Optical/NIR Spectroscopy A3130. John Wilson Univ of Virginia

Transcription

1 Optical/NIR Spectroscopy A3130 John Wilson Univ of Virginia

2 Topics: Photometry is low resolution spectroscopy Uses of spectroscopy in astronomy Data cubes and dimensionality challenge Spectrograph design basics Slit Collimation Dispersion Camera Detector Characterization of spectrographs (wavelength range, resolving power, Efficiency, number of objects) Specific elements: Prisms Plane Reflections Gratings Grisms Volume Phase Holographic Gratings Fabry Perot Interferometry Spectral calibration

3 Spectrograph Design Basics Telescope Focal Plane A = d /d Light from Telescope dl = f cam d

4 Types of Dispersers: Prisms Generally used in minimum deviation mode no astigmatism. Can be used across more than one octave (factor of 2 in wavelength), so great for cross dispersers. Schroeder, Ch. 3 Poor choice for UV (few glasses transmit well in the UV)

5 Types of Dispersers: Prisms Angular Dispersion using Fermat s Principle of Least Time travel time (optical path difference) is equal for closely adjacent paths Path Length Difference (index of refraction x distance) between top ray and bottom ray: 1 n prism t = n air 2L cos ( ) Differentiate with respect to wavelength: Schroeder, Ch. 3 t dn/d = 2L sin ( ) (d /d ) t dn/d = 2L sin ( ) (d /d ) (d /d ) Can simplify to based on geometry and differentiation to: A = (d /d ) = (t/a) (dn/d )

6 Most materials have a dispersion curve (variation of index with wavelength) that conform to: n( ) = A + B/ 2 Differentiating: dn/d = 2B/ 3 So finally: A = (d /d ) = (2t/a)(B/ 3 ) Negative sign: decreases as increases blue light is deviated more

7 FIRE: Folded port InfraRed Echelette on Magellan Telescope Magellan Telesceope FIRE

8 Continuous spectrum 2.5 um mirror R~250 spectrum of (J=16.7) T dwarf with FIRE (Magellan) in prism dispersed mode. (subtraction of two 150 sec exposures) the sky report from fires commissioning run/

9 Types of Dispersers: Diffraction Gratings Diffraction Gratings are poorly named they operate based on constructive and destructive interference of light. TripleSpec (APO) plane reflection grating Diffraction Gratings, particularly plane reflection gratings, are the work horse dispersing optic for moderate high resolution spectrographs in astronomy

10 Types of Dispersers: Diffraction Gratings Single Slit Diffraction Destructive interference when b/2 sin( m ) = m * /2 sin( m ) = m /b Intensity Pattern I( )= I(0) sinc 2 ( ) = I o [(sin )/ ] 2 Where = (2 / )(b/2)sin Figures from Hecht, Optics, Ch. 10

11 Types of Dispersers: Diffraction Gratings Double Slit Diffraction Constructive interference when a sin( ) = m a Notice: As a gets smaller, gets larger a /p212/lectures/node30.html As m, the order, gets larger, gets smaller This assumes parallel light is incident normal to the slits

12 Single slit intensity envelope based on single slit width b Fringes created by double slit based on slit separation a Figure from Hecht, Optics, Ch. 10

13 Multiple Slit Intensity Pattern I( )= I(0) (sin ( )/ ) 2 (sin (N )/sin ) 2 Where = (2 / )(b/2)sin = (2 / )(a/2)sin Figure from Hecht, Optics, Ch. 10

14 Types of Dispersers: Diffraction Gratings Finally we can simply write down the more general grating equation based on path length differences when parallel light is incident on the grating at some arbitrary angle : m / = sin + sin Angular Dispersion for a grating from differentiation: A = d /d = m/( cos ) Chromey Ch. 11

15 Types of Dispersers: Diffraction Gratings Ruling engines burnish (scrape) material away in a highly controlled manner MIT B Ruling Engine Richardson Grating Labs

16 Types of Dispersers: Diffraction Gratings We purchase replicas of gratings: Inverse shape of a submaster, which ultimately derives from a master, recorded into resin A custom coating is applied to the resin to make it reflective Diffraction Grating Handbook Richardson Grating Lab The substrate can be the user s choice, e.g. aluminum or fused silica

17 Types of Dispersers: Diffraction Gratings Blazed Gratings Steer the single slit diffraction peak at zero th order to a more useful order Variety of blaze angles, groove frequencies available Grating equation becomes Chromey Ch. 11 sin ( + ) + sin ( ) = m / Littrow mode : incident and exit beams orthogonal to facet for highest effieciency

18 110 l/mm groove frequency Blaze angle 22 deg Blaze wavelength 6.2 micron order Blaze wavelenth TripleSpec (APO) plane reflection grating /2 = 3.1 Transmission / Efficiency Triplespec Grating Efficiency 2nd Retest v. 1st Retest Mauna Kea Atm Trans UVA Order 3 Unpol 1st Retest UVA Order 4 Unpol 1st Retest UVA Order 5 Unpol 1st Retest Cornell Order 3 Unpol 1st Retest Cornell Order 4 Unpol 1st Retest Cornell Order 5 Unpol 1st Retest UVA Order 3 Unpol 2nd Retest UVA Order 4 Unpol 2nd Retest UVA Order 5 Unpol 2nd Retest UVA Order 6 Unpol 2nd Retest UVA Order 7 Unpol 2nd Retest Cornell Order 3 Unpol 2nd Retest Cornell Order 4 Unpol 2nd Retest Cornell Order 5 Unpol 2nd Retest Cornell Order 6 Unpol 2nd Retest Cornell Order 7 Unpol 2nd Retest 3 6.2/3 = /4 = 1.55 Etc Wavelength (nm)

19 Types of Dispersers: Diffraction Gratings Echelle Gratings Operate in high order and steep incidence angle to generate high angular dispersion and thus high resolution Chromey Ch. 11 A = d /d = m/( cos )

20 HARPS at ESO 3.6 m Telescope R ~ 115,000 with mosaic echelle grating

21 Types of Dispersers: Diffraction Gratings Problem of order overlap m m = (m + 1) (m + 1) There will be a free spectral range in each order where there is no overlap with adjacent orders. Chromey Ch. 11

22 Types of Dispersers: Cross Dispersed Use a second dispersing optic in the opposite dimension to disentangle overlapping orders of a grating Often use a prism Chromey Ch. 11 One can use another grating but be careful not to exceed a factor of 2 (octave) in wavelength

23 Continuous spectrum 2.5 um mirror R~250 spectrum of (J=16.7) T dwarf with FIRE (Magellan) in prism dispersed mode. (subtraction of two 150 sec exposures) the sky report from fires commissioning run/

24 Echelle Mode 2.5 um grating 0.8 R~6000 spectrum of (J=20) quasar with FIRE (Magellan) in echelle mode. (subtraction of two 900 sec exposures) 30/fire on the sky report from firescommissioning run/

25 Types of Dispersers: Grism Grism : Grating + Prism Uses a prism to allow center wavelength of a diffracted order to go straight through Richardson Grating Lab Very useful for providing moderate resolution spectroscopy in a traditional camera layout

26 LMIRCam 3 5 micron imager at LBT Grisms used here at pupil conjugate immediately before final focus at detector

27 Grism 1 (40.0 l/mm) Efficiency (unpolarized) L band Wavelength (um) Order 1 Atm Transmission

28 Order sorting for Grism 2 done with K and M band filters. Grism 2 (32.0 l/mm) Efficiency (unpolarized) K band M band Wavelength (um) Order 1 Order 2 Atm Trans

29 Figure 9. Photo taken during the machining process shows the tip of the diamond tool extending from a holder on the rotating spindle. A spray of light mineral oil from the right acts as a coolant and cutting fluid as well as clearing chips from the workpiece.

30 Zygo interferometer measurement on grism #6 shows surface error in grooves to be 0.10 waves peak to valley at 633 nm over the full 14 x 14 mm aperture of the grism 0.10 waves is 63 nm.

31 SEM photos of grism #3 (40 lines/mm). On the left are details of several grooves showing the very flat and smooth blazed surfaces. On the right is a magnified view of a single groove showing the very sharp groove angle.

32 HD L band Grism

33 Chromey Ch. 11

34 Types of Dispersers: Volume Phase Holographic (VPH) Grating Volume 3 dimensional: thickness few 100 s of m film capable of recording fringes Phase Diffractive element works as a phase grating, not a surface relief grating Optical phase = n*d Chromey Ch. 11 Holographic fringes in the grating are recorded through holography, not mechanical means such as ruling

35 Types of Dispersers: Volume Phase Holographic (VPH) Grating Historical Uses Aircraft Heads up Displays Telecom Industry Laser Pulse Compression

36 Types of Dispersers: Volume Phase Holographic (VPH) Grating Advantages: High theoretical (and realized) efficiency Low scatter Once capped, environmentally stable and cleanable Transmissive optic very helpful in designing spectrographs Can record a broad range of fringe frequency

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38 Types of Dispersers: Volume Phase Holographic (VPH) Grating Disadvantages: Few vendors available to make good VPH s Art and science Depending on design, may not get as broad an efficiency envelope as a ruled grating. Edges of efficiency envelope droop Hard to get higher orders

39 Dichromated Gelatin Gelatin: Gelatin is a mixture of peptides and proteins produced by partial hydrolysis of collagen extracted from the skin, boiled crushed bones, connective tissues, organs and some intestines of animals such as domesticated cattle, chicken, and pigs. The natural molecular bonds between individual collagen strands are broken down into a form that rearranges more easily. Wikipedia No artificial source has been found to have better properties. Dichromated: Dichromate, together with UV or blue light, cross links gelatin. Crosslinked gelatin is insoluble in water. Processing in water bath (swelling), followed by rapid dehydration in alcohol bath (collapse), produces periodic density variation (index of refraction) variation in gelatin. Barden et al. 2000, PASP; Optical Holography

40 Forming an Interference Pattern Light from a Coherent Source Beam Splitter Laser wavelength and relative beam angle determines interference fringe spacing Put the film here! Steering Mirror (x4)

41 Recording a Volume Phase Transmission Grating Fringe planes recorded perpendicular to the film surface result in a transmission grating.

42 Reconstruction of a VPH Transmission Grating Single wavelength in Single wavelength out Multiple wavelengths in Single wavelength out at it s unique angle

43 Volume Phase Holographic Grating Physical Construction Anti reflection Coated Surface (optional) Optical Adhesive Glass cover Glass substrate Thin film with recorded grating Anti reflection Coated Surface (optional)

44 Gratings: Anamorphic Magnification Anamorphic Magnification = Different plate scales in the slit width and slit length directions f = f/# D so focal length for a given camera (f/#) will be different for each direction Schroeder, Astronomical Optics

45 Gratings: Anamorphic Magnification Both normal to camera or normal to collimator orientation satisfy grating equation. But choice influences efficiency, order format, resolution and scattered light Shadowing by this ledge Reflection back towards collimator because of this ledge Allington Smith 2002

46 Gratings: Anamorphic Magnification Peak Efficiency drops, decreases when go off littrow in normal to camera case ( > ) Schroeder, Astronomical Optics

47 Gratings: Anamorphic Magnification Example: Triplespec d1 d1 d2 d2 r = d1 / d2 = / = 0.79

48 Gratings: Anamorphic Magnification Example: Triplespec Slit Width (dispersion direction) 2.8 pix / arcsec Slit Length (x dispersion direction) 3.5 pix / arcsec r = 2.8 / 3.5 = 0.8

49 Final Resolution equation: read Chromey section 10.4 Ways to increase R: Higher order, higher exit angle, smaller seeing As work at larger telescopes need larger collimators (spectrographs) Chromey Ch 11

50 Wavelength Calibration How do we convert from pixel position (x,y) on detector to? Use a calibration source with known spectral lines Empirically derive a solution (f(x,y) = ) using mathematical fitting routines

51 Wavelength Calibration Why isn t the wavelength proportional to pixel position, I.e. linear? Optics of Prism & Gratings, our primary dispersion elements have nonlinear angular dispersions A dn/d (prism) A m/cos (grating)

52 Wavelength Calibration Spectral Source Regime Resolution Arc Lamp (e.g. Argon, Neon, etc.) UV-NIR all, but watch for blended lines OH lines NIR R > 600 to resolve blends Planetary Nebulae (H/He emission lines) VIS-MIR low (insufficient lines for high res)

53 Wavelength Calibration How often must one observe a calibration source? As often as required to be certain the solution has not changed. Depends on instrument mechanics, resolution, and observation program Why would the solution change? Movement of the orders on the detector Mechanical Flexure of Instrument Optics Changing thermal conditions inside instrument