| geompar | stsdas.analysis.isophote | geompar |
geompar -- Set the geometric parameters for the ellipse task.
geompar
This pset is used to set the geometrical parameters associated with the ellipse task.
There are two basic types of geometrical parameters:
After fitting the first isophote, departing from values taken from type 1 parameters, the task proceeds by updating the semi-major axis length following rules defined by type 2 parameters, and using at each step for the first guess ellipse parameters the solution from the previous fit.
The semi-major axis update can be linear or geometric, depending on the linear task parameter. If the geometric growing mode is chosen, the semi-major axis length is increased by a factor of (1. + step), and the process is repeated until either the semi-major axis value reaches the value maxsma, or the last fitted ellipse has more than a given fraction of its sampled points flagged out (see samplepar pset). The process then resumes from the first fitted ellipse (at sma0) inwards, in steps of (1./(1. + step)), until the semi- major axis length reaches the value minsma. In case of linear growing, the increment or decrement value is given directly by step in pixels. If maxsma is set to INDEF, the semi-major axis will grow until a low signal-to-noise criterion is met. See controlpar pset for details.
Type 1 parameters have default values that usually guarantee that the first isophote will be fitted properly. The starting semi-major axis length, sma0, cannot be very small or zero because in the very central regions of a galaxy image the geometry information is usually too much distorted due to pixelation and/or seeing. It cannot be too large either, because at external galaxy regions the low signal-to-noise might preclude a proper fit. An intermediate region will ensure the proper balance between high S/N, low geometrical distortion, and reasonably large number of pixels.
Position angles are defined in the range -90 < PA <= 90. Avoid using as starting position angle pa0 = 0., since the algorithm may not work properly in this case. When the object is such that position angles are near either extreme of the range, noise can make the solution jump back and forth, between successive isophotes, by amounts close to 180 degrees. The output table may be cleaned of those jumps by processing it with task ttools.tcalc using
equals = if PA<0. then PA+180. else PAor a similar expression.
Parameter maxrit sets the maximum semi-major axis length for iterative mode. When fitting at larger semi-major axis lengths, 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. If set to INDEF, iterative mode continues out to where lack of either signal-to-noise or number of valid data points forces the task to stop growing the semi-major axis (see controlpar pset).
The algorithm has no ways of finding where, in the input image section, the galaxy to be measured sits in. That is, x0 and y0 must be properly set from start. Since they are set by default to INDEF, the task has a number of options to set them properly. First, an object locator routine is run, scanning a 10X10 window centered either on the input x0, y0 coordinates or, if any one of them, or both, are set to INDEF, on the input image section center. A number of actions are possible depending on the successful (or not) acquisition of an object. Below it is shown what takes place depending on the values of parameters interactive, recenter and xylearn:
Parameter xylearn is used to automatically update the pset when valid center coordinates become available. If xylearn is set to yes and x0,'y0' are set to INDEF, the task will write to the geompar pset the valid values that will come either from the object locator or the cursor/STDIN input. If xylearn is set to no, nothing happens. This feature is useful when trying several runs of ellipse on the same object. The first time the task is run, the object center must be defined by the user, but in subsequent runs this step is skipped.
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. See the controlpar pset.
Full support for World Coordinate System (WCS) is not available in this version of ellipse. However there is a simpler scheme for handling relative coordinates, that works only in pixel units. Task parameter physical controls the choice between "physical" or "section" coordinate systems. If set to no, pixel coordinates relative to the input image section will be used throughout. If set to yes, pixel coordinates will be relative to the full 2-D frame corresponding to the input image name eventually stripped of subsection specification. This feature can be useful, for instance, when handling a large image with many objects scattered around the field. Each object can be measured by an independent ellipse call in a batch script, using for each object just the subsection that contains the object, thus saving memory and speeding up the fit. The resulting tables with center coordinates will nevertheless share the same coordinate system of the main image. This section handling scheme also supports stepping (the same in both directions X and Y) and multi-dimensional image files.
ellipse, controlpar
