Astronomical Techniques

Transcription

1 Astronomical Techniques Spectrographs & Spectroscopy

2 Spectroscopy What is spectroscopy? A little history. What can we learn from spectroscopy? Play with simple spectrographs. Basic optics of a spectrograph. Gratings, how they work, and the grating equation. Astronomical spectrographs The meanings and importance of slit width Resolution, Linear Dispersion, Anamorphic Magnification,... Multi-object spectroscopy and alternate spectroscopic observing modes. How to plan observing with a spectrograph.

3 What is Spectroscopy Split light by: wavelength frequency energy Electromagnetic Radiation

4 History: Newton Text

5 Newton Explained observed colors of rainbow due to reflection and refraction of sunlight. However a prior correct published explanation existed by the Dominican friartheodoric of Freiberg ( ) Netwon carried out extensive experiments with sunlight and prisms and lenses (Newton 1704, Opticks) Resolution limited by size of beam on prism ~one inch Unable to resolve any absorption bands in solar spectrum

6 Spectrum projected on wall Newton Prism Sunlight illuminates screen with hole Quasi collimated beam ~One inch spot of sunlight on prism

7 History: Fraunhofer The breakthrough came in 1814 by Joseph Fraunhofer Observed the solar spectrum with even higher resolution and better efficiency. Used sunlight, narrow slit, prism, small telescope behind the prism to observe the dispersed light. --more efficient use of the incident light --better spectral resolution. He identified more than 500 dark lines of the solar spectrum ( Fraunhofer, 1817 ). --comparison observation of other sources to identify spectral lines --also used a grating made of wires, and grating equation to define a wavelength scale. Proved that the dark lines (Fraunhofer lines ) are an intrinsic property of the solar spectrum. Also observed bright stars and Venus. How did their spectra look compared to the sun?

8 Fraunhofer also observed a few bright stars and of the planet Venus. How did their spectra look compared to the sun?

9 Fraunhofer also observed a few bright stars and of the planet Venus. How did their spectra look compared to the sun? Not all stars are the same color as the sun Color ~= Temperature => different spectral lines Clouds on Venus reflect/scatter sunlight like clouds on earth

10 1859 Kirchoff and Bunsen found lines observed from various chemical elements match wavelengths and patterns in the sun

11 What breakthrough did we need to understand spectra?

12 What breakthrough did we need to understand spectra? Quantum Mechanics

13 Spectroscopy Lecture 2 Why use spectroscopy? What can it tell us that photometry can t?

14 Spectroscopy Lecture 2 Why use spectroscopy? What can it tell us that photometry can t? Details that are unresolved in the with of the photometric bands Stars: Detailed information about their atmospheres Spectral line strengths => temperature, element abundance surface gravity Galaxies: The mix of star present (when can t resolve individual stars for photometry) Properties of the gas, abundance, temperature, density What physical processes are occuring, star formation, central BH (AGN) Radial velocities => Stellar kinematics from individual stars Kinematics of galaxies from integrated spectra Galaxy velocities => Cosmology, expansion history of the universe large scale structure

15 Stellar spectra webpage

16 Galaxy Spectra

17 Quasars

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19 Recap: What did you learn by looking at spectra by eye last class?

20 Spectrographs Spectroscopy arguably most powerful way to study astronomical objects.

21 D) Spectrographs Spectroscopy arguably most powerful way to study astronomical objects. Today we will discuss... Design of spectrographs Diffraction gratings Spectral resolution Multi-object spectroscopy Solar spectrum (N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF)

22 Design of Spectrographs Light is dispersed according to wavelength and observed (spectroscope) recorded (spectrograph) Astronomical spectrograph placed at focal plane of telescope Newton observed spectra of Sun but spectrographs not commonly used in astronomy until late 1800s

23 Design of Spectrographs Slit placed on object of interest at focal plane block out unwanted light Light collimated then dispersed into spectrum prism or grating Brought to focus on a detector

24 Design of Spectrographs Light dispersed according to wavelength Form multiple images of target and other sources offset in dispersion direction Images overlap confusion between extent of source and wavelength of light

25 Design of Spectrographs Slit removes unwanted light from target and other sources in field Form series of images of slit

26 Design of Spectrographs Slit removes unwanted light from target and other sources in field Form series of images of slit creates continuous spectrum

27 Grating Spectrographs Early spectrographs used prism as dispersive element refractive index of material depends on wavelength recall chromatic aberration

28 Grating Spectrographs Early spectrographs used prism as dispersive element refractive index of material depends on wavelength recall chromatic aberration Modern spectrographs use diffraction grating plate etched with hundreds/thousands of grooves per mm Transmission grating diffracts light passing through it Reflection grating diffracts light reflecting off it Light is dispersed in direction perpendicular to grooves

29 Grating Spectrographs Gratings disperse light via principal of diffraction If groove spacing (σ) similar to λ then light rays diffracted each groove acts as individual light source rays in different directions interfere, only those in certain directions interfere constructively Note sign convention + normal to macroscopic surface

30 Constructive interference occurs when path difference between rays from adjacent grooves is mλ recall e.g. Young's slits +

31 Multi Slit Diffraction Examples

32 Constructive interference occurs when path difference between rays from adjacent grooves is mλ recall e.g. Young's slits Path difference = σsin(α) + σsin(β) (note + sign) +

33 Constructive interference occurs when path difference between rays from adjacent grooves is mλ recall e.g. Young's slits Path difference = σsin(α) + σsin(β) (note + sign) Constructive interference mλ = σsin(α) + σsin(β) Grating Equation +

34 mλ = σsin(α) + σsin(β) Grating Equation For given σ and α, diffraction angle depends on λ light dispersed into spectrum +

35 mλ = σsin(α) + σsin(β) Grating Equation For given σ and α, diffraction angle depends on λ light dispersed into spectrum Constructive interference possible for different integers m Produce fainter spectra either side of central max Called ±1 st order, ±2 nd order etc +

36 Central maximum (m=0) called zeroth order no dispersion image, not a spectrum Typically record one of the orders light wasted in other orders Blazing the grating concentrates ~70% light into one of 1 st order spectra grooves non-symmetrical with blaze angle θ 45

37 Grating Example Light from a star is incident on reflection grating of 600 grooves/mm at angle of 5 to normal. At what angles will photons at 5000Å be dispersed in (i) the 1 st and (ii) 2 nd order spectra? (iii) At what angle will photons of 2500Å be dispersed in 2 nd order?

38 Grating Example Light from a star is incident on reflection grating of 600 grooves/mm at angle of 5 to normal. At what angles will photons at 5000Å be dispersed in (i) the 1 st and (ii) 2 nd order spectra? (iii) At what angle will photons of 2500Å be dispersed in 2 nd order? Groove spacing σ = 1/600 = mm Convert λ to same unit: λ = mm

39 Grating Example Light from a star is incident on reflection grating of 600 grooves/mm at angle of 5 to normal. At what angles will photons at 5000Å be dispersed in (i) the 1 st and (ii) 2 nd order spectra? (iii) At what angle will photons of 2500Å be dispersed in 2 nd order? Groove spacing σ = 1/600 = mm Convert λ to same unit: λ = mm Get β from grating equation: mλ = σsin(α) + σsin(β) sin(β) = (mλ σsin(α))/σ For λ=5000å we have β 1 =12.3 (m=1) and β 2 =30.9 (m=2)

40 Grating Example Light from a star is incident on reflection grating of 600 grooves/mm at angle of 5 to normal. At what angles will photons at 5000Å be dispersed in (i) the 1 st and (ii) 2 nd order spectra? (iii) At what angle will photons of 2500Å be dispersed in 2 nd order? For λ=5000å we have β 1 =12.3 (m=1) and β 2 =30.9 (m=2) sin(β) = (mλ σsin(α))/σ For λ=2500å and m=2 we have β 2 =12.3 Different order spectra overlap!

41 Free Spectral Range For fixed σ and α, at given dispersion angle β, wavelength decreases with increasing order number 1λ 1 (β) = 2λ 2 (β) = 3λ 3 (β) = mλ m (β) where λ m (β) is wavelength of light at angle β in mth order mλ = σsin(α) + σsin(β)

42 Free Spectral Range For fixed σ and α, at given dispersion angle β, wavelength decreases with increasing order number 1λ 1 (β) = 2λ 2 (β) = 3λ 3 (β) = mλ m (β) where λ m (β) is wavelength of light at angle β in mth order N.B. separated for clarity

43 Free Spectral Range For fixed σ and α, at given dispersion angle β, wavelength decreases with increasing order number 1λ 1 (β) = 2λ 2 (β) = 3λ 3 (β) = mλ m (β) where λ m (β) is wavelength of light at angle β in mth order Free spectral range Δλ is difference in λ between adjacent orders at given β tells us how bad overlap is For order m Δλ m = λ m λ (m+1) N.B. for some fixed value of dispersion angle β dropped (β) for clarity

44 Free Spectral Range Free spectral range in m=1 at value of β below is difference in wavelength between red light in m=1 and blue/violet light in m=2

45 Free Spectral Range Δλ m = λ m λ (m+1) But recall 1λ 1 (β) = 2λ 2 (β) = 3λ 3 (β)... mλ m (β) = (m+1)λ (m+1) (β) So m = m 1 m Text m 1 m 1 m = 1 m m 1

46 m = 1 m m 1 e.g. at some β, λ =5000Å giving Δλ =5000Å 2 1 so λ =10,000Å 1 light with λ<10,000å in 1 st order overlaps λ<5,000å in 2 nd order

47 m = 1 m m 1 e.g. at some β, λ 2 =5000Å giving Δλ 1 =5000Å so λ 1 =10,000Å light with λ<10,000å in 1 st order overlaps λ<5,000å in 2 nd order Note free spectral range decreases with increasing m Overlap increases in higher orders Limits useful wavelength range of spectroscope Alleviate by using filters to remove unwanted wavelengths that overlap with region of interest Or we could use a cross dispersing grating or prism, talk about Echelle spectrograph later

48 Blazing the grating concentrates ~70% light into one of 1 st order spectra grooves non-symmetrical with blaze angle θ 45

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51 D3) Spectral Resolution Angular dispersion of grating is variation of β with λ Differentiate grating eqn w.r.t. λ: d d = m cos Angular dispersion is (Units typically radians) inversely proportional to σ proportional to m Increase angular dispersion by increasing number grooves per mm in grating working in higher orders

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53 D3) Spectral Resolution Resolution of a spectrograph described by resolving power R R = λ/δλ δλ is difference in λ between two spectral features that can just be distinguished at focal plane c.f. angular resolution of telescope Low resolving power: R < 10,000 High resolving power: R > 40,000 R depends on angular dispersion and size of slit image physical size of slit optics of spectroscope

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61 The importance of seeing and why its hard to build spectrograph for large telescopes

62 Multi-Object Spectroscopy We can observe more than a single object at the center of the slit Long Slit --- slit covers part or all of a galaxy --- aligned to hit 2 or more stars Multi Slits --- Can get multiple objects over field of view of spectrograph Typically get 100s of objects Fiber spectroscopy s to 1000s of objects, depends on number of fibers Different type of fiber placement in the focal plane - Plug into predrilled holes (SDSS) - Robot to place in focal plane (AAT, 2dF) - Fibers arranged in grid, each fiber can move within its own grid element (BigBoss) Integral field unit

63 What would the spectrum look like if you observed an edge on spiral galaxy with the slit along the major axis? Lets just consider the H alpha emission line.

64 D4) Multi-object Spectroscopy If slits used, produce slit mask Metal plate with slits cut at target positions Place at focal plane Light passes through slits onto grating Form many offset spectra on the detector

65 What would the spectrum look like from a multi-slit spectrograph? Lets consider simple case shown below for the black slits

66 What would the spectrum look like from a multi-slit spectrograph? Lets consider simple case shown below for the black slits Next Whats happens to the spectra from the upper left black slit and the red slit?

67 D4) Multi-object Spectroscopy If slits used, produce slit mask Masks prepared in advance based on imaging data Slits must not overlap in dispersion direction Limits number possible slits for field

68 D4) Multi-object Spectroscopy Alternative place fibre optic cables at object positions Metal plate with holes at target positions Place at focal plane Fibres carry light to grating Form many offset spectra on the detector

69 Fiber optics n2 < n1 Total internal reflection index of refraction where, cos(theta) > n2 / n1 The light losses for internal total reflection are very small (if theta < theta_max) Flux loss occurs as a result of Rayleigh scattering at the glass molecules by impurities, and due to defects in the glass due to stress. (bending abuse) Rayleigh scattering is proportional to lambda**(-4) => worse at short wavelengths. Molecular absorption becomes critical in the IR. Multi-mode fibers used for astronomy. Core diameter typically ~ 2 seeing FWHM ( um diameter) Real fibers are not perfect!!! Degrade (increase) the input focal ratio from the telescope. f/12 in ~> f/7 out, f/5 in ~ f/4 out Fibers scramble the light azimuthly

70 D4) Multi-object Spectroscopy Alternative place fibre optic cables at object positions Masks prepared in advance based on imaging data Targets may be close together in dispersion direction Suffer light losses in fibres

71 SDSS Fibers, Slits and Plates Focal plane is 3deg diameter = 7sq deg area = HUGE Large field => bigger chance of having lots of the object you care about in the field

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73 SDSS Spectrograph optical design Schematic top view Schematic side view

74 Blue Camera

75 CCD dewar Why do we have to keep CCDs cold?

76 SDSS Spectrograph Throughput

77 Blue Chanel Red Chanel He Ar Ne arc emission line calibration image

78 Integral field spectroscopy 2d fibers array focal plane led to slit in spectrograph Difficult to get contiguous coverage of focal plane without using lenslets But lenslets change telescope f/number

79 Can also dither to get complete spatial coverage with IFU

80 IFU Surveys instruments/kmos.html

81 Multi-Object versus Long Slit What are the pros and cons of various spectral observing modes? - Long slit - Multi-Slit - Fibers - IFU

82 Multi-Object versus Long Slit What are the pros and cons of various spectral observing modes? - Long slit - Multi-Slit - Fibers - IFU Things to consider: Spatial coverage of object Sky subtraction Number of objects observed Efficiency, throughput Fiber collisions in focal plane Multi-slit spectra collisions Bright / faint objects contamination, adjacent on slit Bright / faint object optimum exposure time