Provided by: alfa_2.2-1_amd64 bug

NAME

       alfa - automated line fitting algorithm

SYNOPSIS

       alfa [OPTION] [VALUE]... [FILE]

DESCRIPTION

       alfa  rapidly  fits  emission  line spectra, using a genetic algorithm to optimise fitting
       parameters.  It is intended to be entirely automated, but while the default values  should
       work  well  in  many  situations,  a  good  fit to your observed spectrum may require some
       adjustments to the input parameters.  It is optimised for  optical  spectra,  but  can  be
       applied to any wavelength range if a suitable line catalogue is provided.

       alfa  reads one dimensional spectra in either plain text or FITS format.  Plain text input
       should consist of two columns, giving wavelength and flux.  It can also  read  data  cubes
       and  row-stacked  spectra in FITS format.  Results are written out in plain text, to files
       containing the fit (total fit, continuum-subtracted original,  continuum,  sky  lines  and
       residuals), and the line flux measurements.

OPTIONS

       -b, --bad-data [REAL]
              If  all  values  in a spectrum are below the value specified, alfa will not fit it.
              Most useful for avoiding wasting time on low signal regions of data cubes.

       --citation
              Prints out the bibliographic details of the paper to cite if you use alfa  in  your
              research.

       -cl, --clobber
              alfa's  default  behaviour  is  that  it will not overwrite any pre-existing output
              file. If this option is specified, then the output file will be overwritten.

       --collapse
              Sums all spectra in multi-dimensional FITS files into a single  spectrum.   Spectra
              whose  peak  value  is  below  any  value  specified with the --bad-data option are
              excluded.  --collapse has no effect on 1D data.

       -cw, --continuum-window [INTEGER]
              ALFA's continuum fitting takes the 25th percentile  of  flux  values  in  a  moving
              window as representative of the continuum level. This option can be used to set the
              size of the window. The value must be an odd integer. Default: 101

       -dl, --detection-limit [REAL]
              Sets the factor by which a line's flux must  exceed  the  locally  estimated  noise
              level for it to be considered a detection.  Default: 3.0

       -el, --exclude-line [REAL]
              When  reading  in  the  line catalogues, any wavelengths indicated with this option
              will be ignored.  For example, if H alpha were saturated, it could be excluded from
              the  fit  with  --exclude-line  6562.77.   Any  number  of lines can be excluded by
              repeating the option with the appropriate wavelengths.

       -fc, --flux-column [INTEGER]
              When reading FITS tables, this option can be used to specify in which table  column
              the flux values are. For any other file type it will have no effect. Default: 2

       -g, --generations [INTEGER]
              The number of generations used in the genetic algorithm. Default: 500

       -n, --normalise [VALUE]
              Normalise  to Hb=100 assuming that F(Hb)=VALUE.  If VALUE is zero, no normalisation
              is applied.  If this option is not  specified,  fluxes  are  normalised  using  the
              measured value of Hb if it is detected, and not normalised otherwise.

       -nc, --no-continuum
              In  case the spectrum you are fitting is already continuum subtracted, setting this
              option skips alfa's continuum fitting.

       -o, --output-dir [DIRECTORY]
              The directory in which to put the output files. Default: current working directory.

       -of, --output-format [VALUE]
              The format of the output files. Valid values are text, csv, fits, latex. If you are
              using  alfa  in conjunction with neat, plain text and fits formats can currently be
              read in. Default: text

       -pr, --pressure [REAL]
              The fraction of the population retained from each generation. The  product  of  the
              pressure and the population size should be an integer.  Default: 0.3

       -ps, --populationsize [INTEGER]
              The size of the population used in the genetic algorithm. Default: 30

       -rb, --rebin [INTEGER]
              Rebin  the  input  spectrum  by  the  specified  factor. Useful for high resolution
              spectra where line profiles are not instrumental  but  kinematic.  This  option  is
              currently only implemented for 1d spectra.

       -rg, --resolution-guess [VALUE]
              Initial  guess  for  the resolution [lambda/delta lambda]. Default: estimated using
              the sampling of the input spectrum, assuming that it is Nyquist sampled.

       -rtol1, --resolution-tolerance-1 [VALUE]
              Variation allowed in resolution in first pass. Default: equal to 0.9  x  resolution
              guess.

       -rtol2, --resolution-tolerance-2 [VALUE]
              Variation allowed in resolution in second pass. Default: 500.

       -skyc, --sky-catalogue; -sc, --strong-catalogue; -dc, --deep-catalogue [FILENAME]
              The  files  containing the line catalogues to be used for the removal of sky lines,
              the estimation of velocity and resolution, and the full line fitting.  The  default
              catalogues  are  stored  in  /usr/share/alfa  .   If  you  wish  to create your own
              catalogue, the required format is that each line should be 85 characters wide, with
              a  wavelength  in  the first column, and the rest of the characters are not used by
              the code but are transferred to the output files.  They can thus be used, as in the
              supplied  catalogues,  for line transition data.  To use the default catalogues but
              exclude some lines, the --exclude-line option can be used.

       -ss, --subtract-sky
              Fit and subtract night sky emission lines before fitting nebular emission lines.

       -ul, --upper-limits
              Write out upper limits for all lines searched for and not detected. The upper limit
              is  calculated  as  the  detection  limit  (default  3,  set  by --detection-limit)
              multiplied by the uncertainty. In FITS output, this option is ignored -  all  upper
              limits are written out, and are indicated by negative flux values. If the FITS file
              is later anaysed with neat, these negative values are simply ignored.

       -vg, --velocity-guess [VALUE]
              Initial guess for the velocity of the object [km/s]. Default: 0.

       -vtol1, --velocity-tolerance-1 [VALUE]
              Variation allowed in velocity in first pass of the fitting. Default: 900km/s

       -vtol2, --velocity-tolerance-2 [VALUE]
              Variation allowed in velocity in second pass of the fitting. Default: 60km/s

       -wc, --wavelength-column [INTEGER]
              When reading FITS tables, this option can be used to specify in which table  column
              the wavelength values are. For any other file type it will have no effect. Default:
              1

       -ws, --wavelength-scaling [VALUE]
              alfa checks the units of FITS file headers to set the wavelength scale,  defaulting
              to  assuming  Angstroms if no keyword is present.  If your input spectrum is not in
              Angstroms, use this option  to  set  the  value  by  which  wavelengths  should  be
              multiplied to convert them to A.  For example, -ws 10.0 would apply if your spectra
              have wavelengths in nm.

ALGORITHM

       alfa works in three stages.  First, it estimates and subtracts the continuum.  Second,  it
       estimates  the  resolution  of  the spectra and the velocity of the object.  And third, it
       fits the emission lines.  These stages work as follows:

       Continuum subtraction

              alfa estimates the continuum using a percentile filter, taking the 25th  percentile
              in  a  moving  window  101 pixels wide.  Currently these values are hard-coded, but
              will be user-configurable in a future release.  Regions  such  as  the  Balmer  and
              Paschen jumps may be poorly fitted by this method if the spectral resolution is low
              and the continuum gradient is changing fast.   Broad  stellar  emission  lines  and
              telluric  absorption  features  may  also  not  be  well  fitted  by  this  method.
              Inspection of the fitted continuum is recommended.

       Estimation of the resolution and velocity

              If no relevant command line options are specified, alfa begins by assuming that the
              velocity  of the object is zero, and that the spectrum is Nyquist-sampled.  It then
              carries out a fit on  a  subset  of  strong  lines,  using  the  genetic  algorithm
              described below, to obtain an overall estimate for the velocity and the resolution.
              If necessary, the initial guesses can be specified with the  -vg  and  -rg  options
              described  above, and the parameter space for the fine tuning can be specified with
              -vtol1 and -rtol1.

       Fitting of the emission lines

              With the continuum subtracted and  the  resolution  and  velocity  estimated,  alfa
              divides  the  spectrum up into chunks 440 pixels wide, with 20 pixels at either end
              overlapping with adjacent chunks.  Then the genetic algorithm fits all  lines  from
              the  deep  catalogue  that  fall  within  the  central 400 pixels, with the overlap
              regions providing the full line profile for lines close to the edge of  the  chunk.
              The  initial  guess  for  the  resolution  and  velocity  are taken from the global
              estimate for the first chunk, and from the preceding chunk's fine tuned  value  for
              all succeeding chunks.

              With  the parameters optimised in each chunk, uncertainties are estimated using the
              root mean square of the residuals in a 20 pixel window, exlucing  the  two  largest
              residuals  to  mitigate against overestimated uncertainties in the neighbourhood of
              bad pixels or strong lines.

INPUT FILES

       alfa can read either plain text files or FITS format files.   For  plain  text,  the  file
       should  contain a wavelength and a flux, with the wavelength in the same units as the line
       catalogues (the default catalogues have wavelengths in Angstroms).  FITS  files  are  read
       using  the  CFITSIO  library,  so  any  FITS-compliant  file  should  be fine.  However, a
       surprisingly large fraction of all FITS files do not comply with the standard, so in  case
       of problems, trying using fitsverify to check your FITS file.

       The  FITS  file can have one, two or three dimensions.  If it has two, it is assumed to be
       in Row-Stacked Spectra (RSS) format, while if it has three, it is assumed  to  be  a  data
       cube with two axes representing spatial dimensions and the third representing the spectral
       dimension.

       If you don't want to fit the whole dataset, you can specify the range of  pixels  on  each
       axis    that   you   want   alfa   to   read   in   by   appending   the   filename   with
       [startx:endx,starty:endy].  This functionality is part of the  CFITSIO  library,  and  the
       format                        is                        described                       at
       https://heasarc.gsfc.nasa.gov/docs/software/fitsio/c/c_user/node97.html.  alfa itself does
       not read in the coordinates of the section, and so the output file numbering starts from 1
       on each axis regardless of where the image section actually started.  The next release  of
       alfa will have improved support for image sections.

OUTPUT FILES

       alfa  can produce output in plain text, csv, latex or FITS format. FITS is the default and
       recommended format; in future releases, output formats other than FITS will be  deprecated
       and eventually removed.

       When  producing  plain  text, csv or latex output, alfa will produce two files, a fit file
       and a line list file. When producing FITS format, a single file  is  created,  with  three
       extensions,  for  the  fit, the linelist, and a third extension containing the Hbeta flux,
       number of lines detected, the radial velocity and average resolution  over  the  spectrum.
       Output filenames are based on the input filename, with the extensions noted below.

       The fit file (_fit.txt, _fit.csv, _fit.tex) or extension (.fits):

              The  fit  file  contains  the best fitting synthesised spectrum.  It contains seven
              columns, representing the wavelength, the input spectrum, the fitted spectrum,  the
              original  after  continuum  subtraction, the estimated continuum, the fluxes of sky
              lines, and the residuals.  Thus, to see the  fitted  spectrum,  you  need  to  plot
              columns  1  and  3  of this file.  In gnuplot, one can compare the input and fitted
              spectra using this command:
                  plot 'filename_fit' w l, 'filename_fit' using 1:3 w l

       The lines file (_lines.txt, _lines.csv, _lines.tex) or extension (.fits):

              This file contains four columns with parameters of the fitted lines - the  observed
              wavelength,  the  rest wavelength, the flux, and the uncertainty estimated from the
              residuals.  This file can be read directly by neat, which determines abundances for
              photoionised  nebulae. If latex format is requested, the file additionally contains
              atomic data details not currently available in the other formats.

              For RSS files and data cubes, alfa currently produces output for each pixel.  Thus,
              for  a large data cube you may end up with tens of thousands of files in the output
              directory. A routine to combine these outputs into image maps will be provided in a
              future release of alfa.

USAGE NOTES

       alfa's default parameters are supposed to work in most cases, but sometimes you might find
       that it does not converge on the correct wavelength solution.  It searches  initially  for
       velocities in the range +/-900km/s, which is very large for Galactic objects.  So, running
       the code with --resolution-tolerance-1 100. or so may improve your results.

       The genetic parameters (population size, number of generations, pressure) are likely to be
       suitable  for most cases.  There is no algorithm yet known for optimising these parameters
       in a genetic algorithm, so changing them requires trial and error.  In spectra of  regions
       with  lots  of  emission  lines,  such  as  4000-4500  Angstrom,  increasing the number of
       generations can result in a better fit.

   Error codes
       If alfa encounters an error it will return one of the following status codes:

        100 incomplete or invalid command line option
        101 input file does not exist
        102 output directory does not exist
        103 error opening FITS file
        104 no axes found in input file
        105 too many axes found in input file
        106 error reading FITS keywords
        107 error writing FITS output
        108 no line catalogue file specified
        109 line catalogue not found

        200 all fluxes below baddata limit
        201 no known emission lines in wavelength range
        202 failed to estimate velocity and resolution - no lines found

SEE ALSO

       neat

BUGS

       If reporting a bug, please state which version of alfa you were using, and  include  input
       and any output files produced if possible.

AUTHOR

       Roger Wesson