General Overview of the package

  The CCDRED package provides a set of basic commands that perform the various calibration steps. These are the combination of calibration frames, subtraction of the bias level (determined from the overscan area or from a separate bias frame), correction for dark current, division by the flat field, correction for illumination, and correction for the fringe pattern. Also, tools are provided for trimming the frame, and for correction of bad pixels. By combining these basic reduction steps a complete reduction pipeline procedure is built, that enables the user to execute a complete automatic reduction of all science frames.

When the context CCDRED enabled, a keyword structure is created which contains the parameters for the calibration steps and to control these. These parameters determine which and how the available reduction steps will be executed. Obviously, in order to get the desired result, these keywords should be correctly filled. Other keywords contain general information, e.g. about the telescope/instrument being used. Finally, keywords are created to contain the names of important frame descriptors, like the standard MIDAS descriptor for the exposure time ( O_TIME). In case information is absent sensible defaults will be used in mostly. Finally, a number of keywords contain information about the status of the reduction.

All operations on a frame that are successfully executed are recorded in its descriptors. This facility, which includes updating the HISTORY descriptor, avoids repetition of reduction sequences, and provides the user with information about what has been done to the data.

Apart from commands that do the actual work, a number of commands will help the user to manage keywords and descriptors and their contents/values. Basically, this means displaying and changing parameters. Also, commands exist to store the current parameter settings and to retrieve these after a session is restarted.

Most of this manual is geared towards the ``automatic approach'', meaning that it is assumed that the user will use the intelligence that has been built into the system. However, the manual also includes documentation about how to execute single basic steps.

The MIDAS CCD package works in combination with the MIDAS Data Organizer which generates, using a set of selection rules, a MIDAS table containing the science frames and their associated calibration frames. Within this chapter this table is referred to the Association Table. This table is important for the package: a number of the commands will only work if the table exists: pipe line processing is only possible with the Association Table. The MIDAS Data Organizer is extensively documented elsewhere in this MIDAS User's Guide.