Sometimes spectra are very faint, or the desired spectrum is contaminated by other close zero or first orders, or there can be straylight features.These occurrencies are not properly handled by the automatic processing done by omgchain. To circunvent these problems the user can apply omgsource. This task takes a rotated image coming from omgprep, displays it in the terminal and the user can select with the cursor the zero and corresponding first orders to be extracted. The extraction width and the background region definition can be changed as well so as to avoid contaminating features. The task passes the selected values to omgrism and the extraction and calibration continues. The user names the new source list and spectrum files.
The user selected position for the zero order can be refined by omgsource using a centroiding mechanism. Since the position of the zero order determines the wavelength scale, this centroiding can be turned off by the user in the case that close, contaminating features may perturb its results.
An existing source list can be loaded on top of the rotated image so as to check it and to modify it interactively.