Order Extraction

Individual echelle orders are extracted by adding the pixel values over a numerical slit running along the orders, with the position of the slit center defined by equation 7.2. The width of the slit is one pixel and its length, as well as an optional offset to shift the slit prependicular to the dispersion direction, are defined by the user.

The pixel values in the numerical slit are found by linear interpolation of the values in the image. The extracted flux number is the weighted average of these interpolated pixel values. There are three types of weights (w) depending on the selected option:

• option LINEAR: w = 1,
• option AVERAGE: w= 1/L where L is the length of the slit,
• option OPTIMAL: weights optimising the signal to noise ratio of the extracted spectrum. This algorithm is based on (Mukai, 1990)

There are several effects to consider when defining the length of the extraction slit. If the length is too small, the orders are only partially extracted and they present a periodic variation due to the inclination of the orders with respect to the lines in the image. On the other side, if the slit is too large, the extracted flux will include noisy pixels from the flatfielded background, when the flat field correction is applied.

The command EXTRACT/ECHELLE is used for the order extraction. The method is selected using echelle keyword EXTMTD.