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multidrizzle stsdas.analysis.dither


NAME · USAGE_ · PARAMETERS · DESCRIPTION · INSTRUMENT_SPECIFIC_NOTES
UNITS · REFERENCES · EXAMPLES

NAME

MultiDrizzle -- Automated dithered image combination and cleaning

USAGE

multidrizzle input

PARAMETERS

input = "" [string]
The input for MultiDrizzle can be provided in any of several forms:
 - filename of a single image
 - filename of an association (ASN)table
 - wild-card specification for files in directory (using *, ? etc)
 - comma-separated list of filenames
 - '@file' filelist containing list of desired input filenames (and
   optionally also the corresponding inverse variance filenames, if needed
   for the 'final_wht_type' parameter described further below).
The @file filelist needs to be provided as an ASCII text file containing a list of filenames for all input images, with one filename on each line of the file. If inverse variance maps have also been created by the user and are to be used (by specifying IVM to the parameter final_wht_type described further below), then these are simply provided as a second column in the filelist, with each IVM filename listed on the same line as a second entry, after its corresponding exposure filename. If the user specifies IVM for the final_wht_type but does not provide the names of IVM files, Multidrizzle will automatically generate the IVM files itself for each input exposure.
(output = "")[string]
The rootname for the output drizzled products. If an association file has been given as input, this name will be used instead of the product name specified in the ASN file. Similarly, if a single exposure is provided, this rootname will be used for the output product instead of relying on input rootname. If no value is provided when a filelist or wild-card specification is given as input, then a rootname of final will be used.
(mdriztab = "yes") [boolean]
Specifies whether or not to use an MDRIZTAB reference table filename found in the image header to specify the remaining MultiDrizzle parameter settings. If yes, the values in that table will override the settings for the remainder of the parameters.
(refimage = "") [file name]
Optional "reference image" that can be provided, in which case MultiDrizzle will create a final product with the same WCS. This reference image should be a simple FITS file (single-group, no multiple extensions), and should have been already drizzled so that all its distortion has been removed, and its WCS is completely rectified. This is not intended to provide shifts information.
(runfile = "") [file name]
This log file will contain the IRAF CL commands necessary for performing the final combination manually using the "drizzle" task directly. If no name is provided, a file will be created using the rootname of the output file (rootname.run).
(workinplace = "no") [boolean]
This parameter specifies whether to perform all processing, including sky subtraction and update of the DQ array, on the original input or not. If set to yes, then no copy of the input will be created for processing, and the original input will be modified directly by MultiDrizzle.
(updatewcs = "yes") [boolean]
This parameter specifies whether the WCS keywords are to be updated by running makewcs on the input data, or left alone. This should usually be set to yes, unless the WCS keywords have been carefully set by some other method, and need to be passed through to drizzle as is.
(proc_unit = "native") [string]
The source of the distortion coefficients is specified using:
  'native'       - Output DRZ product and input 'values' given in the native units
                   of the input image.
  'electrons'    - Output DRZ product and input 'values' given in units of electrons. 
(coeffs = "header") [string]
The source of the distortion coefficients is specified using:
  'header'  - Look in the image header for the name of the IDCTAB
              and other distortion information to use. Note that
              if the IDCTAB file is not found on disk, then the
              task will exit for ACS data, while for WFPC2 data
              it will default to the older "Trauger" model.
  'cubic'   - use 'cubic' solutions originally provided with 'drizzle'
  'trauger' - use 'trauger' solutions originally provided with 'drizzle'
  'None'    - Do not apply any distortion correction to the images
Alternatively, an arbitrary distortion coefficients file can be specified (including optionally the full pathname). This distortion file is used in computing the WCS of the header.
(context = no) [boolean]
Create a context image during the final drizzle combination?
(clean = no) [boolean]
Remove the temporary files created by MultiDrizzle? The affected files would include the coefficients files and static mask files created by PyDrizzle, along with other intermediate files created by MultiDrizzle. It is often useful to retain the intermediate files and examine them when first learning how to run MultiDrizzle. But when running it routinely, these files can be removed to save space.
(group = "") [string]
Single FITS Extension or group to be drizzled. If a section is provided, then only that chip will be drizzled onto the output frame. Either a FITS extension number or GEIS group number (such as 1), or a FITS extension name (such as sci,1) can be provided. Sets of extensions can also be provided as a comma-separated list of integers (such as 2,3,4). These values will be interpreted as FITS extension numbers (EXTVER keyword values) of extensions which should be drizzled together.
(ra) [real]
Right ascension (in decimal degrees) of the center of the output image. If this is not specified, the code will calculate the center automatically based on the distribution of image dither positions.
(dec) [real]
Declination (in decimal degrees) of the center of the output image. If this is not specified, the code will calculate the center automatically based on the distribution of image dither positions.
(build = yes) [boolean]
Should PyDrizzle should combine the separate drizzle output files into a single multi-extension format FITS file? This combined output file will contain a SCI (science), a WHT (weight), and a CTX (context) extension.
(shiftfile = "") [file name]
Name of optional input file containing the shifts to be applied to the input images to improve the registration of the images. These shifts will be added to those calculated automatically from the image headers. The specification for this file can be found at: http://stsdas.stsci.edu/multidrizzle/doc/pydrizzle_shifts.pdf
(staticfile = "") [file name]
Name of (optional) input static bad-pixel mask. This mask will be applied to all input images. The static bad-pixel mask file must contain extensions with EXTNAMEs of "MASK". For example, an ACS WFC static bad-pixl mask would need a FITS file with two "MASK" extensions. A NICMOS CAL file would need only one "MASK" extension.

For users who created static-bad pixel mask files prior to the release of Multidrizzle version 2.3.6, the conversion of those files to the appropriate format is trivial. The following IRAF command is all that is necessary to convert those files to the appropriate format:

-> hedit filename.fits[SCI,1] extname 'MASK'update+ verify-

(static = yes) [boolean]
Create a static bad-pixel mask from the data? This mask flags all pixels that deviate by more than static_sig sigma below the image median, since these pixels usually result from oversubtraction of bad pixels in the dark image during calibration.
(static_sig = 4.0) [real]
Number of sigma below the RMS to use as the clipping limit for creating the static mask.
(skysub = yes) [boolean]
Perform sky subtraction on the input data?
(skywidth = 0.1) [real]
Bin width, in sigma, used to sample the distribution of pixel flux values in order to compute the sky background statistics.
(skystat = "median|mode|mean") [string]
Statistical method for determining the sky value from the image pixel values.
(skylower = INDEF) [real]
Lower limit of usable pixel values for computing the sky. This value should be specified in units of electrons.
(skyupper = INDEF) [real]
Upper limit of usable pixel values for computing the sky. This value should be specified in units of electrons.
(skyclip = 5) [integer]
Number of clipping iterations to use when computing the sky value.
(skylsigma = 4.0) [real]
Lower clipping limit, in sigma, used when computing the sky value.
(skyusigma = 4.0) [real]
Upper clipping limit, in sigma, used when computing the sky value.
(skyuser = "") [string]
Name of header keyword which records the sky value already subtracted from the image by the user.
(driz_separate = yes) [boolean]
Drizzle each input image onto separate output images? These images are used to create the median image, needed for the cosmic ray rejection step further on.
(driz_sep_outnx) [real]
Size of the X axis of the output images (in pixels) which each input image will be drizzled onto. If no value is specified, it will use the smallest size that can accommodate the full image.
(driz_sep_outny) [real]
Size of the Y axis of the output images (in pixels) which each input image will be drizzled onto. If no value is specified, it will use the smallest size that can accommodate the full image.
(driz_sep_kernel = "turbo") [string]
For the initial separate drizzling operation only, this specifies the form of the kernel function used to distribute flux onto the separate output images. The options are currently:
    square | point | gaussian | turbo | tophat | lanczos3
The default for this step is "turbo" since it is much faster than "square", and it is quite satisfactory for the purposes of generating the median image. More information about the different kernels can be found in the help for the task drizzle.
(driz_sep_wt_scl = "exptime") [real]
Weighting factor for input image. If driz_sep_wt_scl=exptime then the scaling value will be set equal to the exposure time found in the image header. This is the default behavior and is recommended. It is also possible to give wt_scl=expsq for weighting by the square of exposure time. The latter is optimal for read-noise dominated images.
(driz_sep_scale = INDEF) [real]
Linear size of output pixels in arcseconds/pixel for each separate drizzled image (to be used in creating the median for cosmic ray rejection). The default value of INDEF specifies that the undistorted pixel scale for the first input image, as computed by PyDrizzle, will be used as the pixel scale for all the output images.
(driz_sep_pixfrac = 1) [real]
Fraction by which input pixels are "shrunk" before being drizzled onto the output image grid, given as a real number between 0 and 1. This specifies the size of the footprint, or "dropsize", of a pixel in units of the input pixel size. If pixfrac is set to less than 0.001, the kernel is reset to point for more efficient processing. For the step of drizzling each input image onto a separate output image, the default value of 1 is best in order to ensure that each output drizzled image is fully populated with pixels from the input image. For more information, see the help for the task drizzle.
(driz_sep_rot = INDEF) [real]
Position Angle of output image's Y-axis relative to North. A value of 0.0 would orient the final output image with North up. The default of INDEF specifies that the images will not be rotated, but will instead be drizzled in the default orientation for the camera, with the x and y axes of the drizzled image corresponding approximately to the detector axes. This conserves disk space, since these single drizzled images are only used in the intermediate step of creating a median image.
(driz_sep_fillval = "INDEF") [string]
Value to be assigned to output pixels that have zero weight or did not receive flux from any input pixels during drizzling. This parameter corresponds to the fillval parameter of the drizzle task. If the default of INDEF is used and if the weight in both the input and output images for a given pixel are zero, then the output pixel will be set to the value it would have had if the input had a non-zero weight. Otherwise, if a numerical value is provided (eg. 0), then these pixels will be set to that value.
(driz_sep_bits = 0) [integer]
Integer sum of all the DQ bit values from the input image's DQ array that should be considered good when building the weighting mask. This can also be used to reset pixels to good if they had been flagged as cosmic rays during a previous run of MultiDrizzle, by adding the value 4096 for ACS and WFPC2 data.
(median = yes) [boolean]
Create a median image? This median image will be used as the comparison truth image in the cosmic ray rejection step.
(median_newmasks = yes) [boolean]
Create new mask files when creating the median? These masks are generated from the weight files produced previously by the "driz_separate" step, and would contain all the bad pixel information. These pixels will be excluded when calculating the median. Generally this step should be set to "yes", unless it is desired to include bad pixels in generating the median.
(combine_maskpt = 0.7) [real]
Percentage of the weight image below which a pixel is considered a bad pixel when creating a new mask.
(combine_type = "minmed") [string]
This parameter allows the user to choose what method should be used to create the median image. Valid options are:
   median| sum | minmed | minimum
The "minmed" option will produce an image that is generally the same as the median, except in cases where the median is significantly higher than the minimum good pixel value, in which case it will choose the minimum. The sigma thresholds for this decision are provided by the "combine_nsigma" parameter. The grow value for this decision is provided by the combine_grow parameter.
(combine_nsigma = "3") [string]
Sigmas used for accepting minimum values instead of median values when using the minmed combination method. If two values are specified, then the first value will be used in the initial choice between median and minimum, while the second value will be used in the "growing" step to reject additional pixels around those identified in the first step. If only one value is specified, then it is used in both steps.
(combine_nlow) [integer]
This parameter sets the number of low value pixels to reject when the combine_type has been set to median. In this usage, the specified number of low value pixels is discarded prior to the computation of the median image. If set to 0, then no low value clipping gets done when computing the median values for combine_type=median.
(combine_nhigh) [integer]
This parameter sets the number of high value pixels to reject when the combine_type has been set to median. In this usage, the specified number of high value pixels is discarded prior to the computation of the median image . If set to 0, then no high value clipping gets done when computing the median values for combine_type=median.
(combine_lthresh = "INDEF") [string]
This parameter sets the minimum value considered to be good pixels when the combine_type has been set to median. In this usage, the pixels with values below the specified threshold are discarded prior to the computation of the median image .If set to INDEF, then no lower value clipping gets done when computing the median values for combine_type=median. These values should be in units consistent with the proc_unit parameter setting.
(combine_hthresh = "INDEF") [string]
This parameter sets the maximum value considered to be good pixels when the combine_type has been set to median. In this usage, the pixels with values above the specified threshold are discarded prior to the computation of the median image .If set to INDEF, then no upper value clipping gets done when computing the median values for combine_type=median. These values should be in units consistent with the proc_unit parameter setting.
(combine_grow = 1) [real]
Radius (pixels) from flagged pixel for neighbor rejection. This parameter is only applicable when combine_type=minmed.
(blot = yes) [boolean]
Perform the blot operation on the median image? The output will be median smoothed images which match each input chip's image, and are used in the cosmic ray rejection step.
(blot_interp = "poly5") [string]
Type of interpolation to use when blotting drizzled images back to their original WCS. Valid options are:
   nearest    - Nearest neighbor
   linear     - Bilinear interpolation in x and y
   poly3      - Third order interior polynomial in x and y
   poly5      - Fifth order interior polynomial in x and y
   sinc       - Sinc interpolation - accurate but slow
The poly5 interpolation method has been chosen as the default because it is relatively fast and accurate. If sinc interpolation has been selected, then it will use the value of the parameter blot_sinscl to specify the size of the sinc interpolation kernel.
(blot_sinscl = 1.0) [real]
Size of the sinc interpolation kernel in pixels.
(driz_cr = yes) [boolean]
Perform cosmic-ray detection? If set to "yes", it will detect cosmic-rays and create cosmic-ray masks using the algorithms from deriv and driz_cr.
(driz_cr_corr = no) [boolean]
Create a cosmic-ray cleaned input image? The cosmic-ray cleaned _cor image will be generated directly from the input image, and a corresponding _crmask file will be written to document the pixels detected as affected by cosmic-rays.
(driz_cr_snr = "3.3.0") [string]
These values specify the signal-to-noise ratios for the driz_cr task to use in detecting cosmic rays. This parameter value gets passed directly to driz_cr; see the help file for driz_cr for further discussion of this parameter.
(driz_cr_scale = "1.0.7")
Scaling factor applied to the derivative in driz_cr when detecting cosmic-rays. This parameter gets passed directly to driz_cr; see the help file for driz_cr for further discussion of this parameter.
(driz_combine = yes) [boolean]
Perform the final drizzle image combination? This applies the generated cosmic-ray masks to the input images and create a final, cleaned, distortion-corrected product.
(final_wht_type = "EXP") [string]
Specify the type of weighting image to apply with the bad pixel mask for the final drizzle step. The options are:
      EXP  |  ERR  |  IVM
The default of EXP indicates that the images will be weighted according to their exposure time, which is the standard behavior for drizzle. This weighting is a good approximation in the regime where the noise is dominated by photon counts from the sources, while contributions from sky background, read-noise and dark current are negligible. This option is provided as the default since it produces reliable weighting for all types of data, including older instruments (eg., WFPC2) where more sophisticated options may not be available.

Specifying ERR is an alternative for ACS and STIS data, in which case the final drizzled images will be weighted according to the inverse variance of each pixel in the input exposure files, calculated from the error array data extension that is in each calibrated input exposure file. This array encapsulates all the noise sources in each exposure, including read-noise, dark current and sky background, as well as Poisson noise from the sources themselves, and this also includes a dependence upon exposure time. For WFPC2, the ERR array is not produced during calibration, therefore this option is not available. We advise extreme caution when selecting the "ERR" option, since the nature of this weighting scheme can introduce photometric discrepancies in sharp unresolved sources, although these effects are minimized for sources with gradual variations between pixels. The "EXP" weighting option does not suffer from these effects and is therefore the recommended option.

Finally, IVM can be specified, in which case the user can either supply their own inverse-variance weighting map or let Multidrizzle generate one on-the-fly automatically during the final drizzle step. This may be necessary for specific purposes, for example to create a drizzled weight file for software such as Sextractor, which expects a weight image that contains all the background noise sources (sky level, read-noise, dark current, etc) but not the Poisson noise from the objects themselves. The user can create the inverse variance images and then specify their names using the input parameter for MultiDrizzle to specify an @file. This would be a single ASCII file containing the list of input calibrated exposure filenames (one per line), with a second column containing the name of the IVM file corresponding to each calibrated exposure. Each IVM file must have the same file format as the input file, and if given as multi-extension FITS files (for example, ACS or STIS data) then the IVM extension must have the EXTNAME of IVM. If no IVM files are specified on input, then Multidrizzle will rely on the flat-field reference file and computed dark value from the image header to automatically generate an IVM file specific to each exposure.

(final_outnx) [real]
Size of the X axis of the final drizzled image (in pixels). If no value is specified, it will use the smallest size that can accommodate the full image.
(final_outny) [real]
Size of the Y axis of the final drizzled image (in pixels). If no value is specified, it will use the smallest size that can accommodate the full image.
(final_kernel = "square") [string]
Shape of the kernel used by drizzle in the final image combination. With drizzle Version 3.0 and greater, the supported choices are:
    square | point | gaussian | turbo | tophat | lanczos3
(final_wt_scl = "exptime") [real]
Weighting factor for input image. If final_wt_scl=exptime then the scaling value will be set equal to the exposure time found in the image header. This is the default behavior and is recommended. It is also possible to give wt_scl=expsq for weighting by the square of exposure time. The latter is optimal for read-noise dominated images.
(final_scale = INDEF) [real]
Linear size of the output pixels in arcseconds/pixel for the final combined product. The default value of INDEF specifies that the undistorted pixel scale for the first input image, as computed by PyDrizzle, will be used as the pixel scale for the final output image.
(final_pixfrac = 1.) [real]
Fraction by which input pixels are "shrunk" before being drizzled onto the output image grid, given as a real number between 0 and 1. This specifies the size of the footprint, or "dropsize", of a pixel in units of the input pixel size. If pixfrac is set to less than 0.001, the kernel is reset to point for more efficient processing. If more than a few images are being combined, values smaller than 1 (eg 0.7 or 0.8) can be specified, which result in a slightly sharper output image. For more information, read the help for the task drizzle.
(final_rot = 0.) [real]
Position Angle of output image's Y-axis relative to North. The default of 0.0 would orient the final output image with North up. A value of INDEF would specify that the images will not be rotated, but will instead be drizzled in the default orientation for the camera, with the x and y axes of the drizzled image corresponding approximately to the detector axes.
(final_fillval = "INDEF") [string]
Value to be assigned to output pixels that have zero weight or did not receive flux from any input pixels during drizzling. This parameter corresponds to the fillval parameter of the drizzle task. If the default of INDEF is used and if the weight in both the input and output images for a given pixel are zero, then the output pixel will be set to the value it would have had if the input had a non-zero weight. Otherwise, if a numerical value is provided (eg. 0), then these pixels will be set to that value.
(final_bits = 0) [integer]
Integer sum of all the DQ bit values from the input image's DQ array that should be considered good when building the weighting mask. This can also be used to reset pixels to good if they had been flagged as cosmic rays during a previous run of MultiDrizzle, by adding the value 4096 for ACS and WFPC2 data.
(final_units = cps) [string]
This parameter determines the units of the final drizzle-combined image, and can either be counts or cps. It is passed through to drizzle in the final drizzle step.
(gain = "") [real]
Value used to override instrument specific default gain values. The value is assumed to be in units of electrons/count. This parameter should not be populated if the gainkeyword parameter is in use.
(gainkeyword = "") [string]
Keyword used to specify a value to be used to override instrument specific default gain values. The value is assumed to be in units of electrons/count. This parameter should not be populated if the gain parameter is in use.
(rdnoise = "") [real]
Value used to override instrument specific default readnoise values. The value is assumed to be in units of electrons. This parameter should not be populated if the rnkeyword parameter is in use.
(rnkeyword = "") [string]
Keyword used to specify a value to be used to override instrument specific default readnoise values. The value is assumed to be in units of electrons. This parameter should not be populated if the rdnoise parameter is in use.
(exptime = "") [real]
Value used to override default exposure time image header values. The value is assumed to be in units of seconds. This parameter should not be populated if the expkeyword parameter is in use.
(expkeyword = "") [string]
Keyword used to specify a value to be used to override default exposure time image header values. The value is assumed to be in units of seconds. This parameter should not be populated if the exptime parameter is in use.
(crbit = "") [integer]
Integer used to override instrument specific cosmic ray bit values. This value is used by Multidrizzle to update data quality arrays when cosmic rays or other image defects are identified as "bad" in the DRIZ_CR step. To prevent the image's data quality array from being updated set the crbit value to 0.

DESCRIPTION

MultiDrizzle is provided as a "one-touch" interface for performing all the tasks necessary for registering dithered HST images, performing cosmic ray rejection, removing geometric distortion and performing the final image combination using "drizzle". The task is designed so that it can be run in a single step on most common datasets with minimal or no adjustment to any of the default parameters, which are chosen to provide good results for a wide range of different types of datasets. It was written using Python and relies on the Python interface to the IRAF environment, PyRAF, for its operation. As a result, this task needs to be run under PyRAF.

In its simplest form, it can simply be run on all calibrated ACS images in a directory by typing:

--> multidrizzle *flt.fits output=final

The various steps and parameters for the task are best viewed by typing:

--> epar multidrizzle

The task is divided into a series of steps, as follows:

 1. Bad pixel mask creation
 2. Sky subtraction
 3. Drizzle onto separate, registered output images
 4. Combine the separate drizzled images into a median
 5. "blot", or transform back the median to each input image
 6. Compare the median with original images to make cosmic ray masks
 7. Drizzle all images onto a final image using the cosmic ray masks

The principal parameters for each step can be modified to control the behavior of the script for other types of datasets that are not well handled by the default values. It is generally recommended to try running the script first with the default parameters, which should allow the task to process nearly any set of images for an initial review. The script can then be re-run, or restarted at a particular step, with modified parameters if this is necessary.

This task understands how to work with single images, sub-array images, dither associations and mosaic associations. It has been tested predominantly on a wide range of ACS and WFPC2 datasets, but is also intended for use on STIS imaging data. It makes use of image combination method known formally as variable-pixel linear reconstruction and informally as "drizzling" (Fruchter and Hook, 2002, PASP, 114:144). A more detailed explanation of drizzle can be found in the help file for that task.

MultiDrizzle accepts specification of the set of input images in many ways: by specifying a pattern through the suffix parameter to be matched against all images in the current directory, by providing a text file which contains the list of input images, the name of an association table, or a comma-separated list of input images in the filelist parameter. This allows the user to process any subset of images found in the current working directory.

Association tables provide the name of the output product as a row in the table, and MultiDrizzle uses this as the definition of the output product's name. Similarly, the output filename for a single exposure would be automatically built using the rootname of the given filename. However, any other method of specifying the input file would require the specification of the output product's name through the use of the output parameter. If none is provided, a default output filename will be final_drz.fits.

Processing dithered or mosaiced data with PyDrizzle gets handled through the use of ACS/NICMOS-style association tables. These tables are simple binary FITS tables with a few columns to specify the input exposures and the rootname of the output exposure, along with any residual offset or rotation relative to the headers WCS information necessary to get precise alignment when drizzling the data together. MultiDrizzle relies on the PyDrizzle task buildasn to build the input association table from the set of observations specified by either the suffix or filelist parameters and uses that table as the primary input to PyDrizzle.

If a filename is provided for the shiftfile parameter, then that file will be read in and the shifts will be incorporated into the ASN table when it gets built by Multidrizzzle. This allows the user to refine the image registration.

INSTRUMENT SPECIFIC NOTES

ACS ---

 - Multidrizzle is optimized for the processing of ACS data.

WFPC2 -----

 - Only the most common types of WFPC2 observing modes have been tested and are
   supported by Multidrizzle.  Full support for WFPC2 is currently in development

 - Multidrizzle is able to accept GEIS format files as input. However,
   these files are automatically converted to multi extension FITS format
   files.  When file conversion occurs, an association file is created containing the
   names of the new files.

 - In the case of WFPC2,  plate scale differences are taken into account during the sky
   subtraction step. The 'MDRIZSKY' header value for WFPC2 is taken to be on the
   WF3 plate scale.

STIS ----

 - Support for STIS imaging data is limited at this time.  Full support for STIS is
   under development.

 - Until the PA_V3 keyword is automatically included in the header of STIS science
   exposures, the dataset's spt files should be kept in the same directory as the input
   images.  This will allow Multidrizzle to copy the PA_V3 value from the primary header
   of the spt files and add it to the primary header of the science exposures.  This can
   also be done manually by the user prior to running Multidrizzle.

NICMOS ------

 - Support for NICMOS imaging is only preliminary at this time.  Full support for
   NICMOS is under development.

 - Until the PA_V3 keyword is automatically included in the header of NICMOS science
   exposures, the dataset's spt files should be kept in the same directory as the input
   images.  This will allow Multidrizzle to copy the PA_V3 value from the primary header
   of the spt files and add it to the primary header of the science exposures.  This can
   also be done manually by the user prior to running Multidrizzle.

UNITS

Internally to Multidrizzle, all computations occur in units of electrons. In all cases, the units of the Multidrizzle output product will be in electrons per second.

If the "workinplace" parameter is set to yes, the sci extensions of the input files will be in units of electrons upon the completion of Multidrizzle. If the "workinplace" parameter is set to no, the sci extensions will be unchanged at the completion of Multidrizzle.

The sky subtraction algorithm now updates a primary header keyword, MDRIZSKY, of the input image with the value computed for that image. The units of MDRIZSKY are the native units of the input image. The MDRIZSKY values for each input image are also report in the HISTORY cards of the *_drz.fits file.

REFERENCES

The MultiDrizzle script was initially written by Anton Koekemoer (STScI) and was implemented in Python under PyRAF by Warren Hack, Chris Hanley, Ivo Busko and Robert Jedrzejewski (STScI), based on extensive discussions with Andy Fruchter (STScI), Richard Hook (ST-ECF) and other members of the Dither Working Group at STScI. Considerable testing, design review support and code contribution was provided by: Richard Hook (ST-ECF), Jennifer Mack, Shireen Gonzaga, Vera Platais-Kozhurina, and Max Mutchler (STScI).

A description of the use of MultiDrizzle on ACS data in the pipeline can be found in the ACS Data Handbook, available online through the ACS Instrument WWW page at:


    http://www.stsci.edu/hst/acs/documents/

and the WWW page describing the use of MultiDrizzle in the HST ACS Pipeline:

    http://www.stsci.edu/hst/acs/analysis/multidrizzle/

Further examples on how to use MultiDrizzle are documented in the Dither Handbook (Koekemoer et al.), available from the STScI documentation website:

    http://www.stsci.edu/hst/HST_overview/documents/

In addition, software-related documentation for PyDrizzle and MultiDrizzle, along with the latest versions, can be found at:

    http://stsdas.stsci.edu/pydrizzle
    http://stsdas.stsci.edu/multidrizzle

MultiDrizzle can be referenced in publications by citing:


    A. Koekemoer, A. Fruchter, R. Hook, W. Hack, 2002 HST Calibration
        Workshop, p. 337

Additional references for PyDrizzle and Drizzle are provided in the help files for those tasks.

EXAMPLES


1: Combine all the calibrated ACS _flt.fits observations to create the
output whose rootname is 'test_mdriz'.

--> multidrizzle *flt.fits output='test_mdriz'

This will find all the files with suffix of 'flt.fits' in the current
directory, build an ASN table named 'test_mdriz_asn.fits', perform all
processing steps to find and remove cosmic-rays and produce the final,
cleaned, distortion-free image 'test_mdriz_drz.fits'.

2. Process the ACS association table 'j8cw01010_asn.fits' with
MultiDrizzle:

--> multidrizzle j8cw01010_asn.fits

This will find all the calibrated _flt.fits files listed for all the
input exposures in the association table and generate the product listed
as the product member. The input images will always be the calibrated
_flt.fits files generated for each input exposure, rather than any
cosmic-ray cleaned product or repeat-obs summed product generated
by CALACS.

3: Create a listfile containing the names of a subset of the ACS images
in this directory, and combine them into a final image. For example,
if the directory contains both F814W and F606W images, and we only want
to combine the F606W images:

--> multidrizzle "@files_f606w.lis" output='test_f606w'

4: Combine a set of WFPC2 images. Note that it is best to convert these
from FITS to GEIS format first (thus creating the *.c0h files). In
addition, be sure to have the *.c1h files (which contain the badpixel
information). In this case, the task could be run by specifying:

--> multidrizzle *.c0h output='test_wfpc2'

Further examples on how to use MultiDrizzle are documented in the Dither
Handbook (Koekemoer et al.), available from the STScI documentation
website:

    http://www.stsci.edu/hst/HST_overview/documents/

SEE ALSO

pydrizzle, drizzle, blot, dither package


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