The BASE command subtracts polynomial baselines of degree < 30. 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.
One first defines line windows by the command SET WINDOW . Values may be
entered numerically as arguments :
SET WINDOW wl1 wu1 [wl2 wu2 [...] ]
or graphically with the crosshair cursor if SET CURSOR ON
has been
selected. In this 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.
Up to 20 windows may be defined. BASE then fits a polynomial to the rest of the spectrum. 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. . 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...
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.