Transcript Spectroscopic Data

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!!
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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
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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