Sloping Orders

A common problem when extracting echelle orders is that they are significantly sloped with respect to the pixel rows/ columns.

ECHOMOP can cope with highly sloping orders because it works by first locating a single point in each order and then using this as the start point for tracing the order path.

ECHOMOP also calculates the approximate slope of the orders prior to tracing, and uses this information to predict the order position when tracing fails (usually due to cosmic-ray contaminated pixels).

The slope is estimated by taking averaged samples of the frame along two lines. One running from top-left to bottom-right and the other from top-right to bottom-left of the frame.

These diagonals are then searched for peaks, starting from the frame centre outwards towards the corners. This convoluted method is immune from problems due to partial orders which affect other algorithms, as each must be identified twice to allow its slope to be estimated. Partial orders which are only sampled once by the diagonals will not confuse the estimate.

The estimate obtained by this method is however, an average value over all orders. It is therefore not applicable when the orders are highly distorted, or when each order has a different slope.

The slope as calculated by the program may be overridden manually using the FIGARO command SETOBJ:

    SETOBJ object=/rdctn-file/.MORE.ECHELLE.ORDER_SLOPE value=n.nnnn

where n.nnnn= the X/Y pixel ratio.