1. Overview: Astronomical Spectroscopy Basic Spectrograph Optics Objective Prism Spectrographs Slit Spectrographs

2. Contents Light: emission, absorption and radiation Overview of spectroscopy Instruments –Optics (Math & Physics, Prism, Diffraction grating, and so on) –Detectors (Semiconductors, CCD and CMOS, Optical and infrared detectors) Application to astronomy Practical observations and data reduction

3. Vocabulary Spectral resolution, Resolving Power R = /  – is the wavelength and  is the smallest wavelength interval that can be resolved –“low” resolution 10<R<1000 –“moderate” resolution 1000<R<10,000 –“high” resolution R>10,000 Dispersion –  /pixel

4. Refraction The speed of light in a dense medium (air, glass…) is (usually) slower than in a vacuum. Refraction index (ratio of speed of light in a vacuum to the speed in the medium) –air:n = 1.0003 –water:n= 1.33 –salt:n= 1.53 The speed of light in a material depends on wavelength – “dispersion”

5. Prisms Prisms disperse light by refraction. When a beam of white light passes from one medium into another at an angle, the direction of the beam changes due to refraction. Different colors of light are bent at different angles. Generally, red light is bent less, blue light is bent more.

6. 42 degree

7. Objective Prism Spectroscopy Prism installed at the top of the telescope Simplest. Light is already parallel, so no extra lenses. Each point source produces a spectrum No white light reference spot Usually low resolution, good for wide-field surveys and meteors Courtesy of S. Abe

9. Diffraction Gratings Multi-slit diffraction reflection gratings and transmission gratings most astronomical gratings are reflection gratings

10. Reflection Gratings Light reflecting from grooves A and B will interfere constructively if the difference in path length is an integer number of wavelengths. The path difference is dsin  + dsin  (where d is the distance between facets on the grating), so dsin  + dsin  = n  the grating equation n is the “spectral order” and quantifies how many wavelengths of path difference are introduced between successive facets or grooves on the grating) d

12. The Grating Equation d (sin  + sin  ) = n The groove spacing d is a feature of the grating The angle of incidence, , is the same for all wavelengths The angle of diffraction, , must then be a function of wavelength sin  = n /d – sin  d

13. Quiz [1] We are working with a grating with 1000 grooves per millimeter. The incident angle  is 15º. At what angle will light of 400 nm be diffracted in 1 st order (n=1)? 500 nm? 600 nm? Careful: express wavelength and groove spacing in similar units sin  = n /d – sin  Answer: 8.1  (400nm), 14.0  (500nm), 19.9  (600nm)

14. Multiple Grating Orders Multiple spectra are produced by a diffraction grating, corresponding to different orders (n=1,2,3…) Quiz [2] –For a grating of 1000 grooves/mm and 15º incident angle, what wavelength of light will be diffracted to an angle of 14º in second order? Quiz [3] –Fill the blanks in the right figure. sin  = n /d – sin  Answer1 250nm Answer2 300, 200, 100 (from left to right)

15. Slit Spectrographs Entrance Aperture: A slit, usually smaller than that of the seeing disk Collimator: converts a diverging beam to a parallel beam Dispersing Element: sends light of different colors into different directions Camera: converts a parallel beam into a converging beam Detector: CCD, IR array, etc.

16. Why use a slit? to increase resolution –by narrowing the slit –also decreases throughput to block unwanted light –from the sky –other nearby sources to set a reference point AKARI/IRC (coutesy of Y. Ohyama)

17. Collimator The collimator converts the diverging beam of white light from the slit to a parallel beam. The focal ratio of the collimator must be matched to the effective focal ratio of the telescope. The diameter of the collimator determines the diameter of the light beam in the spectrograph. The size of the collimator affects the size of the “slit image” on the detector.

18. Problem: A grating diffracts light into many orders; one order contains only a fraction of the light Fix: Gratings can be designed to concentrate most of the incident intensity into a particular order, by a process called “blazing”. This is a process where the grooves of a grating are cut so that the reflecting surfaces are at a certain angle, the blaze angle. About 90% of the incident light is diffracted preferentially into the first order. Reflection Grating Efficiency

19. Camera Types reflecting camera –broad wavelength coverage –on- of off-axis transmission camera –lenses –generally on-axis, no central obstruction –broad wavelength coverage requires multiple elements

20. Nayuta 2-m Spectrograph Courtesy of S. Ozaki (NAJO)

21. How to Improve the Resolution? based on the grating equation d (sin  + sin  ) = n “  ” is the angle from the slit to the grating normal and “  ” is the angle from the grating normal to the camera.  is usually fixed. The “angular dispersion” of a spectrograph is given by  /  :   sin  = n /d – sin 

22. The resolution varies as –the order number (higher order  more resolution) –the grating spacing (narrower grooves  more resolution) –the camera-collimator angle (as  increases, cos  gets smaller and resolution increases) The effective resolution of a spectrograph is a function of –Not only the grating resolution –But also the size of the slit image and the pixel size resolution

23. Throughput Matters The higher the throughput, the better Limitations: –slit width (get a bigger collimator or better seeing) –efficiency of mirror coatings grating lens transmission detector

24. Limitations for High Dispersion Solution: the echelle grating –works in high orders (n=100) –a second dispersing element spreads the light in a perpendicular direction Problem: detector size, shape –generally square or 1x2 format –a conventional grating spectrograph produces a very LONG high dispersion spectrum that does not fit on a CCD

26. Echelle Gratings To increase spectral resolution, increase the order at which a grating is used For high orders, we must increase  and  in the grating equation (to ~50- 75 o ) The spectral range for each order is small so the orders overlap Separate the orders with a second disperser (cross disperser) acting in a perpendicular direction.

27. Multi-object Spectroscopy Observing one object at a time is inefficient When many stars are available in a field (e.g. a star cluster) use multi-object spectroscopy Put an optical fiber at locations of objects to take spectra. Feed the optical fibers into a spectrograph.

28. Grism A grism is formed by replicating a transmission grating onto the hypotenuse face of a right-angle prism. Grisms are useful in spectrometers that require in-line presentation of the spectrum. The light diffracted by the grating is bent back in line by the refracting effect of the prism. The dispersion of a grism is not linear, since the dispersive effects of the prism and grating are superimposed. 수지

29. Grism Light is incident normally on the back face of the prism. It is refracted at the interface between the prism (index n) and the resin ( 수지, index n E ) and then diffracted at the interface between the resin and the air.

30. Grism Light is incident normally on the back face of the prism. Courtesy of S. Abe & N. Ebizuka, http://nemesis.astro.ncu.edu.tw/~avell

31. Application to Astronomical Objects Stars Astronomical spectroscopy began with Isaac Newton's initial observations of the light of the Sun, dispersed by a prism. These dark bands which appear throughout the solar spectrum were first described in detail by Joseph von Fraunhofer. Most stellar spectra share these two dominant features of the Sun's spectrum: emission at all wavelengths across the optical spectrum (the continuum) with many discrete absorption lines, resulting from gaps of radiation (Fraunhofer lines)

32. The absorption lines in stellar spectra can be used to determine the chemical composition of the star. Of particular importance are the absorption lines of hydrogen. In conjunction with atomic physics and models of stellar evolution, stellar spectroscopy is used to determine properties of stars such as, their distance, age, luminosity and rate of mass loss. The Doppler shift studies can uncover the presence of hidden companions such as black holes and exoplanets.

33. Galaxies and Quasars The spectra of galaxies look similar to stellar spectra, because they consist of the light from millions of stars. Edwin Hubble discovered in the 1920's that galaxies are moving away from the Earth. The further away a galaxy, the faster it is receding (Hubble's Law). When the first spectrum of one of quasars was taken, it was something of a mystery, with absorption lines at wavelengths where none were expected. It was soon realized that what was being seen was a normal galactic spectrum, but highly redshifted. According to Hubble's Law, this implied that the quasar must be extremely distant, and therefore highly luminous.

34. Solar System Objects Planets and asteroids are visible by reflecting the sunshine. The reflected light contains absorption bands due to mafic silicates in the rocks present for rocky bodies (800-1,100nm), or due to the elements and molecules present in the atmospheres of the Gas giants (890nm, CH 4 ). The spectra of comets consist of a reflected solar spectrum from the dusty clouds surrounding the comet, as well as emission lines from gaseous atoms and molecules excited by sunlight fluorescence and/or chemical reactions.

35. 1. Gratings Low resolution 150Lines/mm, 4.3A/pix High resolution 600Lines/mm, 1.05A/pix 2. Slit Wide width ( 은하 관측 ) 72 micrometer Narrow width ( 항성 관측 ) 18 micrometer 3. Wavelength range 3800 A ~ 7500A