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##### Observational Astronomy

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**Observational Astronomy**SPECTROSCOPICdata reduction Piskunov & Valenti 2002, A&A 385, 1095**Echelle spectrum image looks strange …**Spectral orders Spectral line**Data reduction concept**Complex data layout requires in addition to the usual CCD calibrations: • Spectral order localization • Extracting 2D data into a sequence of 1D spectra • Wavelength calibration • Blaze function correction**To performed data reduction we need calibration data**• Bias (shutter closed, 0-time exposures) • Flat field (uniform source, short exposures) • Dark current (shutter closed, different exposures) • Order tracing (flat field with short slit) • Wavelength map(emission line source, short exposures) • Blaze calibration(flat field or a star with minimum sp. lines)**CCD data reduction in one expression**The intensity is given by: s – signal in science exposure b – bias level f – flat field signal g – gain (e-/ADU) d – dark current signal per unit time t – exposure times**Complete data reduction sequence**• Create master bias and master flat • Trace spectral orders • Normalize master flat • Extract wavelength calibrations • Extract science spectra (optimal extraction) • Fit the continuum • Construct the wavelength solution**Creating master bias and master flat**• The goal is to replace the actual calibration data with a model which is free of random noise but carries all the systematic signatures. • S/N of a master must be much larger than the S/N in science frames!!! • Main issue: getting rid of random errors, e.g. cosmic ray hits • Method:filtering within a frame or across a stack of frames • Cross-check between groups of calibration frames**6 times larger vertical scale**Individual flats with master flat subtracted**Flat field**Fragment of a master flat field**Pixel number**Order number Wavelength calibration Emission line spectrum (ThAr) with known laboratory wavelengths 2D polynomial**Wavelength solutionand the PSF**Line analysis for wavelength solution for ESO UVES spectrometer. Points show measured lines (scaled). Blue line shows best Gaussian fit. • Why the Gaussian misses the bulk of points? • Why do you see so much scatter?**Slit function decomposition**Idealized model: Image on the CCD is a sequence of monochromatic images of the entrance slit sampled with detector pixels Watershed pattern is due misalignment between spectral order and detector raws.**Normalizing flat field**Original flat “Spectrum” Model flat Normalized flat (data/model)**Extractingsciencespectrum**• Extraction of science spectrum: • Simple summation • Gaussian fit to the slit function • Optimal extraction**Continuum fit**Blaze function is a good start:**Fringing**Accurate fringing removal requires slit illumination by the FF and by the science object to be identical Wide flat Narrow flat Science spectrum**Exam**• Written exam on Thursday, March 14th, starting 9am sharp • Oral exam: book time with me