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Order definition: Method Hough

The method described above is recommended because it is quick and usually efficient. However an alternative method is available which enables you to process images of lower quality than required by DEFINE/ECHELLE and generally returns more accurate results. The price to be paid however is a higher demand for CPU time. The algorithm accepts frame which can be mildly contaminated by particle hits, bad columns, order gaps due to absorption lines, as found in FLAT or STD. If the frame is of very low quality for the order definition, as it may be the case for a science frame, an initial cleaning is required.

The algorithm assumes that:

The method is enabled by the parameter:

SET/ECHELLE DEFMTD=HOUGH

The command can run in a fully automatic mode if all three parameters NBORDI, WIDTHI, THRESI are set to zero. To enforce the detection of a given number of orders, set the value of NBORDI. If the frame has a low contrast or if some areas of the interorder background are brighter than faint orders in the image, it may be useful to set WIDTHI. Given hwthe half width of the orders and io the mean interorder distance, the optimal value of WIDTHI is in the range ]hw,io - hw[. The threshold THRESI is normally estimated for each order independently. However the value can be enforced by giving a non null value to THRESI.

See also the help file of the commands DEFINE/HOUGH and HOUGH/ECHELLE for more details about the possibilities of this method.

If the image is overscanned, that is includes unsensitive areas in its lower and upper parts, it is better to avoid this areas by use of the command SCAN/ECHELLE.

The best way to use this command is to start with null value for all three parameters NBORDI, WIDTHI, THRESI. For example:

INIT/ECHELLE
SET/ECHELLE DEFMTD=HOUGH ORDREF= ... (order reference frame)
SCAN/ECHELLE ORDREF CURSOR
DEFINE/HOUGH

The successive steps are the following:


next up previous contents
Next: Wavelength calibration Up: Reduction using Standard Stars Previous: Order Definition: Methods STD
Petra Nass
1999-06-15