spectroscopic data
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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.
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 over-scan correctiono trimmingo bad-pixel correctiono cosmic ray hits removalo zero correctiono dark correctiono 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
Spectrum Extraction
Wavelength Calibration
Telluric Correction
Interpretation of Spectra
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
finding the aperture and defining the sky regions apfind & apedit
words in greendenote correspondingIRAF commands for thetask
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 or splot
sensfunc and calibrate
continuum
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
1.0
0.0
area = area
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• Structure of the spectroscopic data• Steps of spectroscopic reduction
Important Terms• aperture tracing• telluric correction• equivalent width
good manual http://iraf.noao.edu/iraf/ftp/iraf/docs/spect.ps.ZChapter/sections covered in this lecture : 11.6