Experimental SAS data for use with ATSAS applications have to be stored in an ASCII text format consisting of three numerical columns:

  1. the scattering vector magnitude,
  2. the experimental scattering intensity,
  3. the experimental uncertainty of the intensity.

In addition to the numerical data, files may contain optional metadata in the form of key–value pairs or beamline-specific annotations, either preceding or following the data block. Such metadata are ignored by most programs unless explicitly stated otherwise.

Angular units and conventions

There is no universally enforced standard for how the scattering vector is encoded in experimental SAXS data files. In practice, at least four commonly used conventions exist, differing both in the mathematical definition and in the physical units.

Many ATSAS applications require knowledge of the angular units in order to interpret the data correctly. Depending on the program, the units may be:

  • specified explicitly by the user via the --unit option,
  • requested interactively,
  • or inferred heuristically from the data.

Both the --unit option and interactive configuration require the user to specify an alpha-numeric identifier:

Identifier Description
1 \(s = 4 \pi \cdot sin(\theta)/\lambda\) in \(\AA^{-1}\).
2 \(s = 4 \pi \cdot sin(\theta)/\lambda\) in \(\text{nm}^{-1}\).
3 \(s = 2 \cdot sin(\theta)/\lambda\) in \(\AA^{-1}\).
4 \(s = 2 \cdot sin(\theta)/\lambda\) in \(\text{nm}^{-1}\).
u, unknown The application may use a heuristic to determine the angular units.

If no explicit unit is provided, or u is entered, the application may attempt to determine the units automatically using a simple heuristic.

Heuristic unit detection

If unit guessing is enabled and no explicit unit is given, the following heuristic may be applied:

  • if the maximum scattering vector value satisfies \(s_{\max} < 1.0\), the data are assumed to be given in inverse ångströms (unit 1);
  • otherwise, the data are assumed to be given in inverse nanometers (unit 2).

Note:

  • for wide-angle scattering data this heuristic fails, and the unit has to be specified explicitly
  • no attempt is ever made to infer conventions 3 or 4 automatically. If your data use these conventions, the correct unit must be specified explicitly.

Applications that read .dat:

  • almerge - Finds the best overlap betweem SAS data sets and merges them together.
  • autorg - Estimation of radius of gyration from SAS data by Guinier approximation.
  • axis - Angular AXIS
  • bodies - Fit geometric shapes to SAS data; predict SAS data of geometric shapes
  • bunch - Rigid body modelling of multidomain proteins.
  • chromixs - SEC-SAXS data analysis
  • crysol - Calculates X-ray scattering patterns from atomic structures
  • databsmw - Molecular weight from experimental data on absolute scale.
  • databsolute - Transform experimental data to absolute scale.
  • dataver - Average two or more experimental data files.
  • datclass - Classification of experimental data into geometric shape classes.
  • datcombine - Combine experimental data files from multiple sources.
  • datcrop - Crop a data file to a specific angular range.
  • datft - Direct Fourier transform of experimental SAS data
  • datgnom - Calculate the p(r) from experimental data files.
  • datmerge - Scale, shift and merge multiple experimental data files
  • datmow - Molecular weight from Apparent Volume
  • datmw - Molecular weight estimation by multiple methods
  • datop - Linear operations on experimental data files.
  • datporod - Determine Porod Volume and Exponent
  • datregrid - Modification of the angular vector.
  • datresample - Resample experimental data
  • datrg - Calculation of radius of gyration from SAS data by Guinier approximation.
  • datvc - Molecular weight from Volume of Correlation
  • em2dam - Converts EM-maps to dummy atom models.
  • ffmaker - Assembles form-factor files for eom and oligomer.
  • launcher - ATSAS launcher
  • mixtures - Analysis of polydisperse systems.
  • nmator - Flexible refinement in torsion-angle space.
  • oligomer - Determine volume fractions of a multi-component mixture.
  • sasplot - SAS Data Plotting
  • shanum - Quantitative assessment of the useful angular data range
  • sreflex - Flexible refinement in cartesian space.

Applications that write .dat:

  • almerge - Finds the best overlap betweem SAS data sets and merges them together.
  • axis - Angular AXIS
  • databsolute - Transform experimental data to absolute scale.
  • dataver - Average two or more experimental data files.
  • datcombine - Combine experimental data files from multiple sources.
  • datcrop - Crop a data file to a specific angular range.
  • datmerge - Scale, shift and merge multiple experimental data files
  • datop - Linear operations on experimental data files.
  • datregrid - Modification of the angular vector.
  • em2dam - Converts EM-maps to dummy atom models.
  • ffmaker - Assembles form-factor files for eom and oligomer.
  • im2dat - Convert an image to a SAS data