| cosmologist |
01/17/2014 07:48AM (Read 6405 times)
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Hi,
I often use splot with "e", to find the equivalent width (EW) of certain features in my spectrum, but I couldn't find any information of how iraf calculates this equivalent width. Whenever I convert my spectra from .fits files to asci files and I try to do the calculations of the EW myself I don't get the exact same values, so I am wondering what algorithm is used by iraf for it.
Thanks!
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| fitz |
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01/17/2014 04:33PM
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See the SPLOT help page section "Centroid, Flux, and Equivalent Width Determinations". Specifically,
CENTROID, FLUX, AND EQUIVALENT WIDTH DETERMINATIONS
There are currently five techniques in SPLOT to measure equivalent widths
and other line profile parameters. The simplest (conceptually) is by
integration of the pixel values between two marked pixels. This is
invoked with the 'e' keystroke. The user marks the two edges of the line
at the continuum. The measured line center, continuum value, line flux, and
equivalent width are given by:
center = sum (w(i) * (I(i)-C(i))**3/2) / sum ((I(i)-C(i))**3/2)
continuum = C(midpoint)
flux = sum ((I(i)-C(i)) * (w(i2) - w(i1)) / (i2 - i2)
eq. width = sum (1 - I(i)/C(i))
where w(i) is the wavelength of pixel i, i1 and i2 are the nearest integer
pixel limits of the integrated wavelength range, I(i) is the data value of
pixel i, C(i) is the continuum at pixel (i), and the sum is over the marked
range of pixels. The continuum is a linear function between the two points
marked. The factor mulitplying the continuum subtracted pixel values
in the flux calculation is the wavelength interval per pixel so that
the flux integration is done in wavelength units. (See the discussion
at the end of this section concerning flux units).
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| cosmologist |
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03/12/2014 03:41AM
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I am trying to write a code that does the same thing but the equivalent width calculated is always wrong (its sometimes negative). There is something that I might have misunderstood. the equation says sum(1-I(i)/C(i)). Since I'm not reading .fits files and reading only text files, I had assumed that I(i) meant the flux value at wavelength i in the original graph and C(i) the same but for the continuum. What exactly do I(i) and C(i) translate to if I'm reading from a text file?
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| fitz |
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03/12/2014 03:59AM
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It depends on what's in your text file. Do you actually have continuum values or just wavelengths and fluxes? If the latter then you need to somehow fit a continuum first and then evaluate the fit function at each wavelength point. If the fluxes have already been calibrated or otherwise flattened then you can use a constant for C(i) (where that constant value depends on how the data were processed, e.g. is the continuum normalization 1.0 or 0.0?)
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| cosmologist |
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03/12/2014 05:59AM
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The text file has just wavelengths and fluxes. The fluxes have already been normalized at a wavelength of 5870 A. Does that mean I don't have to fit it to a continuum and that I can use a constant for C(i) let's say 1 because the flux at 5870 A is 1? Is that correct?
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| fitz |
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03/12/2014 06:05AM
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It sounds like the data are normalized to a continuum level of 1.0, but I'd plot the spectrum first to be sure it looks flattened and it isn't just the case that 1.0 is the max value of a curve.
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| cosmologist |
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03/12/2014 06:59AM
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This is an example of one of my spectra:
https://www.dropbox.com/s/ji6081znmq48j97/NGC1711plot.png
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| fitz |
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03/12/2014 07:14AM
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It looks flux calibrated, but not continuum normalized, you'll need to fit it first to get a continuum value to use.
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| cosmologist |
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03/12/2014 09:57AM
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How I do that? 
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| fitz |
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03/12/2014 02:08PM
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In IRAF you'd use the CONTINUUM task, otherwise Google "continuum normalization" if you plan to write your own code. The process basically involves fitting something like a spline to a sample of high points along the spectrum to establish the continuum, and then dividing your spectrum by this fit to do the normalization (i.e. "flattening").
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splot - equivalent width
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