Lecture 12: Imaging Outline 1 Spectroscopy 2 Filters 3 Photometry Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 1
Lecture 12: Imaging
Outline
1 Spectroscopy2 Filters3 Photometry
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 1
Basic Problem of Optical Spectroscopy
www.csr.utexas.edu/projects/rs/hrs/hyper.html
two spatial/angulardimensions, one wavelengthdimensiondetectors are onlytwo-dimensionalneed to slice hyperspectralcubewill have to scan in onedimensionfilters: scan in wavelengthslit spectrograph: scan in onespatial dimensionor multi-object spectroscopyand integral field units
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 2
Spectrograph Requirementswavelength rangesimultaneous wavelength coveragespectral resolution R = λ/∆λ
spectral profile (point-spread function)scattered light (far wings of spectral profile)wavelength stabilitywavelength accuracy
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 3
Broadband Filters
Different Types of Color Filtersdyed gelatin (Kodak Wratten)
advantages: thin, cheap, large sizesdisadvantages: limited optical quality, heat-sensitive
colored glass (Schott, Corning)advantages: stable, rugged, high transmissiondisadvantages: limited bandpasses, limited sizes
interference filtersadvantages: very narrow filters, almost arbitrary bandpass shapeand wavelengthdisadvantages: expensive, very limited sizes,temperature-sensitive, humidity-sensitive, aging
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 4
Wratten Filters
www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productid=1326
colored plastic sheetsnamed after Frederick Wratten, manufactured by Kodak for about100 yearsrecently: Wratten 2up to 100 mm by 300 mmcan be used for experimentsimproved performance when put between glass plates
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 5
Typical Wratten Filters
www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productid=1326
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 6
Colored Glasstypically useful from 200 - 1000 nmSchott
UG: Black and blue glasses, UV transmittingBG: Blue, blue-green, and multi-band glassesVG Green glassGG: Nearly colorless to yellow glasses, IR transmittingOG: Orange glasses, IR transmittingRG:Red and black glasses, IR transmittingNG:Neutral density glasses with uniform attenuation in the visiblerangeN-WG: Colorless glasses with different cutoffs in the UV,transmitting in the visible range and the IRKG: Virtually colorless glasses with high transmission in the visibleand effective absorption in the IR (heat protection filters)
CorningHoya
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 7
Schott Colored Glass
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 8
Schott Colored Glass
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 9
Schott Colored Glass
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 10
Schott Colored Glass
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 11
Schott Colored Glass
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 12
UBVRI
www.sbig.com/products/filters.htm
UBV by Johnson and Morgan (1953)VRIJKLMNQ (infrared) by Johnson (1960)(combinations of) glass filtersinvented to classify stars with photomultiplierszero point of B-V and U-B color indices defined to be zero for A0V stars
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 13
Limitations of UBVRI Photometrylimited spectral resolutioneffective central wavelength changes with color of starstar’s magnitudes and color depend on the star’s colorshort-wavelength side of U filter extends below atmospherictransmission cutoffproperties of sky define width of bandpass, not filterno clean separation of information from different filtersdifferent detectors have different sensitivitiestoday: Bessel or Cron/Cousins UBVRI with CCDs
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 14
Other Filter Systems
http://www.ucolick.org/b̃olte/AY257/ay257_2.pdf
other filter systems haveless overlap and/or highertransmissionJohnson system designedto measure properties ofstarsThuan-Gunn filters for faintgalaxy observationsStromgren has bettersensitivity to stellarproperties (metallicity,temperature, surfacegravity)Sloan Digital Sky Survey(SDSS) for faint galaxyclassification
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 15
Interference filters
thin film:layer with thickness . λextends in 2 other dimensions� λ
reflection, refraction at all interfaceslayer thickness di . λ⇒interference between reflected andrefracted waves
:
ns
n1, d1
n2, d2
nL, dL
n3, d3
nm
90º
!0
L layers of thin films like Fabry-Perots: thin film stacksubstrate (index ns) and incident medium (index nm) have infinitethicknesscan be taylored to almost any specificationssensitive to temperature, humidity, angle of incidencecan tune filter in wavelength by changing temperature and angleof incidence
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 16
Fabry-Perot Tunable Filter
http://www.arcetri.astro.it/science/solare/IBIS_photo.jpeg
main ingredient is Fabry-Perot with tunable plate separationstability of cavity spacing is criticaloften combine two or more tunable elementsalways need interference prefilter
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 17
Photometrygoal: determine flux of an astronomical object in well-definedwavelength rangeproblems:
seeingextinctionsky backgroundtelescope, instrument, detector
calibration with (standard) starsall-sky photometry: compare objects all over the skydifferential photometry: compare objects on same CCD exposure
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 18
Sky over La Palma
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 19
Atmospherephotometric night = uniform, stable sky conditionsthin clouds and cirrus are hard to see at nightbright, grey and dark observing timeseeing defines size of stellar image for large telescopesimage = convolution of source and Point-Spread Function (PSF)integral over image = integral over source
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 20
Air Massextinction due to atmosphere proportional to amount of air alongline-of-sightairmass = amount of air one looks throughat zenith (z = 0): airmass=1.0absorption coefficient K = 2.5 log(f (z)/f (z = 0) where f (z) ismeasured flux as function of zenith distance zK ∆X = ∆m change in magnitude is absorption coefficient timeschange in airmassmeasure magnitude for different airmasses and extrapolate tozero airmass (no atmosphere!)
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 21
Dark Sky in Alps
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 22
Bright Sky in the Netherlands
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 23
Photometric Observationsobserve object and standard stars with different colorsobserve standard stars at low and high airmass to determine their(color-dependent) extinction coefficientsreduce images with bias, dark, flat fieldmeasure fluxes fi with aperture photometrycalculate instrumental magnitudes: minst = −2.5 log10(fi/texp)
calibrate instrumental magnitude for zero airmass:m0 = minst − K sec zzero point from standard stars: mzp = mstd −mstd,inst
remove zero point: m = mzp + minst
absolute photometry: determine actual magnitudes with transferequations derived from standard starsdifferential photometry does not require calibrations
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 24
Airmass: about sec z
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 25
Sky Background
use annulus around staruse median instead of mean to neglect cosmic rays
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 26
Aperture Photometry
determine location of star by fitting 2-D Gaussian or Moffat PSFmodeldetermine and remove sky background
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 27
Aperture Photometry (continued)optimum aperture size depends on brightness of startrade-off between capturing more starlight and adding noise fromsky background, detector etc.plot result as function of increasing aperture and estimateasymptotic valuefor variability measurements (e.g. exoplanet transit) withdifferential photometry (standard star in same image):
use fixed and identical apertures for target and reference star(s)optimum aperture minimizes scatter in constant parts ofmtarget −mstd
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 28
Color Correctionextinction depends strongly on wavelength (color)measured flux through broad filter depends on stellar spectrumadditional correction: m = m0 + k sec z + k2C sec z where C iscolor index (e.g. C = B − V )
Christoph U. Keller, Leiden Observatory, [email protected] Astronomical Observing Techniques, Lecture 12: Imaging 29