10.1 Details on How to Fit Ellipses

This section gives a more detailed description of the things that is needed to consider when fitting ellipses using the current implementation of GALPHOT. Since the description in the main text (Sect. , p. ) was kept in general terms, we start by introducing the terminology specific to GALPHOT and its implementation. In the following `galaxy' is a generic name for a galaxy observation. An example is r214_GR_1, which is a Gunn r (GR) observation of the galaxy R214. Since this galaxy has been observed more than once in Gunn r, it is given a serial number, in this case 1.

classify is the task that does the classification of objects into `stars' and cosmic-ray-events. It outputs the positions and flagging radii of the two types of objects in the two files galaxy.star and galaxy.cosmic. In addition, the center coordinates of the program galaxy is put in the file galaxy.gal.

harmfit is the task that does the harmonic expansion along concentric circles. It outputs the radial profiles of the Fourier coefficients from the expansion and their uncertainties in the STSDAS table galaxy.har.tab, and also produces a residual image called galaxy.resh.imh.

deviate is the task that flags the pixels that deviate by more than (say) 5 sigma in the above residual image. The output is a list of additional pixels to be flagged called galaxy.pos.del.

ellipfit is the task that does the ellipse fitting. The radial profiles are output in the STSDAS table galaxy.prf.tab, and the residual image is called galaxy.res.imh. ellipfit has some parameters that sometimes need to be fine tune. errcen and errshap are the uncertainties that the first and second order coefficients from harmfit needs to be below to keep the center and shape as free parameters, respectively. As default values for both we used 0.02 for Gunn r and Johnson B, and 0.04 for Johnson U. rcen and rshap can be used to explicitly set $r_{\rm free-center}$ and $r_{\rm free-shape}$ and thus overriding the automatic determination using errcen and errshap. The parameters dposmax, dellmax, and dangmax control how much the center position, the ellipticity, and the position angle, respectively, are allowed to change at a given radius per iteration step. These need to be increased when the initial guess on the profiles computed from the harmfit output are far from the actual values for some reason.

model is a task that can construct a model image from the ellipse fit.

Finally, the task galfit is an `umbrella task' that basically calls the three tasks harmfit, deviate, and ellipfit.

We are now ready to fit an isolated galaxy (Sect. ) or two overlapping galaxies (Sect. ).