LASCO C1 Instrument Profile
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Definition of the Instrument Profile

The C1 instrument is unique among the SOHO instruments in that it contains a Fabry-Perot interferometer which can be used to observe the sun in a vary narrow wavelength band which is tunable over a wide range of visible wavelengths. The Fabry-Perot interferometer (FP) and its associated blocking filters transmits light in a narrow band of wavelengths. The normalized transmission as a function of wavelength is called the instrument profile of the instrument. The instrument profile of an ideal FP is given by a Lorentzian profile:

        L(w) = g/(pi(w^2+g^2))                Eq(1)

where w is wavelength, L(w) is the instrument profile and the full width at half maximum (FWHM) is given by:

        fl == 2g                              Eq(2)

where f_l is the FWHM. (Note that the integral of L(w) over wavelength is normalized to unity.)

Due to various imperfections, the instrument profile of a real FP will not be a simple Lorentzian profile. In the case of the LASCO C1 instrument, it has been found that the instrument profile is well represented by a Voigt function which is a convolution of the Lorentzian profile and a Gaussian profile. The Gaussian profile may be written as:

        G(w) = exp(-w^2/(2 s^2))              Eq(3)
               ----------------
                  s sqrt(2 pi)

where G(w) is the profile and s is the variance of the Gaussian. Note that, as in the case of the Lorentzian, the Gaussian profile is also normalized to unity as is the resultant Voigt profile. Note also that the FWHM of the Gaussian profile is given by f_g where:

        fg = 2s sqrt(2 ln(2))                 Eq(4)

Measurement of the Instrument Profile

Measurements of the Lorentz and Gaussian profile parameters for the LASCO C1 instrument have been made and the table below lists the values (in Angstroms) for the five different wavelength regimes represented by the five individual blocking filters:

              Blocker          fl (A)          fg (A)          f (A)

        5303 FeXIV Green Line   0.064           0.537           0.572
        5694 CaXV  Blue Line    0.242           0.424           0.571
        5890 NaI   Yellow Line  0.161           0.562           0.654
        6374 FeX   Red Line     0.392           0.535           0.781
        6563 Hydrogen Alpha     0.089           0.947           0.995

                                   Table 1

Where f_l is the Lorentzian FWHM, f_g is the Gaussian FWHM, and f is the Voigt FWHM. These values were arrived at by minimising the RMS error in the difference between the measured profile and the NSO[1] solar spectrum convolved with the Voigt instrument function.

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References

[1] Kurucz, R.L., Furnlid, I., Brault, J., Testerman, L, ``Solar Flux Atlas from 296 to 1300 nm'', National Solar Observatory Atlas No. 1, June 1984.

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PR Sep 26, 1998
PR Apr 08, 1999