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Observational Astronomy

SPECTROSCOPICdata reduction

Piskunov & Valenti 2002, A&A 385, 1095

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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