astronomical observing techniques, lecture 12:...

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


http://www.csr.utexas.edu/projects/rs/hrs/hyper.html

Spectrograph Requirementswavelength rangesimultaneous wavelength coveragespectral resolution R = λ/∆λ

spectral profile (point-spread function)scattered light (far wings of spectral profile)wavelength stabilitywavelength accuracy


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


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


http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productid=1326

Typical Wratten Filters

www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productid=1326


http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productid=1326

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


Schott Colored Glass


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


http://www.sbig.com/products/filters.htm

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


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


http://www.ucolick.org/~bolte/AY257/ay257_2.pdf

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


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


http://www.arcetri.astro.it/science/solare/IBIS_photo.jpeg

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


Sky over La Palma


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


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!)


Dark Sky in Alps


Bright Sky in the Netherlands


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


Airmass: about sec z


Sky Background

use annulus around staruse median instead of mean to neglect cosmic rays


Aperture Photometry

determine location of star by fitting 2-D Gaussian or Moffat PSFmodeldetermine and remove sky background


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


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 )


astronomical observing techniques, lecture 12:...

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