controlpar -- Set the algorithm control parameters for the ellipse task.
This pset is used to set the algorithm control parameters associated with the ellipse task. Two basic controls are implemented here:
- At each given isophotal semi-major axis, a convergency criterion for stopping iterations.
- At large semi-major axis lengths, a detection criterion for stopping growing the ellipses to regions of too low signal-to-noise ratio.
The in-isophote controls are provided by parameters conver, minit and maxit. Iterations are stopped when either one of the following criteria are met:
- The largest harmonic amplitude is less than conver times the rms residual of the intensity data around the harmonic fit (see discussion in samplepar pset).
- maxit iterations are reached.
- More than a given fraction of the elliptical sample points have no valid data in then, either because they lie outside the image boundaries or because they where flagged out from the fit (see samplepar pset).
In any case, a minimum number of iterations minit is always performed. A minimum of 10 iterations guarantees that, on average, 2 iterations will be available for fitting each independent parameter (the four harmonic amplitudes and the intensity level). In the first isophote, the minimum number of iterations is 2 * minit, to ensure that, even departing from not-so-good initial values, the algorithm converges to an appropriate solution.
The main control for preventing ellipses to grow to regions of too low signal-to-noise ratio is provided by parameter maxgerr. It specifies the maximum acceptable relative error in the local radial intensity gradient. Experiments (see paper quoted in the ellipse help page) showed that the fitting precision relates to that relative error. Its usual behavior is to increase with semi-major axis, being larger in outer, fainter galaxy regions.
A number of actions may happen when the current gradient error becomes larger than maxgerr (or becomes non-significant and thus is set to INDEF) in the process of increasing semi-major axis length. If the maximum semi-major axis specified by parameter geompar.maxsma is set to INDEF, semi-major axis grow is stopped and the algorithm proceeds inwards to the galaxy image center. If geompar.maxsma is set to some finite value, and this value is larger than the current semi-major axis length, the algorithm enters non-iterative mode and proceeds outwards until reaching geompar.maxsma.
Non-iterative mode can also be entered if the ellipse center wanders from its former position by an amount larger than parameter wander.
In non-iterative mode, the ellipse geometry parameters (center, ellipticity, position angle) are kept fixed at their last fitted values. This may be useful for sampling regions of very low surface brightness, where the algorithm may become unstable and unable to recover reliable geometry information from isophotes. Non-iterative mode can be entered also if the ellipticity exceeds 1.0 or the ellipse center crosses the image boundaries.
The "soft stop" feature is provided as a help in overcoming premature stopping by the maxgerr criterion when measuring complex images that may trigger the criterion because of, e.g. contamination due to stars, HII regions and the like. The gradient error may become artificially large in an "isophote" with such contamination, despite the fact that outer isophotes still exist with enough signal-to-noise to be properly measured. When the soft flag is set to yes, the maxgerr criterion will be triggered only by two consecutive isophotes having their gradient errors exceeding maxgerr.
Parameters hcenter, hellip and hpa are provided for holding the ellipse geometry parameters fixed, effectively removing them from the fit process. Note that, if the geompar.recenter parameter is set to "yes", recentering of the first fitted ellipse will take place even if hcenter is set to "yes".
In some cases the object locator algorithm mail fail, even though there is enough signal-to-noise to start a fit (e.g. in objects with very high ellipticity). In those cases the sensitivity of the algorithm can be decreased, or it can even be turned off, by lowering the value of parameter olthresh.
- (conver = 0.05) [real, min=0.0]
- Convergency criterion. Largest harmonic amplitude must be smaller than conver times the fit rms.
- (minit = 10) [int, min=1]
- Minimum number of iterations at each isophote.
- (maxit = 50) [int, min=2]
- Maximum number of iterations at each isophote.
- (hcenter = no) [boolean]
- Hold ellipse center fixed during the fit ?
- (hellip = no) [boolean]
- Hold ellipticity fixed during the fit ?
- (hpa = no) [boolean]
- Hold position angle fixed during the fit ?
- (wander = INDEF) [real, min=0.0]
- Limit for ellipse center wandering (in pixels). INDEF disables this control.
- (maxgerr = 0.5) [real, min=0.0]
- Maximum acceptable gradient relative error.
- (olthresh = 1.0) [real, min=0.0]
- Threshold for the object locator algorithm. By lowering this value the object locator becomes less strict, in the sense that it will accept lower signal-to-noise data. If set to zero, the x0, y0 values found in the geompar pset are used without questioning.
- (soft = no) [boolean]
- Soft stop ?