Data Reduction Frequently Asked Questions
How does the pipeline choose where on the slit to extract the spectrum in long slit mode?
The FORS2 data reduction pipeline extracts a spectrum for each object it detects within the long slit. The object detection is performed on the rectified image if a GLOBAL_DISTORTION_TABLE is provided, otherwise the spatial distortion is not corrected. For non-rectified long slit data broader peaks might appear due to the remaining curvature. The detection algorithm applied is based on the relative peak intensity of each detected object. First of all, a collapsed image is created averaging along the dispersion axis. On this 1-d spatial profile peaks are identified. A peak is identified by a positive value that is preceded and followed by two lower positive values that decrease with distance. For further details, especially concerning the handling of overlapping spectra, please consult the FORS pipeline manual (subsection "Object detection in spectroscopy" in the "Algorithms" section).