Basic Reduction Steps

The software available in the CCD package takes care of the relative calibrations of the pixel intensities, of averaging, and of cleaning frames. Cleaning in this context means removal of the instrumental signature and other defects from the frames.

A full reduction of CCD data involves the steps outlined below:

• fit and subtract a readout bias given by the overscan strip;
• trim the frame of the overscan strip and other irrelevant columns and/or rows;
• combine the bias, dark, and flat calibration frames (if taken);
• subtract the average bias frame;
• remove the defects from the average dark frame;
• scale the average dark frame and subtract;
• remove the defects from the average flat frame;
• prepare the final flat (subtract dark and normalize; possibly apply illumination correction);
• divide by the flat field;
• fix the bad pixels in the output frame;
• correct for fringe pattern;
• combine science frames.

Some of these steps are optional and depend on the kind of data you have taken. In addition to the operations described here, MIDAS contains a number of other operations like removing of cosmic rays. As indicated above, all steps can be executed in an automatic (batch) mode provided the keyword settings have been done correctly.

In the automatic procedure, the highest level of processing is the loop over all science frames selected in the Association Table. If, for whatever reason, the processing of a frame fails, this frame is skipped and the processing is continued with the next one.

The CCD package system provides a single command for doing the entire calibration of the CCD frames: REDUCE/CCD. The command is a composition of a number of lower level procedures, each of them taking care of a particular part of the calibration procedure, and executable via a separate MIDAS command. These components are: overscan correction, trimming, combining, bias correction, dark subtraction, flat fielding, illumination correction, and fringe correction.

Whether you want one or more calibration steps be executed depends on the settings for the various options. Therefore, before starting be sure that the keywords for the reduction are set correctly. These keywords and their meaning are listed in Table 3.3. Also, the keywords for combining calibration frames should be checked. Furthermore, fill the keyword CCDASSOC with the name of the Association Table, and check that this table is correct. All keywords can be filled and displayed with the commands SET/CCD and SHOW/CCD.

 

Table 3.3: Keywords for setting the reduction process

Keyword	Content	Default	Description
CCD_IN	name	?	input frame or table
SC_TYP	exp. type	*	exposure type of input frame
SC_PROC	yes|no	no	list processing only
SC_SCAN	yes|no	no	applied overscan offset correction
SC_TRIM	yes|no	no	trim the frame from overscan area
SC_FXPIX	yes|no	no	bad pixel correction
SC_BSCOR	yes|no	yes	bias correction
SC_DKCOR	yes|no	yes	dark current correction
SC_FFCOR	yes|no	yes	flat field correction
SC_ILCOR	yes|no	no	illumination correction
SC_FRCOR	yes|no	no	fringing correction
SC_BSFRM	name		bias calibration frame
SC_DKFRM	name		dark current calibration frame
SC_FFFRM	name		flat field calibration frame
SC_ILFRM	name		illumination calibration frame
SC_FRFRM	name		fringing calibration frame