ROWFIT: Fit Each Row of an Image with a Function

ROWFIT substitutes, subtracts, or divides, each row by a fit from a menu of functions (all linear in the coefficients). By default, the image is replaced by the fits, unless the SUB, DIV, or FITONLY keywords are specified. Either a polynomial or a Fourier series can be used for the fitting function, with the number of polynomials (or sins and cosines) specified by the NCOEF=m keyword. The default is a polynomial of order 0 (i.e. a constant term).

The XBUF=buf keyword allows using any physical quantity (instead of the default column number) as independent variable for the fit. In this case the pixel values of the spectrum in buffer 'buf' will be use as the values of the independent variable at each column. Useful application include the fitting and subtraction of continuum or absorption spectra from emission line objects, sky-level estimation and sky subtraction in long-slit spectroscopy, or the fitting of many highly non-linear functions. The WBUF=wbuf keyword allows the user to give individual pixel weights for the fit, with the weights for the given pixel given by the value in buffer 'wbuf'.

The parameters of the fit can be saved for inspection or for later use with the LOAD=buf keyword. The image loaded into buffer 'buf' will have, for each row, the coefficients of the row fit and their estimated error as well as the standard deviation from the fit in the following format:

  Column  0  holds the standard deviations from the fits,
  Column  1  holds the first coefficient of the fits (The constant term),
  Column -1  holds the estimated error on the first coefficient,
  Column  2  holds the second coefficient of the fits (The linear term),
  Column -2  holds the estimated error on the second coefficient,
  Column  3  holds the third coefficient of the fits (The quadratic term),
  Column -3  holds the estimated error on the third coefficient,
and so on until column +/- the m coefficients fixed with the NCOEF=m keyword.

The LOADed coefficients can be smoothed, averaged with other fits, or manipulated in any fashion before used to operate in other images with the UNLOAD=buf keyword. In this case ROWFIT evaluates (DOES NOT FIT) the function defined by the coefficients in buffer 'buf' and operates (substitutes, divides or subtracts the function) on the 'source' buffer. These coefficients can also be used to remove distortions from an image when used in command ALIGN.

Note that when evaluating a function with the UNLOAD= keyword the user has to specify the kind of function (FOUR or POLY) and the abscissae buffer (XBUF=) when appropriate.