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10PROCESSING OF SPECTRAL DATA

Once we have collected spectral data, we need to reduce them to numbersand plots that are useful to astronomers, physicists, chemists, and scientists ingeneral. Atlas plots of the spectrum of the radiation at both low and high dispersionsare impo rtant. The former gives an overview of the intensity distribution, andthe latter shows details such as line shapes and widths. Som etimes the data cansimply be plotted without any modification or analysis. However, the spectral datawe get from an instrument are not a true representation of the radiation comingin; rather, they are that radiation as modified by the instrument. As we havediscussed, the spectrometer output for purely monochromatic radiation input isnot an infinitesimally narrow spectrum "line," because of the finite aperture andresolving power of the spectrometer. We want the final atlas to show as closely aspossible the true spectrum of the radiation itself, with instrumental effects removedas far as possible. Of course, presenting the raw spectrum itself is desirable as well,in order to show wh at data the experimenter has to w ork w ith.

Another need is for a line list, which includes all lines evident on the high-resolution atlas and which incorporates accurate fitted line parameters, includingfrequency and wavelength, intensity, width, area, and damp ing, or an equivalent setof line parameters.

In this chapter, we shall discuss how to make atlases and line lists. All ofour examples are drawn from data taken on the McM ath-P ierce Fourier transformspectrometer at the National Solar Observatory, Kitt Peak. The data-processingcode is GREMLIN, the successor to many versions of DECOM P authored by J. W.

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170 10. Processing o f Spectral Data

Brault over the last 25 years. The re are other codes extant for similar data reductionthat are adaptable to everything we say here.10.1 Emission Background Subtraction and Intensity Correction

We start with the interferogram transformed into a spectrum , in whatever formatis required by our reduction program . We display the desired spectral region andnote its various features, such as locations of strong lines, bands, backgroundcon tinuum, and noise level. Figu re 10.1 is the furnace spec trum in emission ofZrO in the region 10 000 cm~^ to 20 000 c m ~ ^ The data have been averaged tobring out the overall intensity pattern . Very strong lines do not stand out in thisrepresentation.

12500 15000 17500Wavenumber(cm'')

20000

Fig. 10.1 ZrO furnace em ission spectrum.

There are atom ic emission lines, quite a few molecular bands degraded to thered, a few weak ab sorption b ands of oxygen at about 13 000 cm " and an underlyingcontinuum. The next task is to subtract the continuum. We make a spectrum thatrepresents the continuum by slicing up the file into sections with equal numbersof points, picking the smallest-intensity point in each section and passing a curvethrough these minima. This backgrou nd curve is filtered or sm oothed if necessaryto remove any remaining local structure or artifacts and is then subtracted fromthe raw spec trum . In some cases , it may be necessary to perform an iteration ofbackground subtraction before the spectrum has a level baseline. Th e result lookslike Fig. 10.2.

Next, the instrumental transmission function is ratioed with the spectrum. Toget the function, we need the calibrated spectrum of a standard lamp and theobserved spectrum of this lamp made under the same experimental conditions asthe unknow n spectrum. Th e general proced ure is: Call up the data file of theobserved standard lamp spectrum ; call up the calibrated datafileof this lamp; ratio

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10.1 Emission Background Subtraction and Intensity C orrection 111

the two files to get the transm ission function of the spectrom eter; ratio this functionwith the unknown spectrum data file. We use the ZrO spectrum as the unknowndata file and a ribbon filament lamp as the standard. The calibrated lamp spectrum,observed lamp spectrum , and their ratio the instrumental transmission function are shown in Fig. 10.3, the final corrected ZrO spectrum is shown in Fig. 10.4.

1.0w .82 .6t .4< .2

.0

-- I I -" 1 1 1 1

t^M u1 1 1 1 1 1 1 1 1 1 1 1

lilLHtljIiljlMii 1

1 ^

1 f12500 15000 17500

Wavenumber (cm')20000

Fig. 10.2 ZrO furnace spectrum with background subtracted.

-SJ(XF