Order extraction

You now have a choice. For quick-look extraction, you can create a mask image whose data values indicate which order (if any) each pixel belongs to. This mask is created by `echmask' and applied by `maskext'. `echmask' allows separate extraction of object and sky but requires the number of rows of object and sky data to be independent of order number. This is not acceptable when (as is usually the case) it is important to minimise the sky noise and to maximise the signal.

For final data reduction or where the use of a mask is not acceptable the orders can be straightened using `cdist' (UCLES is extremely stable and preliminary results indicate that many images can be co-added prior to application of `cdist', so the cost in processing time should be acceptable). Having straightened the orders, `echselect' can be run to identify, for each order, the rows to be used for the object and those to be used for sky.

The result in both cases is a `collapsed échellogram' where X is wavelength and Y is order number. Your object image will of course give rise to both an object and a sky échellogram.