Profiles

This section lists the profiles currently supported by libprofit.

sersic

An implementation of the Sersic luminosity profile. The sersic profile describes the intensity of a galaxy depending on its distance to the center.

The sersic profile accepts the following parameters:

  • xcen: x centre of the Sersic profile (can be fractional pixel positions).
  • ycen: y centre of the Sersic profile (can be fractional pixel positions).
  • mag: Total magnitude of the Sersic profile. Converted to flux using flux = 10^(-0.4*(mag - magzero)), where magzero is that of the containing model.
  • re: Effective radius
  • nser: Sersic index of the Sersic profile.
  • ang: The orientation of the major axis of the Sersic profile, in degrees. The starting point is the positive Y image axis and grows counter-clockwise.
  • axrat: Axial ratio of the Sersic profile defined as minor-axis/major-axis, i.e. axrat = 1 is a circle and axrat = 0 is a line.
  • box: The boxiness of the Sersic profile that traces contours of iso-flux, defined such that r = (x^(2+box)+y^(2+box))^(1/(2+box)). When box = 0 the iso-flux contours will be normal ellipses, but modifications between -1 < box < 1 will produce visually boxy distortions. Negative values have a pin-cushion effect, whereas positive values have a barrel effect (the major and minor axes staying fixed in all cases).

The sersic profile implements recursive sub-pixel sampling for better results in areas closer to the profile center. This sub-sampling can be controller by the following additional parameters:

  • rough: Don’t perform any sub-sampling, ignore the rest of the parameters.
  • rscale_switch: Radius scale fraction within which sub-sampling is performed. Pixels outside this radius are not sub-sampled.
  • max_recursions: The maximum levels of recursions allowed.
  • resolution: Resolution (both horizontal and vertical) to be used on each new recursion level.
  • acc: Accuracy after which recursion stops.

The sersic profile also implements far-pixel filtering, quickly zeroing pixels that are too far away from the profile center. This filtering can be controller by the following parameters:

  • rscale_max: Maximum re-based distance to consider for filtering.
  • rescale_flux: Whether the calculated profile flux should be scaled to take into account the filtering performed by re_max.

Finally, an adjust parameter allows the user whether adjustments of most of the parameters described above should be done automatically depending on the profile parameters. libprofit makes a reasonable compromise between speed and accuracy, and therefore this option is turned on by default.

moffat

The moffat profile works in exactly the same way as the sersic profile. It also supports sub-pixel sampling using the same parameters. Because of the nature of the profile the re and nser parameters from the sersic profiles are not present, and instead the following new parameters appear:

  • fwhm: Full-width at half maximum of the profile across the major-axis of the intensity profile.
  • con: Profile concentration.

ferrer

Again, the ferrer profile works in exactly the same way as the sersic profile. It replaces the re and nser parameters from the sersic profile with:

  • rout: The outer truncation radius.
  • a: The global power-law slope to the profile center
  • b: The strength of truncation as the radius approaches rout.

coresersic

The coresersic profile works in exactly the same way as the sersic profile. In addition to the re and nser parameters from the sersic profile it also adds:

  • rb: The transition radius of the sersic profile.
  • a: The strength of the transition from inner core to outer sersic
  • b: The inner power-law of the core sersic.

king

The king profile works in exactly the same way as the sersic profile. It replaces the re and nser parameters from the sersic profile with:

  • rc: The effective radius of the sersic component.
  • rt: The transition radius of the sersic profile
  • a: The power-law of the King.

brokenexp

The broken exponential profile works in exactly the same way as the sersic profile. It replaces the re and nser parameters from the sersic profile with:

  • h1: The inner exponential scale length.
  • h2: The outer exponential scale length (must be equal to or less than h1).
  • rb: The break radius.
  • a: The strength of the truncation as the radius approaches rb.

sky

The sky profile provides a constant value for an entire image.

  • bg: Value per pixel for the background. This should be the value as measured in the original image, i.e. there is no need to worry about the effect of the model’s magzero.

psf

The psf profile adds the model’s psf to the model’s image at a specific location and for a given user-defined magnitude.

  • xcen: The x position at which to generate the centred PSF (can be fractional pixels).
  • ycen: The y position at which to generate the centred PSF (can be fractional pixels).
  • mag: The total flux magnitude of the PSF.

null

The null profile leaves the image area untouched. It is only useful for testing purposes.