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Spectroscopic Data. ASTR 3010 Lecture 15 Textbook Ch.11. Spectroscopic Measurement. CCD or other digital Detector. Schematic Diagram of a Typical Spectroscopic Data. Only Readout the relevant portion of CCD array. Spectroscopic data are typically long and slender.

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spectroscopic data

Spectroscopic Data

ASTR 3010

Lecture 15

Textbook Ch.11

spectroscopic measurement
Spectroscopic Measurement

CCD or other digital Detector

only readout the relevant portion of ccd array
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
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
unique spectroscopic calibration data
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)
steps of spectroscopic data reduction
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.

extraction of spectra
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
spectral aperture is not always a horizontal line!

need to trace the aperture along the dispersion axis aptrace

wavelength calibration using arc images
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
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
Normalization or Flux Calibration

continuum

sensfunc and calibrate

continuum or splot

telluric correction
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
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
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
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
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
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