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11.2.1 Ripple Correction

A distinctive feature of echelle gratings is the variation in sensitivity as a function of wavelength within a spectral order, commonly known as the blaze function. The adjustment applied to eliminate this characteristic is referred to as a ripple correction. The use of the term ripple'' becomes apparent when the net fluxes in successive orders are plotted as a function of wavelength. A series of scalloped or ripple patterns appear which must be corrected for prior to the application of the absolute calibration.

The ripple correction and all associated equations are defined in Cassatella (1996, 1997a, 1997b). The basic form of the ripple correction as a function of order number and wavelength is:

where x is expressed as:

the parameter is given as a function of order number:

and the central wavelength corresponding to the peak of the blaze is:

Note that unlike the SWP camera, the LWP and LWR ripple corrections do not exhibit a dependence of central wavelength on THDA; instead the observed central wavelengths vary linearly with time. In addition, the LWR parameter evinces a bimodal behavior which has been fit with two separate functions (i.e., a linear and a quadratic polynomial). Here, m is order number, is wavelength in Ångstroms, T is the THDA, and D is the observation date in decimal years. The ripple correction is applied to the net flux prior to the application of the heliocentric velocity correction to the wavelengths. The various ripple coefficients used in the above equations are given in Table 11.10 for each camera.

 Coefficients LWP LWR SWP m < 101 A0 0.406835 3.757863 1.360633 0.926208 A1 0.01077191 -0.0640201 -4.252626e-3 0.0007890132 A2 -5.945406e-5 3.5664390e-4 0.0 0.0 W0 230868.177 230538.518 137508.316 W1 0.0 0.0 0.0321729 W2 -0.0263910 -0.0425003 0.0 W3 56.433405 90.768579 2.111841

Next: 11.2.2 Absolute Flux Calibration Up: 11.2 High-Dispersion Absolute Flux Previous: 11.2 High-Dispersion Absolute Flux
Karen Levay
12/4/1997