Finalizing the image

After subtracting the sky background, we applied SExtractor again to automatically find and mask the background stars according to their intensity, size and shape. The masked positions and areas were put into a list. The objects missed by SExtractor, especially around and on top of the galaxy were manually masked and added to the list. Figure 1 shows a rotated and centralized negative image of the galaxy IC 3322A J after masking.
Rotating and centralizing the galaxy is necessary to be able to create a set of vertical surface brightness profiles, which we use for fitting instead of a full two dimensional pixel distribution. We rotated the images according to the smallest rotation angle so that the galaxy is positioned horizontally in the plane. The disk was symmetrically divided, and using one of the multi-colour views in IRAF and blinking the halves of the images, the galaxy was centralized by eye, allowing for subpixel shifts.

Figure 1: Final masked image of IC 3322A J.