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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 14 January 20163
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May 3, 2023 1
Observational Astronomy
SPECTROSCOPICdata reduction
Piskunov & Valenti 2002, A&A 385, 1095
May 3, 2023 2
Echelle spectrum image looks strange …
Spectral orders
Spectral line
Data reduction conceptComplex data layout requires in addition to the usual CCD calibrations:Spectral order localizationExtracting 2D data into a sequence of 1D spectraWavelength calibrationBlaze function correction
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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)
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CCD data reduction in one expressionThe intensity is given by:
s – signal in science exposureb – bias levelf – flat field signalg – gain (e-/ADU)d – dark current signal per unit timet – exposure times
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Complete data reduction sequence
1. Create master bias and master flat2. Trace spectral orders3. Normalize master flat4. Extract wavelength calibrations5. Extract science spectra (optimal
extraction)6. Fit the continuum7. Construct the wavelength solution
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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
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Example of constructing master flat
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Stack of individual flats (fragments)
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6 times larger vertical
scale
Individual flats with master flat subtracted
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Flat field Fragment of a master flat field
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Order tracing
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Order tracing (2)
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Wavelength calibration
Pixel number
Order number
2D polynomial
Emission line spectrum (ThAr) with known laboratory wavelengths
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Wavelength solutionand the PSF
1. Why the Gaussian misses the bulk of points?
2. Why do you see so much scatter?
Line analysis for wavelength solution for ESO UVES spectrometer.Points show measured lines (scaled).Blue line shows best Gaussian fit.
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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.
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Normalizing flat fieldOriginal flat
Model flat
“Spectrum”
Normalized flat (data/model)
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Extractingsciencespectrum
Extraction of science spectrum:-Simple summation-Gaussian fit to the slit function-Optimal extraction
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Continuum fitBlaze function is a good start:
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Fringing
Accurate fringing removal requires slit illumination by the FF and by the science object to be identical
Wide flat
Narrow flat
Science spectrum
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Exam Written exam on Thursday, March
14th, starting 9am sharp Oral exam: book time with me