1. Spectrographs

2. Literature: Astronomical Optics, Daniel Schroeder Astronomical Observations, Gordon Walker Stellar Photospheres, David Gray

3. Spectral Resolution d 1 2 Consider two monochromatic beams They will just be resolved when they have a wavelength separation of d Resolving power: d = full width of half maximum of calibration lamp emission lines R = d

4. R = 15.000  = 0.73 Å R = 100.000  = 0.11 Å R = 500.000  = 0.022 Å

5. Spectral Resolution The resolution depends on the science: 1. Active Galaxies, Quasars, high redshift (faint) objects: R = 500 – 1000 2. Supernova explosions: Expansion velocities of ~ 3000 km/s d / = v/c = 3000/3x10 5 = 0.01 R > 100

6. R = 3.000 R = 30.000

7. 35.0000.160100000 60.0000.09130000 100.0000.05310000 140.0000.046000 200.0000.0283000 R  th (Ang) T (K) 3. Thermal Broadening of Spectral lines:

8. 3000001K 1000003G0 1200025F5 375080F0 2000150A0 R1R1 Vsini (km/s)Sp. T. 4. Rotational Broadening: 1 2 pixel resolution, no other broadening

9. 5. Chemical Abundances: Hot Stars: R = 30.000 Cool Stars: R = 60.000 – 100.000 Driven by the need to resolve spectral lines and blends, and to accurately set the continuum.

10. 6 Isotopic shifts: Example: Li 7 : 6707.76 Li 6 : 6707.92 R> 200.000

12. 7 Line shapes (pulsations, spots, convection): R=100.000 –200.000 Driven by the need to detect subtle distortions in the spectral line profiles.

13. Line shapes due to Convection Hot rising cell Cool sinking lane The integrated line profile is distorted. Amplitude of distortions ≈ 10s m/s

14. R = 200.000 R > 500.000

15. 8 Stellar Radial Velocities:  RV (m/s) ~ R –3/2 (  ) –1/2  wavelength coverage R  (m/s) 100 000 1 60 000 3 30 000 7 10 000 40 1 000 1200

16. collimator Spectrographs slit camera detector corrector From telescope Anamorphic magnification: d 1 = collimator diameter d 2 = mirror diameter r = d 1 /d 2

17. slit camera detector corrector From telescope collimator Without the grating a spectograph is just an imaging camera

18. A spectrograph is just a camera which produces an image of the slit at the detector. The dispersing element produces images as a function of wavelength without disperser with disperser slit fiber

19. Spectrographs are characterized by their angular dispersion dd   d Dispersing element dd d A =

20. f dldl dd d dldl d = f In collimated light

21. S dd d dldl d = S In a convergent beam

22. Plate Factor P = ( f A ) –1 = ( f ) dd d P = ( f A ) –1 = ( S ) dd d P is in Angstroms/mm P x CCD pixel size = Ang/pixel

23. w h f1f1 d1d1 A D f d2d2   w´w´ h´h´  D = Diameter of telescope d 1 = Diameter of collimator d 2 = Diameter of camera f = Focal length of telescope f 1 = Focal length of collimator f 2 = Focal length of camera A = Dispersing element f2f2

24. w h f1f1 d1d1 A D d2d2 f w´w´ h´h´ f2f2    w = slit width h = slit height Entrance slit subtends an angle  and  ´ on the sky:  = w/f  ´ = h/f Entrance slit subtends an angle  and  ´ on the collimator:  = w/f 1  ´ = h/f 1

25. w ´ = rw(f 2 /f 1 ) = r  DF 2 h ´ = h(f 2 /f 1 ) =  ´ DF 2 F 2 = f 2 /d 1 r = anamorphic magnification due to dispersing element = d 1 /d 2

26. w ´ = rw(f 2 /f 1 ) = r  DF 2 This expression is important for matching slit to detector: 2  = r  DF 2 for Nyquist sampling (2 pixel projection of slit). 1 CCD pixel (  ) typically 15 – 20  m Example 1:  = 1 arcsec, D = 2m,  = 15  m => rF 2 = 3.1 Example 2:  = 1 arcsec, D = 4m,  = 15  m => rF 2 = 1.5 Example 3:  = 0.5 arcsec, D = 10m,  = 15  m => rF 2 = 1.2 Example 4:  = 0.1 arcsec, D = 100m,  = 15  m => rF 2 = 0.6

27. 5000 A 4000 A n = –1 5000 A 4000 A n = –2 4000 A 5000 A n = 2 4000 A 5000 A n = 1 Most of light is in n=0

28. bb   The Grating Equation m  = sin  + sin  b 1/  = grooves/mm 

29. dd d = m  cos  = sin  + sin  cos  Angular Dispersion: Linear Dispersion: d dx d dd = dd = 1 f cam 1 d  /d dx = f cam d  Angstroms/mm

30. Resolving Power: w ´ = rw(f 2 /f 1 ) = r  DF 2 dx = f 2 dd d  f 2 dd d  r  DF 2 R = / d = A r 1  d1d1 D  = rr A D d1d1 For a given telescope depends only on collimator diameter Recall: F 2 = f 2 /d 1

31. D(m)  (arcsec) d 1 (cm) 2110 4120 10152 100.526 300.577 300.2538 R = 100.000A = 1.7 x 10 –3

32. Adaptive Optics corrects for the atmospheric motion and allows one to achieve near diffraction limit

33. What if adaptive optics can get us to the diffraction limit? Slit width is set by the diffraction limit:  = D R = r A D d1d1 D = A r d1d1 R d1d1 100000 0.6 cm 1000000 5.8 cm

34. For Peak efficiency the F-ratio (Focal Length / Diameter) of the telescope/collimator should be the same collimator 1/F 1/F 1 F 1 = F F 1 > F 1/f is often called the numerical aperture NA

35. F 1 < F d/1d/1 But R ~ d 1 /  d 1 smaller =>  must be smaller

37. Normal gratings: ruling 600-1200 grooves/mm Used at low blaze angle (~10-20 degrees) orders m=1-3 Echelle gratings: ruling 32-80 grooves/mm Used at high blaze angle (~65 degrees) orders m=50-120 Both satisfy grating equation for = 5000 A

38.       Grating normal Relation between blaze angle , grating normal, and angles of incidence and diffraction Littrow configuration:  = 0,  =  =  m = 2  sin  A = 2 sin  R = 2d 1 tan  D A increases for increasing blaze angle R2 echelle, tan  = 2,  = 63.4 ○ R4 echelle tan  = 4,  = 76 ○ At blaze peak  +  = 2  m b = 2  sin  cos  b = blaze wavelength

40. 3000 m=3 5000 m=2 40009000 m=1 6000 14000 Schematic: orders separated in the vertical direction for clarity 1200 gr/mm grating 2 1 You want to observe 1 in order m=1, but light 2 at order m=2, where 1 ≠ 2 contaminates your spectra Order blocking filters must be used

41. 4000 m=99 m=100 m=101 5000 9000 14000 Schematic: orders separated in the vertical direction for clarity 79 gr/mm grating 30002000 Need interference filters but why throw away light? In reality:

42. collimator Spectrographs slit camera detector corrector From telescope Cross disperser

44. yy ∞ 2 y m-2 m-1 m m+2 m+3   Free Spectral Range  m Grating cross-dispersed echelle spectrographs

45. Prism cross-dispersed echelle spectrographs yy ∞ –1 y

46. Cross dispersion yy ∞   · –1 = Increasing wavelength grating prism grism

47. Cross dispersing elements: Pros and Cons Prisms: Pros: Good order spacing in blue Well packed orders (good use of CCD area) Efficient Good for 2-4 m telescopes Cons: Poor order spacing in red Order crowding Need lots of prisms for large telescopes

48. Cross dispersing elements: Pros and Cons Grating: Pros: Good order spacing in red Only choice for high resolution spectrographs on large (8m) telescopes Cons: Lower efficiency than prisms (60-80%) Inefficient packing of orders

49. Cross dispersing elements: Pros and Cons Grisms: Pros: Good spacing of orders from red to blue Cons: Low efficiency (40%)

50. Important Data reduction issues: 1. Blaze function 2. Scattered Light 3. Reflections

55. „Picket Fence“ or reflected light for Littrow configuration

56. Spectrum of a White Light Source (Flat Lamp) Picket fence:

57. Scattered light Scattered light is light that is scattered into the interorder spacing of echelle spectrographs. All instruments have scattered light at some level or another. This must be removed in the reduction process. Why? A cross section across rows of the spectrum of the white light source Bias level of CCD

58. To determine the abundance of an element in the stellar spectrum you need to measure the equivalent width w IdId IcIc w = I c – I IcIc d ∫ w I d + I s I c + I s IsIs w = I c + I s – (I +I s ) I c + I s w = I c – I I c + I s Scattered light reduces equivalent width ∫ ∫ d d Width of a perfectly black line of rectangular profile that would remove the same amount of flux I

59. So you want to build a spectrograph: things to consider Chose R  product – R is determined by the science you want to do –  is determined by your site (i.e. seeing) If you want high resolution you will need a narrow slit, at a bad site this results in light losses Major consideration: Costs, the higher R, the more expensive

60.  normal  Do I need to tilt the grating to make it fit in my room?

61. Reflective or Refractive Camera? Is it fed with a fiber optic? Camera pupil is image of telescope mirror. For reflective camera: Image of Cassegrain hole of Telescope camera detector slit Camera hole Iumination pattern

62. Reflective or Refractive Camera? Is it fed with a fiber optic? Camera pupil is image of telescope mirror. For reflective camera: Image of Cassegrain hole camera detector A fiber scrambles the telescope pupil Camera hole ilIumination pattern

63. Cross-cut of illumination pattern For fiber fed spectrograph a refractive camera is the only intelligent option fiber e.g. HRS Spectrograph on HET: Mirror camera: 60.000 USD Lens camera (choice): 1.000.000 USD Reason: many elements (due to color terms), anti reflection coatings, etc. Lost light due to hole in mirror

64. Stability: Mechanical and Thermal? HARPS HARPS: 2.000.000 Euros Conventional: 500.000 Euros

65. Tricks to improve efficiency: Overfill the Echelle d1d1 d1d1 R ~ d 1 /  w´ ~  /d 1 For the same resolution you can increase the slit width and increase efficiency by 10-20%

66. Atmospheric Seeing Blurs the Image on Slit slit Lost light R = / d = A r 1  d1d1 D But… You catch more photons, but a wider slit means lower resolution

67. Need to turn this Into this

68. Tricks to improve efficiency: Image slicing The slit or fiber is often smaller than the seeing disk: Image slicers reformat a circular image into a line

69. A modern Image slicer

70. Fourier Transform Spectrometer

71. Interferogram of a monchromatic source: I(  ) = B( )cos(2  n  )

72. Interferogram of a two frequency source: I(  ) = B 1 (  )cos(2  1  ) + B 2 ( 2 )cos(2  2  )

73. Interferogram of a two frequency source: I(  ) =  B i ( i )cos(2  i  ) = B( )cos(2  )d  –∞ +∞ Inteferogram is just the Fourier transform of the brightness versus frequency, i.e spectrum

75. Words of Advice If it is too good to be true it probably isn‘t Lessons learned: 1. „The Phosphorus Stars“ 2. „The Lithium Stars“ 3. „The non-pulsating, pulsating A stars“ „You have to be careful that you do not fool yourself and unfortunately, you are the easiest person to fool“ - Richard Feynman