For each SCIENCE frame reduced in optimum extraction mode, the pipeline creates a result table of category "ORDER_TRACE" which contains detailed information about the position of the extracted signal. The position information is visualized in the QC1 plot "signal position". There is one such plot per CCD (i.e. per reduced science spectrum).

The information is plotted in a matrix which has seven columns for the order-by-order plots, 1 column for aggregate plots, and a final column for summary information.

Find a complete sample plot for order position here.

There is tutorial on position and FWHM plots which gives more information about the proper and problematic cases.

Order-by-order plots.
- For each order, the position of the signal in the extraction slit is determined by the pipeline (with input from the order table) for about 100 chunks. These positions are plotted as red dots (column :P_CH of ordertrace table).

- In the next step, a 2nd order polynomial is fitted to these values where all values with an error above a threshold (2 sigma rms) are excluded. These points are taken from column :P_F0 and plotted green.

- In the last step, a kappa-sigma routine is used to clean the positions from outliers with high weight factors. The final result is in column :P_F1 and plotted blue.

These plots are provided for each echelle order in the spectrum. Abscissa is X coordinate (dispersion direction) in pixels, ordinate is pseudo-Y coordinate (cross-dispersion) in pixels. 'Pseudo' means 'along the slit' since inclination of the orders is already removed. Any inclination or curvature along the orders visible in the QC1 plot is therefore due to higher-order effects or to 2D structure of the object. The Y window is 5 pixels high.

The fitting and clipping routine has been optimized to obtain a signal tracking solution stable against cosmic hits, bad columns, absorption or emission lines etc.

Aggregate plot (per 7orders). For each set of 7 orders constituting a row in the QC1 plot matrix, all :P_CH values (red dots) are plotted together in the 8th column, with their mean value indicated. This 'aggregate' plot allows, in cases of faint signal with large scatter of points, a better evaluation of the reliability of the results.

Aggregate plot (all orders). For the same reason, there is a similar plot (in column 9) with all :P_CH values (red dots) combined.

Aggregate plots (combined view). The aggregate plots in column 8, viewed together, permit a quick-look evaluation of certain typical extraction problems. E.g., they show whether atmospheric dispersion has shifted the bluest orders strongly. Also it is clearly visible if a certain object has no flux below a certain blue wavelength.

Summary information (mean values). Finally there is a plot with the mean position values (orderwise). These values are also plotted in the main reduction QC1 plot and are stored in the FITS header (keywords HIERARCH ESO QC OBJnn POS where nn is the order number). Here basically two effects, atmospheric dispersion and insufficient flux, become visible.