Spectroscopy

1. Spectroscopy

2. Spectroscopy • Spectral analysis is probably the most important method for learning about the physics of the astronomical sources • Simplest method to get spectral information is using filters • In this case the size of the spectral elemet we can resolve is the width of the filter • More detailed information is obtained if the light is sent through a dispersive element

3. Newton‘s experiment

4. Dispersive elements • The core optical element of an astronomical spectrograph is its dispersive element • With a dispersive element, the angle at which the light leaves it, is wavelength dependent. • There are three kinds of dispersive elements: • Prisms • Grating • Grisms

5. Prism 1 • When light moves from one medium to a denser medium, it is slowed down and as a result either bent (refracted) or reflected. • The refractive index of a material is a measure of the light speed in that material. It depends on the wavelength. • The white light entering the prism is a mixture of different wavelengths, each of which gets bent slightly differently. Blue light is slowed more down than red light and will therefore be bent more than red light.

6. Prism 2

7. Grating 1 • The diffraction grating is the primary dispersing element in most astronomical spectrometers • It has significantly larger limiting resolution than a prism of comparable size • A grating is also versatile in the spectral formats it can provide • And it can be quite efficient over a reasonable spectral range, though usually not as efficient as a prism

8. Grating 2 • A diffraction grating is a reflecting or transparent element, whose optical properties of an external (or internal) surface are periodically modulated. • Most commonly the diffraction gratings are realized as fine parallel and equally spaced ruled grooves on a material surface.

9. Grating equation 1 • The diffraction of light in a plane reflectance grating is governed by sin + sin = m/d is the angle of incidence m is the angle of diffraction m denotes the order number of the diffracted beam d is the groove spacing of the grating

10. Grating equation 2

11. Échelle grating 1 • For high resolution astronomical work échelle is the preferred choise over a grating used in low order • The reasons for this are: • Larger luminosity • Two dimensional format that permits broad spectral coverage

12. Échelle grating 2 • A diffraction grating in which the grooves are quite widely spaced and have a zigzag or step-like cross-sectional profile. • The light to be dispersed is made to fall on the grating at right angles to the faces of the grooves. This has the effect of producing a series of overlapping spectra with a high degree of resolution. • A second, low-dispersion grating, or a prism, arranged perpendicular to the échelle, serves to separate out the overlapping spectra.

13. Échelle spectrum

14. Grism Schematic grism and the optical light path through it • A grism is a transmission grating replicated onto a prism that has its angle chosen in such a way that the desired order of the grating passes through the grism undeviated. • The grating groove spacing and the prism angle for a given prism material define the location of the spectrum on the detector, and the grating groove angle and refractive index of the grating define the grating blaze function. • = prism angle =groove angle =deflection angle np=refractive index d=grating groove spacing

15. Resolving power • Resolving power (R) tells how small details we can resolve in the spectrum • It is defined as  • So for example • R=1000 at 6500 Å gives =6.5 Å or 300km/s • R=10 000 at 6500 Å gives =0.65 Å or 30km/s • R=100 000 at 6500 Å gives =0.065 Å or 3km/s

16. Which resolving power to use for your observations? • Always „the larger the better“ is not the answer • High resolution needs a lot of photons, so to get any signal one needs a bright source and/or a large telescope • Also, in some cases there is no need for high resoltion. If the process you want to study produces velocities of 1000km/s, there is not much point studying it with resolution of 1 km/s

17. Spectroscopy at NOT • NOT has 4 instruments with spectroscopic capabilities: • Optical multi purposes spectrograph, ALFOSC • Low to medium resoltion IR spectrograph, NOTCam • 2 high resolution échelle spectrographs, FIES & SOFIN

18. ALFOSC • 15 possible grisms • Wavelengths covered between 3200-11000 Å • Resolving powers between 190 and 10 000 • Long slits and 6.6 " -8 .6 " & 15" slits for échelle spectroscopy and spectropolarimetry • Also multi object spectroscopy (MOS)

19. NOTCam, low resolution • One grism with an echelle grating • Intermediate resolution (2-pixel R=2500, with dispersion 2.5-4.1 Angstrom/pixel) in J (5th and 6th order), H (4th order) and K (3rd order) when used with the WFC • The grism is used in combination with the JHK filter set. • Slit width of 128 microns (1 arcsec)

20. NOTCam, medium resolution • Also the High-Resolution Camera (HRC) can be used to obtain spectra. • Wavelength ranges in the JHK bands: 1.26-1.34 micron (Pa-beta), 1.57-1.67 micron, 2.07-2.20 micron (Br-gamma). • Respective dispersions are 0.87, 1.08, 1.39 A/pix. With the dedicated 0.5 arcsec slit, the resolutions are R=5700, 5700, 4900 respectively.

21. FIES 1 • Cross-dispersed high-resolution echelle spectrograph, R=60 000 • Spectral range 4000-8300 Å is covered without gaps in a single setting • Mounted in a well insulated building separate from and adjacent to the NOT dome.

22. FIES 2 • The optical fibre connecting the telescope with the spectrograph is permanently mounted with a movable 45-degree mirror • Being a permanently mounted instrument, FIES can be used at any time, also for short periods of time while other instruments are mounted. • Equipped with a 100 micron fibre, corresponding to 1.3 arcseconds on the sky at a spectral resolution of R =40 000. • We expect a new and more extensive fiber unit to be installed, which contains a new 100 micron fiber (but with exit slit) offering a resolution of R = 60 000

23. SOFIN 1 • Échelle spectrograph • Wavelength coverage 3500-11000Å • Three different spectral resolutions R = 30 000, 80 000, and 170 000 • One of the few very high resolution spectrographs in the northern hemisphere.

24. SOFIN 2 • The resolution is altered by changing one of the three different optical cameras • The higher the spectral resolution the smaller the part of the spectral range which can be covered by the ccd. The change of the spectral setting is done by turning the échelle grating and the cross-dispersion prism.

25. Polarimetry

26. Polarised light • The electric field vector may be arbitrarily divided into two perpendicular components x and y • For a simple harmonic wave, the two components have exactly the same frequency. • Components have two other defining characteristics that can differ: • Amplitude • Phase • The shape traced out in a fixed plane by the electric vector as such a plane wave passes over it is a description of the polarisation state • Plots show electric field vector (blue), its x and y components (red/green) and the path traced by the tip of the vector in the plane (purple) Polarisation states Linear Circular Elliptical

27. Why is it usefull? • Mechanisms for creating polarisation: • Scattering by small dust particles or free electrons • Various absorption and emission processes in magnetic fields • Polarisation provides information about: • The geometry of the light emitting source • And the radiation process involved

28. How to observe polarisation • Polarimeters consist of optical elements such as retarders and polarisers that change the polarisation state of the incoming light in a controlled way • The detector only measures intensities • The various intensity measurements are then combined to get the polarisation state f the incoming light Polarimeter Spectrometer Telescope Detector Calibration opics

29. Wave plates 1 • A wave plate (or retarder) works by shifting the phase of the light wave between two perpendicular polarization components. A typical wave plate is simply a birefringent crystal with a carefully chosen thickness. • The wave plate is characterized by the amount of relative phase that it imparts on the two components

30. Wave plates 2 • Quater wave plate creates a quarter wavelength phase shift and can change linearly polarized light to circular and vice versa. • Half wave plateretards one polarisation by a half wavelength, or 180 degrees. This type of wave plate rotates the polarization direction of linear polarized light.

31. Polarisers 1 • Polariser is an optical element that produces (at least partially) polarised light from unpolarised input beam • Many types of polarisers, for example • Polaroid • Christal polarisers

32. Polaroid • First polaroid by Edwin Land in 1928 used herapathite chrystals, which were spread as a thin layer between supporting sheets • The chrystals have a magnetic dipole moment, so that if the suspension is placed in a very strong magnetic field they become oriented to form a uniform dichroic layer • Nowadays other materials are used, e.g., polyvinyl alcohol and polyvinylene

33. Chrystal polarisers A calcite crystal laid upon a paper with some letters showing the double refraction Wollaston prism

34. Polarimetry with ALFOSC • ALFOSC has a polarimetry unit FAPOL which can be equiped either with half wave plate (linear polarisation) or quater wave plate (cicular + linear polarisation) • A calcite plate is needed in the aperture wheel to provide a simultaneous measurement of the ordinary and the extraordinary components. • The unvigneted field is limited by the size of the calcites to 140" in diameter. • Both photo- and spectropolarimetry can be obtained

35. Turpol • Simultaneous five-colour polarimeter (UVBRI) • Double image chopping polarisation levels 0.01% detectable • Rotatable quater-wave plate: simultaneous cicular and linear polarissation • Best efficiency for only linear obtained with a half-wave retarder • Calcite plate polarising beam splitter yields direct elimination of sky back-grund polarization