Optical/NIR Spectroscopy A3130 John Wilson Univ of Virginia
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
Broad & Narrow band Photometry: Low resolution methods of spectroscopy RIT
Broad & Narrow band Photometry: Low resolution methods of spectroscopy Planets around HR 8799: Comparing intensity in various photometric bands with model atmospheres Skemer et al 2012
Broad & Narrow band Photometry: Low resolution methods of spectroscopy HST WFC3 Narrow Band Filters: Resolving Power R = / 15.2 nm Filter F126N R ~ 83 1258.5 nm http://www.stsci.edu/hst/wfc3/documents/handbooks/currentihb/c0 7_ir06.html#400352
Uses in Astronomy: Understanding astrophysical processes Spectral Classification of Y dwarfs Sub stellar objects with T_eff near room temperature (300K) Kirkpatrick et al. 2012
Uses in Astronomy: Determining a Redshift Detection of a red shifted Lyalpha line for a galaxy in the Hubble Ultra Deep Field An investment of 14.8 hours on VLT. Airglow lines Lehnert 2010
Uses in Astronomy: Determining a Rotational Velocity Discovery of fast rotating B stars with APOGEE Display rigidly rotating magnetospheres common to this type of object Double horned line profiles with 1000 km/sec velocity width Eikenberry et al 2014, submitted
Spectrograph Design Basics Telescope Focal Plane A = d /d Light from Telescope dl = f cam d
No Slit A spectrograph is simply a camera that produces images of the focal plane spread out in wavelength (color)
The slit is critical for defining an appropriately sized field of view at the focal plane so the desired images, each spanning a discrete span in wavelength (color), don t overlap.
Spectrograph Design Basics: Slit Why do we need a slit? Overlapping spectra Resolution will vary with the seeing instead of being stably defined by the geometry of the slit and spectrograph Difficult data reduction without it STIS UV Prism Mode Image of OB Association NGC 604 http://www.stsci.edu/hst/stis/documents/handbooks/currentihb/c04 _spectros5.html
Spectrograph Design Basics: Slit What is a slit? A rectangular aperture that limits the spatial location on the sky which feeds the spectrograph
Spectrograph Design Basics: Slit What is a slit? A narrow rectangular aperture that limits the spatial location on the sky which feeds the spectrograph TripleSpec Slit on Saturn
Spectrograph Design Basics: Slit Why do we need a slit? Overlapping spectra Resolution defined by the seeing A better way: use a multi object spectrograph with slit masks LRIS (Keck) slit mask and spectra
Spectrograph Design Basics: Slit Long Slit spectrographs Workhorses for understanding nearby galaxies/extended objects Slide from M. Whittle
Aside: Data Cubes and Dimensionality Challenge 6 dimensions of information: 2 D detectors 2 spatial 1 spectral 1 temporal 2 polarization Consider the following trades: Wide wavelength coverage (or high resolution in multiple orders) for a single object Shorter wavelength coverage (or lower resolution) for numerous short slits throughout a 2 d field. Long slit for long spatial coverage in 1 d
Spectrograph Design Basics: Collimator The Collimator creates uniformly parallel light Parallel light is necessary for using most dispersers It should have the same focal ratio (f/#) as the beam incident on the slit It should be large enough to sufficiently illuminate the disperser Can be either refractive or reflective (or both)
Spectrograph Design Basics: Collimator D tel D col f/7.5 = f col / D col f/7.5 = f tel / D tel
Spectrograph Design Basics: Disperser The Disperser is an optic that changes the direction of light as a function of wavelength. Characterized by Angular Dispersion A = d /d Different types of dispersers have different forms of angular dispersion
Spectrograph Design Basics: Disperser Examples Prisms Plane Reflection Grating Volume Phase Holographic Grating Grisms
Spectrograph Design Basics: Camera The Camera converts angles to position at the focal plane (detector) to form the spectrum.
Spectrograph Design Basics: Camera du Pont Telescope Nothing magical here exactly what the telescope does when it collects collimated light from distant objects Rays colored by field (position on the sky)
Spectrograph Design Basics: Camera Camera considerations: Needs to be large enough to collect the angularly dispersed light from the disperser the farther away from the disperser, the larger it needs to be dl = f cam d Focal length f cam will dictate the width (dl) of the slit image on the detector. The choice of camera focal length needs to take into account the detector pixel size for proper sampling.
Spectrograph Design Basics: Detector Detector considerations: Size constrained by $$, vendor offerings Pixel size again, often constrained by vendor offerings, usually 10 20 micron/ pixel dl = f cam d Response v. wavelength Operating Temperature Noise Characteristics
Characterization of Spectrographs Wavelength Range, how many modes (instrument settings) needed Efficiency sensitivity Slit Width, how it matches to the median seeing Sampling of the slit width is it at least critically sampled (2 pix per slit width)? Multiplexing Capability (Number of Objects/Length of Slit/IFU?) Resolution Resolving Power /
Characterization of Spectrographs Resolving Power What resolving power does your science require? R ~ 300 is sufficient for characterizing broad spectral features for classification of unknown objects Wilson et al. 2001
Characterization of Spectrographs Resolving Power 12000 10000 R = 1000 R = 2000 R = 3000 What resolving power does your science require? Detection between airglow lines? Relative Intensity 8000 6000 4000 2000 12000 0 1.5 1.55 1.6 1.65 1.7 1.75 1.8 Wavelength 10000 R = 1000 R = 2000 R = 2500 Relative Intensity 8000 6000 4000 2000 0 1.63 1.64 1.65 1.66 1.67 1.68 Wavelength Plots courtesy T. Herter
Characterization of Spectrographs Resolving Power What resolving power does your science require? Abundance determination of atomic lines usually requires R >= 15,000 CN S OH CN CN CN Fe S Fe S Fe CN Fe Fe Fe CN? CN Fe Fe Fe OH OH CN CN
Characterization of Spectrographs Resolving Power Empircal measurements in zemax of for discrete wavelengths
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)
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 )
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