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