ECHSELECT allows an interactive selection of the rows containing the object and sky data. The object and sky results are normalized to the number of rows used in each and then placed into separate output files. The sky frame can then be subtracted from the object frame to produce a sky corrected spectrum.
$ ECHSELECT
(IMage) Name of distortion-corrected echellogram [OBJC] -
(PREvious) Read in the previous selection? [NO] -
(WHole) Sum all cross-sections to give cut through image [NO] -
(XStart) First x-axis value for collapse of image [1] - 901
(XEnd) Last x-axis value for collapse of image [2040] - 1000
(MStart) Number of the first order fitted [82] -
(MDelta) Order number increment (-1 or +1) [-1] -
(OBJout) Name of created object echellogram [OBJECT_C] - OBJE
(SKYout) Name of created sky echellogram [OBJECT_C] - OBJK
The analysis adds XEnd-XStart columns to obtain an intensity profile of the object along the slit. A cursor is used to specify the object and sky rows in each order; commands can be listed by typing `H' or `?'. For a correctly oriented image the first order (MStart) has the highest number and the order increment (MDelta) will be -1. The sky image can then be subtracted from the object image.
Once the selections have been made for one object frame it is possible to apply the same extraction to other frames by answering YES to the PREVIOUS keyword.