Spectroscopic Data

1. Spectroscopic Data ASTR 3010 Lecture 15 Textbook Ch.11

2. Spectroscopic Measurement CCD or other digital Detector

3. Schematic Diagram of a Typical Spectroscopic Data

4. Only Readout the relevant portion of CCD array • Spectroscopic data are typically long and slender. to save the readout time and storage space…

5. Actual Spectroscopic Data • Similar to Imaging Data, a similar set of calibration images is obtained for spectroscopic observation  same pre-processing!! • over-scan correction • trimming • bad-pixel correction • cosmic ray hits removal • zero correction • dark correction • flat-fielding

6. Unique spectroscopic calibration data • Unlike imaging observations, two additional kinds of calibration data are obtained for spectroscopic observations. • arc images (wavelength correction) • smooth spectrum standards (telluric feature correction) • (and possibly flux standard stars = standard stars in imaging)

7. Steps of spectroscopic data reduction bad pixel, bias, over-scan, trim, zero, flat-fielding find  trace  extract  normalize using arc images (e.g., gas emission lines) using stars with known smooth spectra line width measurement, Teff, [Z/H], vsini, etc.

8. Extraction of Spectra words in green denote corresponding IRAF commands for the task finding the aperture and defining the sky regions apfind& apedit

9. spectral aperture is not always a horizontal line! need to trace the aperture along the dispersion axis aptrace

10. 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

11. How do we extract (find and trace) arc spectra? • or very faint objects? • Use well exposed bright star spectrum as a reference!

12. Normalization or Flux Calibration continuum sensfunc and calibrate continuum or splot

13. Telluric Correction Telluric standard stars : stars with no (or very little) spectral line feature. low metallicity stars, white dwarfs, fast rotating stars telluric

14. 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

15. 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

16. 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?

17. 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.

18. In summary… Important Concepts Important Terms aperture tracing telluric correction equivalent width • Structure of the spectroscopic data • Steps of spectroscopic reduction • good manual http://iraf.noao.edu/iraf/ftp/iraf/docs/spect.ps.Z • Chapter/sections covered in this lecture : 11.6