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Spectroscopic Data ASTR 3010 Lecture 15 Textbook Ch.11 Spectroscopic Measurement Schematic Diagram of a Typical Spectroscopic Data Only Readout the relevant portion of CCD array • Spectroscopic data are typically long and slender. to save the readout time and storage space… Actual Spectroscopic Data • Similar to Imaging Data, a similar set of calibration images is obtained for spectroscopic observation same pre-processing!! o o o o o o o over-scan correction trimming bad-pixel correction cosmic ray hits removal zero correction dark correction flat-fielding Unique spectroscopic calibration data • Unlike imaging observations, two additional kinds of calibration data are obtained for spectroscopic observations. o arc images (wavelength correction) o smooth spectrum standards (telluric feature correction) o (and possibly flux standard stars = standard stars in imaging) Steps of spectroscopic data reduction Array pre-processing bad pixel, bias, over-scan, trim, zero, flat-fielding Spectrum Extraction find trace extract normalize Wavelength Calibration Telluric Correction Interpretation of Spectra using arc images (e.g., gas emission lines) using stars with known smooth spectra line width measurement, Teff, [Z/H], vsini, etc. Extraction of Spectra words in green denote corresponding IRAF commands for the task finding the aperture and defining the sky regions apfind & apedit spectral aperture is not always a horizontal line! need to trace the aperture along the dispersion axis aptrace Wavelength calibration using Arc images • Excited lines from a combination of noble gases (He, Ne, Ar, Xe, Th, etc.) using arc images, you can convert pixel numbers to wavelength! identify How do we extract (find and trace) arc spectra? • or very faint objects? • Use well exposed bright star spectrum as a reference! Normalization or Flux Calibration continuum sensfunc and calibrate continuum or splot Telluric Correction Telluric standard stars : stars with no (or very little) spectral line feature. low metallicity stars, white dwarfs, fast rotating stars telluric Plotting the extracted spectra and … splot display the extracted spectrum and do simple operations on the spectrum such as continuum normalization and equivalent width measurements. ‘h’ + ‘k’ EW fit with A Gaussian to continuum=1 Equivalent Width • Quantitative indicator of the strength of a spectral feature • EW = area of the spectral feature area = area 1.0 0.0 EW unit is the wavelength unit of the spectrum One caveat in spectroscopic pre-processing… Flat images… flat images sometime taken w/wo the dispersing element. Which one is better? Is only one kind useful? Flats in spectroscopic data this spectrum is a combination of the above detector flat + spectral response of the system. Flat-fielding is to remove the pixel-to-pixel sensitivity variation! Other gradual, global variation can be taken out later. In summary… Important Concepts Important Terms • Structure of the spectroscopic data • Steps of spectroscopic reduction • aperture tracing • telluric correction • equivalent width good manual http://iraf.noao.edu/iraf/ftp/iraf/docs/spect.ps.Z Chapter/sections covered in this lecture : 11.6