Choosing between ELLPRO and ELLFOU

  ELLFOU and ELLPRO work in very different ways. ELLPRO works by creating trial ellipses which it places on the galaxy. It then carefully adjusts the shape and position of the ellipses until the brightness variation around the ellipse is minimised. ELLFOU identifies the locations of all the pixels in a given isophotal range, takes a subset of those and then trys to fit an ellipse through the pixels that minimises the pixel/ellipse distance sum. These approaches are known as intensity and contour analysis respectively.

You might well ask then why two routines are provided. The reason is simply that galaxies vary enormously. If these routines are used with a normal galaxy of approximately elliptical shape they will normally both behave well. But this is not always the case. Limits to this sensible behaviour are outlined below.