuracy…Stellar atmospheres, finest abundances, Exo - Earths, Physical Constants, Expansion..
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Assuming an area of 900 m2 and a pixel size of 0.0125 AAt 5000 A (R=100000 with a sampling of 4 pixels), neglecting RON, DK etc.. Using Eff=0.2: Nphot=0.0125*T*Eff*Area*5.7*1010*10(-0.4*Mn
) /lambda(A) and SNR =
N
phot
/Sqrt(Nphot)We get in ½ hour: Mag. SN/pixel10 2150 11 1355 12 85513 54014 34015 21516 13817 86
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Considering for Doppler shifts a rule of thumb of
Doppler Precision ~ 100/(S/N) m/sec (Liske et al, for QSO, HARPS for stars)… Doppler Precision of 20/30 cm/sec for extragalactic sources/observation
AT MOST can be reached. For stars, likely the limit will be given by detector saturation:
S/N 2000 for exposure (4*10
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e-) is probably a reasonable limit, or 5 cm/s. Line center: using S=(0.69/SNR)*SQRT(Pixel*FWHM) we obtain 16 m/sec for FWHM=3Km/s and SNR=1000
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Implications
Fibre fed, scrambling (efficiency); but is there a real alternative??Temperature and pressure controlled (costs, obvious for IR)
Calibration (Costs or observing time)
NO MOVING functions inside the spectrograph – full spectral range (loss of flexibility) - increasing in costs, and complexity. (but would you like to spend a 0.5 ME(TBD) worth night to re-observe the same object because range was not covered?
If you do everything you can hardly be the best for all applications
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Comments/questions
Be more ambitious?! Open space, go beyond extensions..Modularity: Be ready for AO?Modularity: Be ready for larger wavelength coverage?Modularity: Use different spectrographs on the same field?
(FLAMES,GMT?)
Slit ??
Polarimetry
? How?Any critical timescale (scientific ground) ?What will be surely NOT done in the next 12 yrs?Any spectrum you take, calibrate it well (MM)?
Neglecting GAIA impact?? (new QSOs, new objects ??)
When will be the time to go back to low Z to understand high Z, to stars to understand planets ? Etc..
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Possible ‘concept’?
Multi-fibre injectionR~100000 4 Pixels 10 slices, R~200000 2 Pixels, R=10000 without slicing and multi object (likely 7-8) or IFUUpgradable
to use the AO with VHR modeUpgradable to use
polarimetry
when available
Upgradable in wavelength range: 3-4 spectrographs divided in Lambda, NOT necessarily all arriving at the same time. May be NOT all with the same resolution?If we are serious about expansion, asap is importantIf we want be ambitious we MUST invest.. Sky subtraction, stellar models.. Etc..
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