The tracing of the paths of the orders across the data frame is often a source of difficulty as it is fairly easy for blemishes in the frame to fatally deflect order tracing algorithms from the actual path of the order. ECHOMOP provides a variety of options to help combat these problems. ECHOMOP order tracing first locates the positions of the orders at the centre of the frame, and estimates the average order slope. It uses this information to predict the existence of any partial orders at the top/bottom of the frame which may have been missed by the examination of the central columns during order location.
Tracing then proceeds outwards from the centre of each order. At each step outwards a variable size sampling box is used to gather a set of averages for the rows near the expected order centre. The centre of this data is then evaluated by one of the following methods:
The trace algorithm will loop increasing its sampling box size automatically when it fails to find a good centre. The sample box can increase up to a size governed by the measured average order separation.
When a set of centres have been obtained for an order, a polynomial is fitted to their coordinates. The degree is selectable. For ideal data, these polynomials will represent an accurate reflection of the path of the order across the frame. For real data it is usually helpful to refine these polynomials by clipping the most deviant points, and re-fitting. Options are provided to do this automatically or manually.
When dealing with distorted data it is often necessary to use a high degree polynomial to accurately fit the order traces. This in turn can lead to problems at the edges of the frame when the order is often faint.
Typically the polynomial will `run away' from the required path. The simplest solution is of course to re-fit with a lower order polynomial, however, this may not be satisfactory if the high degree is necessary to obtain a good fit over the rest of the order.
In these circumstances, and others where one or more orders polynomials have `run away', ECHOMOP provides an automatic consistency checker. The consistency checking task works by fitting polynomials to order-number and Y-coordinate at a selection of positions across the frame. The predicted order centres from both sets of polynomials are then compared with each other and then mean and sigma differences calculated. The `worst' order is then corrected by re-calculating its trace polynomial using the remaining orders (but excluding its own contribution). This process is repeated until the mean deviation between the polynomials falls below a tunable threshold value. The consistency checker will also cope with the `bad' polynomials which can result when partial orders have been automatically fitted.
Viewing traced paths
The tracing of the échelle order paths is central to the entire extraction process and care should be taken to ensure the best traces possible. ECHOMOP provides a large number of processing alternatives to help ensure this can be done. Most of these provide information such as RMS deviations etc., when run. In general however, the best way of evaluating the success or failure of the tracing process is to visually examine the paths of the trace fitted polynomials. Three methods of viewing the traced paths are provided.
For a single order, a more detailed examination of the relation of a trace polynomial to the points it fits, can be obtained using the V(view) command in the task ech_fitord/ECHMENU Option 3.
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