The apertures corresponding to each one of the fibres are distributed onto the CCD detector. We have to define and trace these apertures before than to extract them.
The apertures are commonly affected by the field distortions 0 by the optics ot the system and so each spectrum aren't line up along the rows/columns in the dispersion direction. The apertures will follow distorted paths which can be fitted by a polynomial.
For quite faint objects can be necessary a reference image (typically, a flat image, image taken illuminating the system with a uniform lamp -generally tungsten-), which parameters of the tracing polynomial are used to the object image. This is justifiable given the stability of the INTEGRAL/WYFFOS system.
Just before the scattered light subtraction, we define the apertures very narrow (around 0.5 pixels). After the scattered light subtraction, we redefine the apertures to the standard widths (2.8, 3.2 and 5.4 pixels for the SB1, SB2 and SB3 bundles).
This operation can be done with apall (see Appendix I) task (load noao, imred and specred packages).