Spectroscopy & Spectrographs Roy van Boekel & Kees Dullemond Overview Spectrum,

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Spectroscopy & Spectrographs Roy van Boekel & Kees Dullemond Overview Spectrum,

Petersen, Deborah, Features Editor has reference to this Academic Journal, PHwiki organized this Journal Spectroscopy & Spectrographs Roy van Boekel & Kees Dullemond Overview Spectrum, spectral resolution Dispersion (prism, grating) Spectrographs longslit echelle fourier trans as long as m Multiple Object Spectroscopy Spectroscopy: what do we measure Spectrum = the intensity (or flux) of radiation as a function of wavelength “Continuous” sampling in wavelength (as opposed to imaging, where we integrate over some finite wavelength range) Note: In practice, when using CCDs as long as spectroscopy, one also integrates over finite wavelength ranges – they are just very narrow compared to the wavelength itself: Pixel width Sampling is continuous but the spectral resolution is limited by the design of the spectrograph Spectrum in classical sense holds no direct spatial in as long as mation. Many spectrographs allow retrieving spatial info in 1 dimension, some even in 2 (“integral field units”)

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Spectral resolution Smallest separation in wavelength that can still be distinguished by instrument, usually given as fraction of in addition to denoted by R: or alternatively useful, though somewhat arbitrary working definition Basic spectrograph layout a means to isolate light from the source in the focal plane, usually a slit “collimator” to make parallel beams on the dispersive element dispersive element, e.g. a prism or grating. Reflection gratings much more frequently used than transmission gratings “Camera”: imaging lens to focus beams in the (detector) focal plane + detector to record the signal Dispersion Splitting up light in its spectral components achieved by one of two ways: differential refraction prism interference reflection/transmission grating fourier trans as long as m (Farby-Perot)

Prism Refractive index n of material depends on wavelength Several approximate as long as mulae exist to describe n(). “Sellmeier” equation is accurate over a large wavelength range in addition to used by manufacturers of optical glasses: Bi in addition to Ci are empirically determined coefficients. With 3 terms the Sellmeier approximation is accurate to 1 part in ~510-6 in the whole optical in addition to near-infrared range Prism general light path through prism: one can show that: dispersion is maximum as long as a symmetrical light path dispersion is maximum as long as grazing incidence. Corresponding top angle depends on refractive index of material. E.g. ~74° as long as heavy flint glass However: most light is reflected instead of refracted as long as grazing incidence. In practice, smaller are used (60° in addition to 30° are common choices) Prism dispersion curve strongly non-linear, dispersion in blue much stronger than in red part of spectrum

Prism spectrograph layout Credit: C.R. Kitchin “Astrophysical techniques” CRC Press, ISBN 13: 978-1-4200-8243-2 Young’s double slit experiment d double slit screen lens incident wave Young’s double slit experiment d double slit screen lens incident wave

Young’s double slit experiment N=2 Now a triple slit experiment d triple slit screen lens incident wave Now a triple slit experiment N=3

Adding more slits N=4 Adding more slits N=5 Adding more slits N=6

Adding more slits N=16 General as long as mula of pattern N=4 Exercise: Show that the 1/N2 normalization is correct. Width of the peaks N=4 For one has

Width of the peaks N=4 For one has with Width of the peaks N=4 For one has with Peak width is there as long as e: (Later: Relevance as long as spectral resolution) Now do 3 different wavelengths N=4 Green is here the reference wavelength . Blue/red is chosen such that its 1st order peak lies in green’s first null on the left/right of the 1st order.

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Now do 3 different wavelengths N=8 Keeping 3 wavelengths fixed, but increasing N Now do 3 different wavelengths N=16 Keeping 3 wavelengths fixed, but increasing N Now do 3 different wavelengths N=4 Green is here the reference wavelength . Blue/red is chosen such that its 1st order peak lies in green’s first null on the left/right of the 1st order.

Now do 3 different wavelengths N=8 Green is here the reference wavelength . Blue/red is chosen such that its 1st order peak lies in green’s first null on the left/right of the 1st order. Let’s look at the 2nd order N=8 Green is here the reference wavelength . Blue/red is chosen such that its 1st order peak lies in green’s first null on the left/right of the 1st order. Let’s look at the 2nd order N=8 Green is here the reference wavelength . Blue/red is chosen such that its 1st order peak lies in green’s first null on the left/right of the 1st order. m=0 m=1 m=2 m=3 m=4

Example of low-R Infrared spectroscopy Origin of dust species in disk around young stars, solar system comets, in addition to building blocks of planets Young star undergoes accretion outburst Amorphous dust turns into crystals Credit: Spitzer Science Center Abraham et al. 2009

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