11.2.2 Absolute Flux Calibration Function

The high-dispersion inverse sensitivity curve is defined to be the product of the low-dispersion inverse sensitivity curve and a wavelength-dependent high-to-low absolute calibration function (Cassatella 1994, 1996, 1997a, 1997b):

C = n / N

where C is the calibration function, n is the low-dispersion net flux normalized to the exposure time, and N is the high-dispersion ripple-corrected net flux also normalized to the exposure time. The calibration function represents the efficiency of high-dispersion spectra relative to low-dispersion and was determined empirically using pairs of high- and low-dispersion spectra obtained close in time so as to minimize the effects of the time-dependent sensitivity degradation. C is represented functionally as a polynomial in the following form:

$$C_{\lambda} = C_{0} + C_{1} \lambda + C_{2} \lambda^{2} + C_{3} \lambda^{3}$$

where $\lambda$ is wavelength in Ångstroms. The coefficients used in the calibration function are given in Table 11.11.

 

 

Coefficients LWP LWR SWP

C₀ 251.383956 251.383956 1349.8538

C₁ -0.053935103 -0.053935103 -2.0078566

C₂ 0.0 0.0 1.10252585e-3

C₃ 0.0 0.0 -2.0939327e-7