This corrects data for line curvature, using an arc spectrum. The function is the same as for comb, but it works in the perpendicular direction (again this is for long-slit spectra).
The aim is to give ARC2D a spectrum with fairly straight arc lines, so it can `block' more cross-sections together to obtain a higher signal to noise. The operation is similar to parts (1) to (3) of ARC2D, except that ARCSDI does not need to know the line identifications. To apply the correction, use OLD mode, the correction file (i.e. the file with the polynomial coefficients) is ARCSDI.GMC. The main menu is the same as for COMB.
The arc lines are located using much of the same code as in ARC2D described below, including the fitting of Gaussians. The reason for doing preliminary correction of this nature, rather than using ARC2D straight away, is that for data which is fairly noisy, it is desirable to give ARC2D as many cross-sections at a time for the fitting. It is intended that ARCSDI be used with a long exposure from a chimney arc. Another problem overcome by this is that of vignetting which occurs in the comparison arc optics of the RGO spectrograph at the AAT. `Chimney' arcs are reflected off the dome, and do not suffer from this problem (the same optical path is used as for the observations). It is often an advantage to use an arc from another wavelength range, if this has more suitable lines, or if they are better distributed along the spectrum.
Once the lines are located, they are treated in much the same way as the positions of continua in COMB. Chebyshev polynomials are fitted to these and the results used to determine the required shifts.
FIGARO A general data reduction system