## Removing Baselines

The BASE command subtracts polynomial baselines of degree . The fitting algorithm uses Chebyshev polynomials, and does not allow any extrapolation outside the fitting range. It is thus important to fit the baseline out to the maximum extension of the wanted spectrum. If extrapolation is needed, a constant value will be used outside the fitting range, equal to the polynom value at the boundary. The algorithm warns if the polynomial degree is too high. See section for a typical baseline fitting session.

The user first defines line windows by the command SET WINDOW with the following syntax:

      SET WINDOW [wl1 wu1 [wl2 wu2 [...]]] [/VAR array]
                 [/POLYGON [N] [filename1...filenameN]]
                 [/NOCURSOR]

The POLYGON option enable the definition of 2D polygons on images obtain with LOAD; PLOT /INDEX when working on all the index. Line window values may be entered numerically as arguments, red from variables and line polygons may be red from input files. If available, the cursor may be used to define the windows or polygons. In the window case, enter the values in the same order as above by typing “N” or “ ” (space bar); “C” cancels the last value entered; “H” types a help message and “E” terminates the operation. In the case of polygons, each left clic defines a gon and a right clic terminates the operations. The polygons may leak out of the image. Several polygons may be defined in case the line appear at very different velocities.

Up to 100 windows or 5 polygons may be defined. BASE then fits a polynomial to the part of the spectrum outside the line windows. However, only the “visible” parts of the spectrum are used and bad channels are taken out. The degree of the polynomial is defined by SET BASE n, or temporarily by the BASE command itself with its argument.

Sinusoidal baselines may also be subtracted, using the command BASE SINUS Amplitude Period Phase where Amplitude, Period and Phase are initial guesses for a minimization routine. A linear baseline is added to the sinusoid in any case.

When working on an individual spectrum (not the index), the /PLOT option plots the fitted baseline in the current box. The area in the windows as well as the rms noise, are computed. A baseline can be computed for one spectrum, and then subtracted from a different one using BASE LAST. This may be helpful for example at Pico-Veleta where you may remove from the 100 kHz backend the baseline determined from the 1 MHz one. Be sure that you do not change the X-unit between the time you computed the baseline and the time you remove it...

When working on the whole index, the baseline are fitted spectrum per spectrum and the baseline–corrected spectra are stored in the 2D array ready for plotting with the next PLOT /INDEX command. However, baseline fitting results are lost and an explicit loop on the index entry must be use including a new baseline computation) to store the results with the WRITE command.