Subtracts a scaled dark or flash calibration image from a series of images


CALCOR subtracts dark or flash calibration data from a series of bias-corrected images. The calibration data are multiplied by a constant before subtraction, so that calibration data which have been normalised to counts per unit of time per pixel, can be scaled to the "exposure" times suitable for correcting the input data. If the calibration frame data levels are already correct to perform the necessary correction then the data should be scaled by a factor of one. In addition to subtracting the calibration data CALCOR also processes saturated values protecting them from modification. This protection is necessary if the saturated pixels are not to become differentiated.


calcor in out cal expose [preserve] [title]


Name of the image containing the calibration data, this would normally be the output from MAKECAL. The data should be normalised to one exposure unit. It is expected that the calibration image contains dark or flash exposure CCD data which have been bias corrected.

If USESET is true, CAL should be a group expression referring to one calibration frame matching each of the Set Index attributes represented in the IN list; again the name of the file produced by MAKECAL will normally be suitable.

The name of this file may be specified using indirection through a file. [Global calibration image]

A list of (comma separated) values specifying the numbers by which the calibration data need to be multiplied before subtraction from the input data. These are the "exposure" factors for the dark counts expected in the input data or the flash exposure times. If the calibration data have been normalised to reflect the number of counts per second of time, then this is the number of seconds of flash exposure or the number of seconds duration between readouts, if it is a dark counts image. If the calibration image has been produced so that the correct levels are already present, then these values should be returned as one. A quick method of specifying that all the images have the same "exposure" factors is to return a single value, this will then be used for all input images.

The given values must be in the same order as the input images. Indirection through an ASCII file may be used. If more than one line is required to enter the information then a continuation line may be requested by adding "-" to the end of the last value.

Names of the images to be processed. The calibration data will be scaled and subtracted from these. The image names should be separated by commas and may include wildcards.

NOTE the use of wildcards with this program is NOT recommended unless the input images all have the same calibration exposure factors. The processing order of any wildcarded images cannot be guaranteed.

Whether to keep (i.e. not delete) the input images (parameter IN) or not. Deleting the input images has the advantage of saving disk space, but should probably only be used if this program is part of a sequence of commands and the intermediary data produced by it are not important.

The calibration master frame (parameter CAL) is never deleted.

The default for this parameter is TRUE and this cannot be overridden except by assignment on the command line or in response to a forced prompt. [TRUE]

Name of the CCDPACK logfile. If a null (!) value is given for this parameter then no logfile will be written, regardless of the value of the LOGTO parameter.

If the logging system has been initialised using CCDSETUP then the value specified there will be used. Otherwise, the default is "CCDPACK.LOG". [CCDPACK.LOG]

Every CCDPACK application has the ability to log its output for future reference as well as for display on the terminal. This parameter controls this process, and may be set to any unique abbreviation of the following:
  • TERMINAL – Send output to the terminal only

  • LOGFILE – Send output to the logfile only (see the LOGFILE parameter)

  • BOTH – Send output to both the terminal and the logfile

  • NEITHER – Produce no output at all

If the logging system has been initialised using CCDSETUP then the value specified there will be used. Otherwise, the default is "BOTH". [BOTH]

Names of the output images. These may be specified as list of comma separated names, using indirection if required, OR, as a single modification element (of the input names). The simplest modification element is the asterisk "" which means call each of the output images the same name as the corresponding input images. So:

IN >

signifies that all the images in the current directory should be used and the output images should have the same names.

Other types of modification can also occur, such as:

OUT > tmp_

which means call the output images the same as the input images but put tmp_ in front of the names. Replacement of a specified string with another in the output file names can also be used:

OUT > tmp_|debias|flattened|

this replaces the string debias with flattened in any of the output names tmp_.

NOTE the use of wildcards with this program is not recommended unless the input images all have the same calibration exposure factors. The order of processing of any wildcarded images cannot be guaranteed.

If the input data type is to be preserved and used for processing then this parameter should be set TRUE. If this parameter is set FALSE then the input data will be processed and returned in a suitable floating point representation. This option is useful if the output data will have a significant number of BAD values due to numeric errors (over or under flow), or if unacceptable loss of precision will occur if the data are processed in their initial data type (due to rounding errors).

Note if a global value for this parameter has been set, using CCDSETUP, then this will be used. [TRUE]

The data saturation value, if it has been applied. See SETSAT. [1.0D6]
If the input data have had a saturation value applied then this parameter should be given as TRUE. If the input data have been processed within CCDPACK then the saturation value will have been stored within the CCDPACK extension, if this is so then this value will be used. Note that data with different saturation properties (i.e. values) which have not been set within CCDPACK will require separate processing (i.e. in groups with the same properties – see notes). [FALSE]
Title for the output images. [Output from CALCOR].
Whether to use Set header information or not. If USESET is false then any Set header information will be ignored. If USESET is true, then the CAL parameter is taken to refer to a group of files, and each IN file will be processed using a calibration image with a Set Index attribute which matches its own. An IN file with no Set header is considered to match a CAL file with no Set header, so USESET can safely be set true when the input files contain no Set header information.

If a global value for this parameter has been set using CCDSETUP then that value will be used. [FALSE]


calcor frame1 frame2 calibration 250
This example runs CALCOR in its most basic mode. The input data in image frame1 has the data in image calibration subtracted, after multiplying by 250. The resultant data is written to image frame2. Note that if saturation values have been applied to the data in frame1 within CCDPACK, then this will be handled automatically. The output data will be of the same type as the input data.
calcor in=^frames.dat out=_darksub cal=dark_master expose=^dark_exposures
In this example a list of images are sequentially processed. The list of image names is stored in the file frames.dat. The output images are named after the corresponding input image with the characters _darksub appended. The dark times for each input frame are read from the file dark_exposures. This is the recommended method for processing lists of input images.
calcor l1551_f11 l1551_f11_ds dark_master 1.0 preserve=false logto=both logfile=l1551_darkcor.log title=dark_corrected_data
This example follows a similar theme to the first example, except that the output data type is now _REAL or _DOUBLE, depending on the precision required to process the data. The calibration correction data are assumed to have the right exposure factor. The output image is given the title "dark_corrected_data" and the parameters used by CALCOR are stored in the logfile l1551_darkcor.log.
calcor in=ngc4151r_f1 cal=flash_master out=ngc4151r_f1_dc expose=310.0 setsat saturation=32767
In this example a saturation value external to CCDPACK has been applied to the input image. This is indicated by setting SETSAT TRUE and by supplying the saturation value. Values which are greater than or equal to the saturation value are left unmodified by the calibration frame subtraction. This may leave the saturated values "displaced" from the local values, causing a discontinuity in the local isophotes, but is the only method by which the saturated pixels may still be readily identified after the subtraction of the calibration frame.

See also

Flash or dark calibration”, MAKECAL.


Behaviour of parameters

Most parameters retain their current value as default. The "current" value is the value assigned on the last run of the application. If the application has not been run then the "intrinsic" defaults, as shown in the parameter help, apply. The exceptions to this rule are:

Retaining parameter values has the advantage of allowing you to define the default behaviour of the application but does mean that additional care needs to be taken when using the application on new datasets/different devices, or after a break of sometime. The intrinsic default behaviour of the application may be restored by using the RESET keyword on the command line.

Certain parameters (LOGTO, LOGFILE, USESET, PRESERVE and CAL) have global values. These global values will always take precedence, except when an assignment is made on the command line. In general global values may be set and reset using the CCDSETUP and CCDCLEAR commands, however, the CAL parameter may only be set by a run of the application MAKECAL.

Implementation Status: