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ccdproc noao.imred.ccdred


NAME · USAGE_ · PARAMETERS · DESCRIPTION · 1._FIXPIX · 2._OVERSCAN
3._TRIM · 4._ZEROCOR · 5._DARKCOR · 6._FLATCOR · 7._ILLUMCOR
8._FRINGECOR · 9._READCOR · 10._SCANCOR · 11._PROCESSING_STEPS
12._PROCESSING_ARITHMETIC · 13._IN_THE_ABSENCE_OF_IMAGE_HEADER_INFORMATION
EXAMPLES · TIME_REQUIREMENTS · REVISIONS · SEE_ALSO

NAME

ccdproc -- Process CCD images

USAGE

ccdproc images

PARAMETERS

images
List of input CCD images to process. The list may include processed images and calibration images.
output = ""
List of output images. If no list is given then the processing will replace the input images with the processed images. If a list is given it must match the input image list. Note that any dependent calibration images still be processed in-place with optional backup.
ccdtype = ""
CCD image type to select from the input image list. If no type is given then all input images will be selected. The recognized types are described in ccdtypes .
max_cache = 0
Maximum image caching memory (in Mbytes). If there is sufficient memory the calibration images, such as zero level, dark count, and flat fields, will be cached in memory when processing many input images. This reduces the disk I/O and makes the task run a little faster. If the value is zero image caching is not used.
noproc = no
List processing steps only?

PROCESSING SWITCHES

fixpix = yes
Fix bad CCD lines and columns by linear interpolation from neighboring lines and columns? If yes then a bad pixel mask, image, or file must be specified.
overscan = yes
Apply overscan or prescan bias correction? If yes then the overscan image section and the readout axis must be specified.
trim = yes
Trim the image of the overscan region and bad edge lines and columns? If yes then the data section must be specified.
zerocor = yes
Apply zero level correction? If yes a zero level image must be specified.
darkcor = yes
Apply dark count correction? If yes a dark count image must be specified.
flatcor = yes
Apply flat field correction? If yes flat field images must be specified.
illumcor = no
Apply illumination correction? If yes illumination images must be specified.
fringecor = no
Apply fringe correction? If yes fringe images must be specified.
readcor = no
Convert zero level images to readout correction images? If yes then zero level images are averaged across the readout axis to form one dimensional zero level readout correction images.
scancor = no
Convert zero level, dark count and flat field images to scan mode flat field images? If yes then the form of scan mode correction is specified by the parameter scantype .

PROCESSING PARAMETERS

readaxis = "line"
Read out axis specified as "line" or "column".
fixfile
Bad pixel mask, image, or file. If "image" is specified then the name is specified in the image header or instrument translation file. If "BPM" is specified then the standard BPM image header keyword defines a bad pixel mask. A bad pixel mask is a compact format (".pl" extension) with zero values indicating good pixels and non-zero values indicating bad pixels. A bad pixel image is a regular image in which zero values are good pixels and non-zero values are bad pixels. A bad pixel file specifies bad pixels or rectangular bad pixel regions as described later. The direction of interpolation is determined by the mask value with a value of two interpolating across columns, a value of three interpolating across lines, and any other non-zero value interpolating along the narrowest dimension.
biassec
Overscan bias strip image section. If "image" is specified then the overscan bias section is specified in the image header or instrument translation file. Only the part of the bias section along the readout axis is used. The length of the bias region fit is defined by the trim section. If one wants to limit the region of the overscan used in the fit to be less than that of the trim section then the sample region parameter, sample , should be used. It is an error if no section or the whole image is specified.
trimsec
image section for trimming. If "image" is specified then the trim image section is specified in the image header or instrument translation file.
zero = ""
Zero level calibration image. The zero level image may be one or two dimensional. The CCD image type and subset are not checked for these images and they take precedence over any zero level calibration images given in the input list.
dark = ""
Dark count calibration image. The CCD image type and subset are not checked for these images and they take precedence over any dark count calibration images given in the input list.
flat = ""
Flat field calibration images. The flat field images may be one or two dimensional. The CCD image type is not checked for these images and they take precedence over any flat field calibration images given in the input list. The flat field image with the same subset as the input image being processed is selected.
illum = ""
Illumination correction images. The CCD image type is not checked for these images and they take precedence over any illumination correction images given in the input list. The illumination image with the same subset as the input image being processed is selected.
fringe = ""
Fringe correction images. The CCD image type is not checked for these images and they take precedence over any fringe correction images given in the input list. The fringe image with the same subset as the input image being processed is selected.
minreplace = 1.
When processing flat fields, pixel values below this value (after all other processing such as overscan, zero, and dark corrections) are replaced by this value. This allows flat fields processed by ccdproc to be certain to avoid divide by zero problems when applied to object images.
scantype = "shortscan"
Type of scan format used in creating the CCD images. The modes are:
"shortscan"
The CCD is scanned over a number of lines and then read out as a regular two dimensional image. In this mode unscanned zero level, dark count and flat fields are numerically scanned to form scanned flat fields comparable to the observations.
"longscan"
In this mode the CCD is clocked and read out continuously to form a long strip. Flat fields are averaged across the readout axis to form a one dimensional flat field readout correction image. This assumes that all recorded image lines are clocked over the entire active area of the CCD.
nscan
Number of object scan readout lines used in short scan mode. This parameter is used when the scan type is "shortscan" and the number of scan lines cannot be determined from the object image header (using the keyword nscanrows or it's translation).

OVERSCAN FITTING PARAMETERS

There are two types of overscan (or prescan) determinations. One determines a independent overscan value for each line and is only available for a readaxis of 1. The other averages the overscan along the readout direction to make an overscan vector, fits a smoothing function to the vector, and then evaluate and then evaluates the smooth function at each readout line or column. The line-by-line determination only uses the function parameter and the smoothing determinations uses all the following parameters.

function = "legendre"
Line-by-line determination of the overscan is specified by:

         mean - the mean of the biassec columns at each line
       median - the median of the biassec columns at each line
       minmax - the mean at each line with the min and max excluded

The smoothed overscan vector may be fit by one of the functions:

     legendre - legendre polynomial
    chebyshev - chebyshev polynomial
      spline1 - linear spline
      spline3 - cubic spline
order = 1
Number of polynomial terms or spline pieces in the overscan fit.
sample = "*"
Sample points to use in the overscan fit. The string "*" specified all points otherwise an icfit range string is used.
naverage = 1
Number of points to average or median to form fitting points. Positive numbers specify averages and negative numbers specify medians.
niterate = 1
Number of rejection interations to remove deviant points from the overscan fit. If 0 then no points are rejected.
low_reject = 3., high_reject = 3.
Low and high sigma rejection factors for rejecting deviant points from the overscan fit.
grow = 0.
One dimensional growing radius for rejection of neighbors to deviant points.
interactive = no
Fit the overscan vector interactively? If yes and the overscan function type is one of the icfit types then the average overscan vector is fit interactively using the icfit package. If no then the fitting parameters given below are used.

DESCRIPTION

Ccdproc processes CCD images to correct and calibrate for detector defects, readout bias, zero level bias, dark counts, response, illumination, and fringing. It also trims unwanted lines and columns and changes the pixel datatype. It is efficient and easy to use; all one has to do is set the parameters and then begin processing the images. The task takes care of most of the record keeping and automatically does the prerequisite processing of calibration images. Beneath this simplicity there is much that is going on. In this section a simple description of the usage is given. The following sections present more detailed discussions on the different operations performed and the order and logic of the processing steps. For a user's guide to the ccdred package see guide . Much of the ease of use derives from using information in the image header. If this information is missing see section 13.

One begins by setting the task parameters. There are many parameters but they may be easily reviewed and modified using the task eparam . The input CCD images to be processed are given as an image list. Previously processed images are ignored and calibration images are recognized, provided the CCD image types are in the image header (see instruments and ccdtypes ). Therefore it is permissible to use simple image templates such as "*.imh". The ccdtype parameter may be used to select only certain types of CCD images to process (see ccdtypes ).

The processing operations are selected by boolean (yes/no) parameters. Because calibration images are recognized and processed appropriately, the processing operations for object images should be set. Any combination of operations may be specified and the operations are performed simultaneously. While it is possible to do operations in separate steps this is much less efficient. Two of the operation parameters apply only to zero level and flat field images. These are used for certain types of CCDs and modes of operation.

The processing steps selected have related parameters which must be set. These are things like image sections defining the overscan and trim regions and calibration images. There are a number of parameters used for fitting the overscan or prescan bias section. These are parameters used by the standard IRAF curve fitting package icfit . The parameters are described in more detail in the following sections.

In addition to the task parameters there are package parameters which affect ccdproc . These include the instrument and subset files, the text and plot log files, the output pixel datatype, the amount of memory available for calibration image caching, the verbose parameter for logging to the terminal, and the backup prefix. These are described in ccdred .

Calibration images are specified by task parameters and/or in the input image list. If more than one calibration image is specified then the first one encountered is used and a warning is issued for the extra images. Calibration images specified by task parameters take precedence over calibration images in the input list. These images also need not have a CCD image type parameter since the task parameter identifies the type of calibration image. This method is best if there is only one calibration image for all images to be processed. This is almost always true for zero level and dark count images. If no calibration image is specified by task parameter then calibration images in the input image list are identified and used. This requires that the images have CCD image types recognized by the package. This method is useful if one may simply say "*.imh" as the image list to process all images or if the images are broken up into groups, in "@" files for example, each with their own calibration frames.

When an input image is processed the task first determines the processing parameters and calibration images. If a requested operation has been done it is skipped and if all requested operations have been completed then no processing takes place. When it determines that a calibration image is required it checks for the image from the task parameter and then for a calibration image of the proper type in the input list.

Having selected a calibration image it checks if it has been processed by looking for the image header flag CCDPROC. If it is not present then the calibration image is processed. When any image has been processed the CCDPROC flag is added. For images processed directly by ccdproc the individual processing flags are checked even if the CCDPROC flag is present. However, the automatic processing of the calibration images is only done if the CCDPROC flag is absent! This is to make the task more efficient by not having to check every flag for every calibration image for every input image. Thus, if additional processing steps are added after images have been partially reduced then input images will be processed for the new steps but calibration images will not be processed automatically.

After the calibration images have been identified, and processed if necessary, the images may be cached in memory. This is done when there are more than two input images (it is actually less efficient to cache the calibration images for one or two input images) and the parameter max_cache is greater than zero. When caching, as many calibration images as allowed by the specified memory are read into memory and kept there for all the input images. Cached images are, therefore, only read once from disk which reduces the amount of disk I/O. This makes a modest decrease in the execution time. It is not dramatic because the actual processing is fairly CPU intensive.

Once the processing parameters and calibration images have been determined the input image is processed for all the desired operations in one step; i.e. there are no intermediate results or images. This makes the task efficient. If a matching list of output images is given then the processed image is written to the specified output image name. If no output image list is given then the corrected image is output as a temporary image until the entire image has been processed. When the image has been completely processed then the original image is deleted (or renamed using the specified backup prefix) and the corrected image replaces the original image. Using a temporary image protects the data in the event of an abort or computer failure. Keeping the original image name eliminates much of the record keeping and the need to generate new image names.

1. Fixpix

Regions of bad lines and columns may be replaced by linear interpolation from neighboring lines and columns when the parameter fixpix is set. This algorithm is the same as used in the task fixpix . The bad pixels may be specified by a pixel mask, an image, or a text file. For the mask or image, values of zero indicate good pixels and other values indicate bad pixels to be replaced.

The text file consists of lines with four fields, the starting and ending columns and the starting and ending lines. Any number of regions may be specified. Comment lines beginning with the character # may be included. The description applies directly to the input image (before trimming) so different files are needed for previously trimmed or subsection readouts. The data in this file is internally turned into the same description as a bad pixel mask with values of two for regions which are narrower or equal across the columns and a value of three for regions narrower across lines.

The direction of interpolation is determined from the values in the mask, image, or the converted text file. A value of two interpolates across columns, a value of three interpolates across lines, and any other value interpolates across the narrowest dimension of bad pixels and using column interpolation if the two dimensions are equal.

The bad pixel description may be specified explicitly with the parameter fixfile or indirectly if the parameter has the value "image". In the latter case the instrument file must contain the name of the file.

2. Overscan

If an overscan or prescan correction is specified (overscan parameter) then the image section (biassec parameter) defines the overscan region.

There are two types of overscan (or prescan) determinations. One determines a independent overscan value for each line and is only available for a readaxis of 1. The other averages the overscan along the readout direction to make an overscan vector, fits a smoothing function to the vector, and then evaluate and then evaluates the smooth function at each readout line or column.

The line-by-line determination provides an mean, median, or mean with the minimum and maximum values excluded. The median is lowest value of the middle two when the number of overscan columns is even rather than the mean.

The smoothed overscan vector determination uses the icfit options including interactive fitting. The fitting function is generally either a constant (polynomial of 1 term) or a high order function which fits the large scale shape of the overscan vector. Bad pixel rejection is also available to eliminate cosmic ray events. The function fitting may be done interactively using the standard icfit iteractive graphical curve fitting tool. Regardless of whether the fit is done interactively, the overscan vector and the fit may be recorded for later review in a metacode plot file named by the parameter ccdred.plotfile . The mean value of the bias function is also recorded in the image header and log file.

3. Trim

When the parameter trim is set the input image will be trimmed to the image section given by the parameter trimsec . This trim should, of course, be the same as that used for the calibration images.

4. Zerocor

After the readout bias is subtracted, as defined by the overscan or prescan region, there may still be a zero level bias. This level may be two dimensional or one dimensional (the same for every readout line). A zero level calibration is obtained by taking zero length exposures; generally many are taken and combined. To apply this zero level calibration the parameter zerocor is set. In addition if the zero level bias is only readout dependent then the parameter readcor is set to reduce two dimensional zero level images to one dimensional images. The zero level images may be specified by the parameter zero or given in the input image list (provided the CCD image type is defined).

When the zero level image is needed to correct an input image it is checked to see if it has been processed and, if not, it is processed automatically. Processing of zero level images consists of bad pixel replacement, overscan correction, trimming, and averaging to one dimension if the readout correction is specified.

5. Darkcor

Dark counts are subtracted by scaling a dark count calibration image to the same exposure time as the input image and subtracting. The exposure time used is the dark time which may be different than the actual integration or exposure time. A dark count calibration image is obtained by taking a very long exposure with the shutter closed; i.e. an exposure with no light reaching the detector. The dark count correction is selected with the parameter darkcor and the dark count calibration image is specified either with the parameter dark or as one of the input images. The dark count image is automatically processed as needed. Processing of dark count images consists of bad pixel replacement, overscan and zero level correction, and trimming.

6. Flatcor

The relative detector pixel response is calibrated by dividing by a scaled flat field calibration image. A flat field image is obtained by exposure to a spatially uniform source of light such as an lamp or twilight sky. Flat field images may be corrected for the spectral signature in spectroscopic images (see response and apnormalize ), or for illumination effects (see mkillumflat or mkskyflat ). For more on flat fields and illumination corrections see flatfields . The flat field response is dependent on the wavelength of light so if different filters or spectroscopic wavelength coverage are used a flat field calibration for each one is required. The different flat fields are automatically selected by a subset parameter (see subsets ).

Flat field calibration is selected with the parameter flatcor and the flat field images are specified with the parameter flat or as part of the input image list. The appropriate subset is automatically selected for each input image processed. The flat field image is automatically processed as needed. Processing consists of bad pixel replacement, overscan subtraction, zero level subtraction, dark count subtraction, and trimming. Also if a scan mode is used and the parameter scancor is specified then a scan mode correction is applied (see below). The processing also computes the mean of the flat field image which is used later to scale the flat field before division into the input image. For scan mode flat fields the ramp part is included in computing the mean which will affect the level of images processed with this flat field. Note that there is no check for division by zero in the interest of efficiency. If division by zero does occur a fatal error will occur. The flat field can be fixed by replacing small values using a task such as imreplace or during processing using the minreplace parameter. Note that the minreplace parameter only applies to flat fields processed by ccdproc .

7. Illumcor

CCD images processed through the flat field calibration may not be completely flat (in the absence of objects). In particular, a blank sky image may still show gradients. This residual nonflatness is called the illumination pattern. It may be introduced even if the detector is uniformly illuminated by the sky because the flat field lamp illumination may be nonuniform. The illumination pattern is found from a blank sky, or even object image, by heavily smoothing and rejecting objects using sigma clipping. The illumination calibration image is divided into the data being processed to remove the illumination pattern. The illumination pattern is a function of the subset so there must be an illumination correction image for each subset to be processed. The tasks mkillumcor and mkskycor are used to create the illumination correction images. For more on illumination corrections see flatfields .

An alternative to treating the illumination correction as a separate operation is to combine the flat field and illumination correction into a corrected flat field image before processing the object images. This will save some processing time but does require creating the flat field first rather than correcting the images at the same time or later. There are two methods, removing the large scale shape of the flat field and combining a blank sky image illumination with the flat field. These methods are discussed further in the tasks which create them; mkillumcor and mkskycor .

8. Fringecor

There may be a fringe pattern in the images due to the night sky lines. To remove this fringe pattern a blank sky image is heavily smoothed to produce an illumination image which is then subtracted from the original sky image. The residual fringe pattern is scaled to the exposure time of the image to be fringe corrected and then subtracted. Because the intensity of the night sky lines varies with time an additional scaling factor may be given in the image header. The fringe pattern is a function of the subset so there must be a fringe correction image for each subset to be processed. The task mkfringecor is used to create the fringe correction images.

9. Readcor

If a zero level correction is desired (zerocor parameter) and the parameter readcor is yes then a single zero level correction vector is applied to each readout line or column. Use of a readout correction rather than a two dimensional zero level image depends on the nature of the detector or if the CCD is operated in longscan mode (see below). The readout correction is specified by a one dimensional image (zero parameter) and the readout axis (readaxis parameter). If the zero level image is two dimensional then it is automatically processed to a one dimensional image by averaging across the readout axis. Note that this modifies the zero level calibration image.

10. Scancor

CCD detectors may be operated in several modes in astronomical applications. The most common is as a direct imager where each pixel integrates one point in the sky or spectrum. However, the design of most CCD's allows the sky to be scanned across the CCD while shifting the accumulating signal at the same rate. Ccdproc provides for two scanning modes called "shortscan" and "longscan". The type of scan mode is set with the parameter scanmode .

In "shortscan" mode the detector is scanned over a specified number of lines (not necessarily at sideral rates). The lines that scroll off the detector during the integration are thrown away. At the end of the integration the detector is read out in the same way as an unscanned observation. The advantage of this mode is that the small scale, zero level, dark count and flat field responses are averaged in one dimension over the number of lines scanned. A zero level, dark count or flat field may be observed in the same way in which case there is no difference in the processing from unscanned imaging and the parameter scancor may be no. If it is yes, though, checking is done to insure that the calibration image used has the same number of scan lines as the object being processed. However, one obtains an increase in the statistical accuracy of if they are not scanned during the observation but digitally scanned during the processing. In shortscan mode with scancor set to yes, zero level, dark count and flat field images are digitally scanned, if needed, by the same number of scan lines as the object. The number of scan lines is determined from the object image header using the keyword nscanrow (or it's translation). If not found the object is assumed to have been scanned with the value given by the nscan parameter. Zero, dark and flat calibration images are assumed to be unscanned if the header keyword is not found.

If a scanned zero level, dark count or flat field image is not found matching the object then one may be created from the unscanned calibration image. The image will have the root name of the unscanned image with an extension of the number of scan rows; i.e. Flat1.32 is created from Flat1 with a digital scanning of 32 lines.

In "longscan" mode the detector is continuously read out to produce an arbitrarily long strip. Provided data which has not passed over the entire detector is thrown away, the zero level, dark count, and flat field corrections will be one dimensional. If scancor is specified and the scan mode is "longscan" then a one dimensional zero level, dark count, and flat field correction will be applied.

11. Processing Steps

The following describes the steps taken by the task. This detailed outline provides the most detailed specification of the task.

(1)
An image to be processed is first checked that it is of the specified CCD image type. If it is not the desired type then go on to the next image.
(2)
A temporary output image is created of the specified pixel data type (ccdred.pixeltype ). The header parameters are copied from the input image.
(3)
If trimming is specified and the image has not been trimmed previously, the trim section is determined.
(4)
If bad pixel replacement is specified and this has not been done previously, the bad pixel file is determined either from the task parameter or the instrument translation file. The bad pixel regions are read. If the image has been trimmed previously and the bad pixel file contains the word "untrimmed" then the bad pixel coordinates are translated to those of the trimmed image.
(5)
If an overscan correction is specified and this correction has not been applied, the overscan section is averaged along the readout axis. If trimming is to be done the overscan section is trimmed to the same limits. A function is fit either interactively or noninteractively to the overscan vector. The function is used to produce the overscan vector to be subtracted from the image. This is done in real arithmetic.
(6)
If the image is a zero level image go to processing step 12. If a zero level correction is desired and this correction has not been performed, find the zero level calibration image. If the zero level calibration image has not been processed it is processed at this point. This is done by going to processing step 1 for this image. After the calibration image has been processed, processing of the input image continues from this point. The processed calibration image may be cached in memory if it has not been previously and if there is enough memory.
(7)
If the image is a dark count image go to processing step 12. If a dark count correction is desired and this correction has not been performed, find the dark count calibration image. If the dark count calibration image has not been processed it is processed at this point. This is done by going to processing step 1 for this image. After the calibration image has been processed, processing of the input image continues from this point. The ratio of the input image dark time to the dark count image dark time is determined to be multiplied with each pixel of the dark count image before subtracting from the input image. The processed calibration image may be cached in memory if it has not been previously and if there is enough memory.
(8)
If the image is a flat field image go to processing step 12. If a flat field correction is desired and this correction has not been performed, find the flat field calibration image of the appropriate subset. If the flat field calibration image has not been processed it is processed at this point. This is done by going to processing step 1 for this image. After the calibration image has been processed, processing of the input image continues from this point. The mean of the image is determined from the image header to be used for scaling. If no mean is found then a unit scaling is used. The processed calibration image may be cached in memory if it has not been previously and if there is enough memory.
(9)
If the image is an illumination image go to processing step 12. If an illumination correction is desired and this correction has not been performed, find the illumination calibration image of the appropriate subset. The illumination image must have the "mkillum" processing flag or the ccdproc will abort with an error. The mean of the image is determined from the image header to be used for scaling. If no mean is found then a unit scaling is used. The processed calibration image may be cached in memory if it has not been previously and there is enough memory.
(10)
If the image is a fringe image go to processing step 12. If a fringe correction is desired and this correction has not been performed, find the fringe calibration image of the appropriate subset. The illumination image must have the "mkfringe" processing flag or the ccdproc will abort with an error. The ratio of the input image exposure time to the fringe image exposure time is determined. If there is a fringe scaling in the image header then this factor is multiplied by the exposure time ratio. This factor is used for scaling. The processed calibration image may be cached in memory if it has not been previously and there is enough memory.
(11)
If there are no processing operations flagged, delete the temporary output image, which has been opened but not used, and go to 14.
(12)
The input image is processed line by line with trimmed lines ignored. A line of the input image is read. Bad pixel replacement and trimming is applied to the image. Image lines from the calibration images are read from disk or the image cache. If the calibration is one dimensional (such as a readout zero level correction or a longscan flat field correction) then the image vector is read only once. Note that IRAF image I/O is buffered for efficiency and accessing a line at a time does not mean that image lines are read from disk a line at a time. Given the input line, the calibration images, the overscan vector, and the various scale factors a special data path for each combination of corrections is used to perform all the processing in the most efficient manner. If the image is a flat field any pixels less than the minreplace parameter are replaced by that minimum value. Also a mean is computed for the flat field and stored as the CCDMEAN keyword and the time, in a internal format, when this value was calculated is stored in the CCDMEANT keyword. The time is checked against the image modify time to determine if the value is valid or needs to be recomputed.
(13)
The input image is deleted or renamed to a backup image. The temporary output image is renamed to the input image name.
(14)
If the image is a zero level image and the readout correction is specified then it is averaged to a one dimensional readout correction.
(15)
If the image is a zero level, dark count, or flat field image and the scan mode correction is specified then the correction is applied. For shortscan mode a modified two dimensional image is produced while for longscan mode a one dimensional average image is produced.
(16)
The processing is completed and either the next input image is processed beginning at step 1 or, if it is a calibration image which is being processed for an input image, control returns to the step which initiated the calibration image processing.

12. Processing Arithmetic

The ccdproc task has two data paths, one for real image pixel datatypes and one for short integer pixel datatype. In addition internal arithmetic is based on the rules of FORTRAN. For efficiency there is no checking for division by zero in the flat field calibration. The following rules describe the processing arithmetic and data paths.

(1)
If the input, output, or any calibration image is of type real the real data path is used. This means all image data is converted to real on input. If all the images are of type short all input data is kept as short integers. Thus, if all the images are of the same type there is no datatype conversion on input resulting in greater image I/O efficiency.
(2)
In the real data path the processing arithmetic is always real and, if the output image is of short pixel datatype, the result is truncated.
(3)
The overscan vector and the scale factors for dark count, flat field, illumination, and fringe calibrations are always of type real. Therefore, in the short data path any processing which includes these operations will be coerced to real arithmetic and the result truncated at the end of the computation.

13. In the Absence of Image Header Information

The tasks in the ccdred package are most convenient to use when the CCD image type, subset, and exposure time are contained in the image header. The ability to redefine which header parameters contain this information makes it possible to use the package at many different observatories (see instruments ). However, in the absence of any image header information the tasks may still be used effectively. There are two ways to proceed. One way is to use ccdhedit to place the information in the image header.

The second way is to specify the processing operations more explicitly than is needed when the header information is present. The parameter ccdtype is set to "" or to "none". The calibration images are specified explicitly by task parameter since they cannot be recognized in the input list. Only one subset at a time may be processed.

If dark count and fringe corrections are to be applied the exposure times must be added to all the images. Alternatively, the dark count and fringe images may be scaled explicitly for each input image. This works because the exposure times default to 1 if they are not given in the image header.

EXAMPLES

The user's guide presents a tutorial in the use of this task.

1. In general all that needs to be done is to set the task parameters and enter

cl> ccdproc *.imh &

This will run in the background and process all images which have not been processed previously.

TIME REQUIREMENTS

o SUN-3, 15 MHz 68020 with 68881 floating point hardware (no FPA)
o 8 Mb RAM, 2 Fuji Eagle disks.
o Input images = 544 x 512 short
o Output image = 500 x 500 real
o Operations are overscan subtraction (O), trimming to 500x500 (T),
  zero level subtraction (Z), dark count scaling and subtraction (D),
  and flat field scaling and subtraction (F).
o UNIX statistics
  (user, system, and clock time, and misc. memory and i/o statistics):

[OTF] One calibration image and 9 object images:
No caching:  110.6u 25.5s 3:18 68% 28+ 40K 3093+1645io   9pf+0w
Caching:     111.2u 23.0s 2:59 74% 28+105K 2043+1618io   9pf+0w

[OTZF] Two calibration images and 9 object images:
No caching:  119.2u 29.0s 3:45 65% 28+ 50K 4310+1660io   9pf+0w
Caching:     119.3u 23.0s 3:07 75% 28+124K 2179+1601io   9pf+0w

[OTZDF] Three calibration images and 9 object images:
No caching:  149.4u 31.6s 4:41 64% 28+ 59K 5501+1680io  19pf+0w
Caching:     151.5u 29.0s 4:14 70% 27+227K 2346+1637io 148pf+0w

[OTZF] 2 calibration images and 20 images processed:
No caching:  272.7u 63.8u 8:47 63% 28+ 50K 9598+3713io  12pf+0w
Caching:     271.2u 50.9s 7:00 76% 28+173K 4487+3613io  51pf+0w

REVISIONS

CCDPROC V2.11.2
A new "output" parameter is available to specify an output image leaving the input image unchanged. If this parameter is not specified then the previous behavior of "in-place" operation with an optional backup occurs.
CCDPROC V2.11
The bad pixel fixing was modified to allow use of pixel masks, images, or the text file description. Bad pixel masks are the desired description and use of text files is only supported for backward compatibility. Note that support for the trimmed or untrimmed conversion from text files has been eliminated.

Line-by-line overscan/prescan subtraction is now provided with three simple algorithms.

CCDPROC: V2.10.3
The output pixel datatypes (specified by the package parameter pixeltype have been extended to include unsigned short integers. Also it was previously possible to have the output pixel datatype be of lower precision than the input. Now the output pixel datatype is not allowed to lose precision; i.e. a real input image may not be processed to a short datatype.

For short scan data the task now looks for the number of scan lines in the image header. Also when a calibration image is software scanned a new image is created. This allows processing objects with different numbers of scan lines and preserving the unscanned calibration image.

It is an error if no biassec is specified rather than defaulting to the whole image.

The time, in a internal format, when the CCDMEAN value is calculated is stored in the CCDMEANT keyword. The time is checked against the image modify time to determine if the value is valid or needs to be recomputed.

SEE ALSO

instruments, ccdtypes, flatfields, icfit, ccdred, guide, mkillumcor,
mkskycor, mkfringecor

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