# TWEAKREG: Alignment of Images

Combining images using astrodrizzle requires that the WCS information in the headers of each input image align to within sub-pixel accuracy. The `tweakreg` task allows the user to align sets of images to each other and/or to and external astrometric reference frame or image.

`TweakReg` - A replacement for IRAF-based `tweakshifts`

Authors: | Warren Hack, Mihai Cara
License: | http://www.stsci.edu/resources/software_hardware/pyraf/LICENSE

`drizzlepac.tweakreg.``TweakReg`(files=None, editpars=False, configobj=None, imagefindcfg=None, refimagefindcfg=None, **input_dict)

  Tweakreg provides an automated interface for computing residual shifts between input exposures being combined using `AstroDrizzle`. The offsets computed by Tweakreg correspond to pointing differences after applying the WCS information from the input image’s headers. Such errors would, for example, be due to errors in guide-star positions when combining observations from different observing visits or from slight offsets introduced upon re-acquiring the guide stars in a slightly different position.

  Parameters:

  file : str or list of str (Default = `'*flt.fits'`)

  > Input files (passed in from files parameter) This paramater 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 optional inverse variance map filenames)

  > 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.

  editpars : bool (Default = False)

  > A parameter that allows user to edit input parameters by hand in the GUI. True to use the GUI to edit parameters.

  configObj : configObject (Default = None)

  > An instance of `configObject` which overrides default parameter settings.

  imagefindcfg : dict, configObject (Default = None)

  > An instance of `dict` or `configObject` which overrides default source finding (for input images) parameter settings. See help for `imagefindpars` PSET for a list of available parameters. Only the parameters that are different from default values need to be specified here.

  refimagefindcfg : dict, configObject (Default = None)

  > An instance of `dict` or `configObject` which overrides default source finding (for input reference image) parameter settings. See help for `refimagefindpars` PSET for a list of available parameters. Only the parameters that are different from default values need to be specified here.

  inputDict : dict, optional

  > An optional list of parameters specified by the user, which can also be used to override the defaults.

  > Note

  > This list of parameters can include the `updatewcs` parameter, even though this parameter no longer can be set through the TEAL GUI.

  > Note

  > This list of parameters can contain parameters specific to the `TweakReg` task itself described here in the “Other Parameters” section and may not contain parameters from the `refimagefindpars` PSET.

  > Note

  > For compatibility purpose with previous `TweakReg` versions, `inputDict` may contain parameters from the the `imagefindpars` PSET. However, if `imagefindcfg` is not `None`, then `imagefindpars` parameters specified through `inputDict` may not duplicate parameters specified through `imagefindcfg`.

  Other Parameters:

  refimage : str (Default = ‘’)

  > Filename of reference image. Sources derived from this image will be used as the reference for matching with sources from all input images unless a separate catalog is provided through the `refcat` parameter.

  expand_refcat : bool (Default = False)

  > Specifies whether to add new sources from just matched images to the reference catalog to allow next image to be matched against an expanded reference catalog.

  enforce_user_order : bool (Default = True)

  > Specifies whether images should be aligned in the order specified in the `file` input parameter or `TweakReg` should optimize the order of alignment by intersection area of the images. Default value (`True`) will align images in the user specified order, except when some images cannot be aligned in which case `TweakReg` will optimize the image alignment order. Alignment order optimization is available only when `expand_refcat` = `True`.

  exclusions: string (Default = ‘’)

  > This parameter allows the user to specify a set of files which contain regions in the image to ignore (or to include) when finding sources. This file MUST have 1 line for each input image with the name of the input file in the first column. Subsequent columns would be used to specify an inclusion and/or exclusion file for each chip in `'SCI,<n>'` index order. If a chip does not require an exclusion file, the string `None` or `INDEF` can be used as a placeholder for that chip. Each exclusion file can be either a mask provided as a simple FITS file or a region file in DS9-format.

  > When a mask file is provided, `TweakReg` will look for the first image-like extension with image data of the same dimensions as the input image. Zeros in the mask will be interpreted as “bad” (excluded from search) pixels while non-zero pixels will be interpreted as “good” pixels. It is recommended that mask files be FITS files without extensions and mask data (preferably of integer type) reside in the primary HDU.

  > If a region file is provided then it should conform to the ‘region’ file format generated by DS9. The region files can contain both regular (“include”) regions as well as “exclude” regions. Regular (“include”) regions indicate the regions of the image that should be searched for sources while “exclude” regions indicate parts of the image that should not be used for source detection. The “ruler”, “compass”, and “projection” regions are not supported (ignored). When all regions in a region file are “exclude” regions, then it will be assumed that the entire image is “good” before the exclude regions are processed. In other words, an “include” region corresponding to the entire image will be prepended to the list of exclude regions.

  > Note

  > Regions in a region file are processed in the order they appear in the region file. Thus, when region files contain both “include” and “exclude” regions, the order in which these regions appear may affect the results.

  > Warning

  > `TweakReg` relies on `pyregion` package for work with region files. At the time of writing, `pyregion` uses a different algorithm from DS9 for converting regions from sky coordinates to image coordinate (this conversion is performed before regions are converted to masks). For these reasons, regions provided in sky coordinates may not produce the expected (from DS9) results. While in most instances these discrepancies should be tolerable, it is important to keep this in mind.

  > During testing it was observed that conversion to image coordinates is most accurate for polygonal regions and less accurate for other regions Therefore, if one must provide regions in sky coordinates, it is types. recommended to use polygonal and circular regions and to avoid elliptical and rectangular regions as their conversion to image coordinates is less accurate. One may use `mapreg` task in the `drizzlepac` package to convert region files from sky coordinates to image coordinates. This will allow one to see the actual regions that will be used by source finding routine in `TweakReg`.

  updatewcs : bool (Default = No)

  > NOT available through TEAL GUI interface. This parameter can only be set through the Python interface to Tweakreg by passing it in as part of the input_dict in order to insure that running `updatewcs` does not overwrite a previously determined solution written out to the input file headers.

  writecat : bool (Default = Yes)

  > Specify whether or not to write out the source catalogs generated for each input image by the built-in source extraction algorithm.

  clean : bool (Default = No)

  > Specify whether or not to remove the temporary files created by `tweakreg`, including any catalog files generated for the shift determination.

  interactive : bool (Default = Yes)

  > This switch controls whether the program stops and waits for the user to examine any generated plots before continuing on to the next image. If turned off, plots will still be displayed, but they will also be saved to disk automatically as a PNG image with an autogenerated name without requiring any user input.

  verbose : bool (Default = No)

  > Specify whether or not to print extra messages during processing.

  runfile : string (Default = ‘tweakreg.log’)

  > Specify the filename of the processing log.

  *UPDATE HEADER*

  updatehdr : bool (Default = No)

  > Specify whether or not to update the headers of each input image directly with the shifts that were determined. This will allow the input images to be combined by `AstroDrizzle` without having to provide the shiftfile as well.

  wcsname : str (Default = ‘TWEAK’)

  > Name of updated WCS.

  reusename : bool (Default = False)

  > Allows overwriting of an existing WCS with the same name as specified by `wcsname` parameter.

  *HEADERLET CREATION*

  headerlet: bool (Default = No)

  > Specify whether or not to generate a headerlet from the images at the end of the task? If turned on, this will create a headerlet from the images regardless of the value of the `updatehdr` parameter.

  attach: bool (Default = Yes)

  > If creating a headerlet, choose whether or not to attach the new headerlet to the input image as a new extension.

  hdrfile: string (Default = ‘’)

  > Filename to use for writing out headerlet to a separate file. If the name does not contain `.fits`, it will create a filename from the rootname of the input image, the value of this string, and it will end in `'_hlet.fits'`. For example, if only `'hdrlet1'` is given, the full filename created will be `'j99da1f2q_hdrlet1_hlet.fits'` when creating a headerlet for image `'j99da1f2q_flt.fits'`.

  clobber: bool (Default = No)

  > If a headerlet with ‘hdrfile’ already exists on disk, specify whether or not to overwrite that previous file.

  hdrname: string (Default = ‘’)

  > Unique name to give to headerlet solution. This name will be used to identify this specific WCS alignment solution contained in the headerlet.

  author: string, optional (Default = ‘’)

  > Name of the creator of the headerlet.

  descrip: string, optional (Default = ‘’)

  > Short (1-line) description to be included in headerlet as `DESCRIP` keyword. This can be used to provide a quick look description of the WCS alignment contained in the headerlet.

  catalog: string, optional (Default = ‘’)

  > Name of reference catalog used as the basis for the image alignment.

  history: string, optional (Default = ‘’)

  > Filename of a file containing detailed information regarding the history of the WCS solution contained in the headerlet. This can include information on the catalog used for the alignment, or notes on processing that went into finalizing the WCS alignment stored in this headerlet. This information will be reformatted as 70-character wide FITS HISTORY keyword section.

  *OPTIONAL SHIFTFILE OUTPUT*

  shiftfile : bool (Default = No)

  > Create output shiftfile?

  outshifts : str (Default = ‘shifts.txt’)

  > The name for the output shift file created by `tweakreg`. This shiftfile will be formatted for use as direct input to `AstroDrizzle`.

  outwcs : str (Default = ‘shifts_wcs.fits’)

  > Filename to be given to the OUTPUT reference WCS file created by `tweakreg`. This reference WCS defines the WCS from which the shifts get measured, and will be used by `AstroDrizzle` to interpret those shifts. This reference WCS file will be a FITS file that only contains the WCS keywords in a Primary header with no image data itself. The values will be derived from the FIRST input image specified.

  *COORDINATE FILE DESCRIPTION*

  catfile : str (Default = ‘’)

  > Name of file that contains a list of input images and associated catalog files generated by the user. Each line of this file will contain the name of an input image in the first column. The remaining columns will provide the names of the source catalogs for each chip in order of the science extension numbers ((SCI,1), (SCI,2), ...).

  > A sample catfile, with one line per image would look like:

  >     image1_flt.fts  cat1_sci1.coo  cat1_sci2.coo
  >     image2_flt.fts  cat2_sci1.coo  cat2_sci2.coo

  xcol : int (Default = 1)

  > Column number of X position from the user-generated catalog files specified in the catfile.

  ycol : int (Default = 2)

  > Column number of Y position from the user-generated catalog files specified in the catfile.

  fluxcol : int (Default = None)

  > Column number for the flux values from the user-generated catalog files specified in the catfile. These values will only be used if a flux limit has been specified by the user using the `maxflux` or `minflux` parameters.

  maxflux : float (Default = None)

  > Limiting flux value for selecting valid objects in the input image’s catalog. If specified, this flux will serve as the upper limit of a range for selecting objects to be used in matching with objects identified in the reference image. If the value is set to `None`, all objects with fluxes brighter than the minimum specified in `minflux` will be used. If both values are set to `None`, all objects will be used.

  minflux : float (Default = None)

  > Limiting flux value for selecting valid objects in the input image’s catalog. If specified, this flux value will serve as the lower limit of a range for selecting objects to be used in matching with objects identified in the reference image. If the value is set to `None`, all objects fainter than the limit specified by `maxflux` will be used. If both values are set to `None`, all objects will be used.

  fluxunits : str {‘counts’, ‘cps’, ‘mag’} (Default = ‘counts’)

  > This allows the task to correctly interpret the flux limits specified by `maxflux` and `minflux` when sorting the object list for trimming of fainter objects.

  xyunits : str {‘pixels’, ‘degrees’} (Default = ‘pixels’)

  > Specifies whether the positions in this catalog are already sky pixel positions, or whether they need to be transformed to the sky.

  nbright : int (Default = None)

  > The number of brightest objects to keep after sorting the full object list. If nbright is set equal to `None`, all objects will be used.

  *REFERENCE CATALOG DESCRIPTION*

  refcat : str (Default = ‘’)

  > Name of the external reference catalog file to be used in place of the catalog extracted from one of the input images. When `refimage` is not specified, reference WCS to be used with reference catalog will be derived from input images.

  refxcol : int (Default = 1)

  > Column number of RA in the external catalog file specified by the refcat.

  refycol : int (Default = 2)

  > Column number of Dec in the external catalog file specified by the refcat.

  refxyunits : str {‘pixels’,’degrees’} (Default = ‘degrees’)

  > Units of sky positions.

  rfluxcol : int (Default = None)

  > Column number of flux/magnitude values in the external catalog file specified by the refcat.

  rmaxflux : float (Default = None)

  > Limiting flux value used to select valid objects in the external catalog. If specified, the flux value will serve as the upper limit of a range for selecting objects to be used in matching with objects identified in the reference image. If the value is set to `None`, all objects with fluxes brighter than the minimum specified in `rminflux` will be used. If both values are set to `None`, all objects will be used.

  rminflux : float (Default = None)

  > Limiting flux value used to select valid objects in the external catalog. If specified, the flux will serve as the lower limit of a range for selecting objects to be used in matching with objects identified in the reference image. If the value is set to `None`, all objects fainter than the limit specified by `rmaxflux` will be used. If both values are set to `None`, all objects will be used.

  rfluxunits : {‘counts’, ‘cps’, ‘mag’} (Default = ‘mag’)

  > This allows the task to correctly interpret the flux limits specified by `rmaxflux` and `rminflux` when sorting the object list for trimming of fainter objects.

  refnbright : int (Default = None)

  > Number of brightest objects to keep after sorting the full object list. If refnbright is set to `None`, all objects will be used. Used in conjunction with refcat.

  *OBJECT MATCHING PARAMETERS*

  minobj : int (Default = 15)

  > Minimum number of identified objects from each input image to use in matching objects from other images.

  searchrad : float (Default = 1.0)

  > The search radius for a match.

  searchunits : str (Default = ‘arcseconds’)

  > Units for search radius.

  use2dhist : bool (Default = Yes)

  > Use 2d histogram to find initial offset?

  see2dplot : bool (Default = Yes)

  > See 2d histogram for initial offset?

  tolerance : float (Default = 1.0)

  > The matching tolerance in pixels after applying an initial solution derived from the ‘triangles’ algorithm. This parameter gets passed directly to `xyxymatch` for use in matching the object lists from each image with the reference image’s object list.

  separation : float (Default = 0.0)

  > The minimum separation for objects in the input and reference coordinate lists. Objects closer together than ‘separation’ pixels are removed from the input and reference coordinate lists prior to matching. This parameter gets passed directly to `xyxymatch` for use in matching the object lists from each image with the reference image’s object list.

  xoffset : float (Default = 0.0)

  > Initial estimate for the offset in X between the images and the reference frame. This offset will be used for all input images provided. If the parameter value is set to `None`, no offset will be assumed in matching sources in `xyxymatch`.

  yoffset : float (Default = 0.0)

  > Initial estimate for the offset in Y between the images and the reference frame. This offset will be used for all input images provided.If the parameter value is set to None, no offset will be assumed in matching sources in `xyxymatch`.

  *CATALOG FITTING PARAMETERS*

  fitgeometry : str {‘shift’, ‘rscale’, ‘general’} (Default = ‘rscale’)

  > The fitting geometry to be used in fitting the matched object lists. This parameter is used in fitting the offsets, rotations and/or scale changes from the matched object lists. The ‘general’ fit geometry allows for independent scale and rotation for each axis.

  residplot : str {‘No plot’, ‘vector’, ‘residuals’, ‘both’} (Default = ‘both’)

  > Plot residuals from fit? If ‘both’ is selected, the ‘vector’ and ‘residuals’ plots will be displayed in separate plotting windows at the same time.

  nclip : int (Default = 3)

  > Number of clipping iterations in fit.

  sigma : float (Default = 3.0)

  > Clipping limit in sigma units.

  *ADVANCED PARAMETERS AVAILABLE FROM COMMAND LINE*

  updatewcs : bool (Default = No)

  > This parameter specifies whether the WCS keywords are to be updated by running updatewcs on the input data, or left alone. The update performed by updatewcs not only recomputes the WCS based on the currently used `IDCTAB`, but also populates the header with the `SIP` coefficients. For `ACS/WFC` images, the time-dependence correction will also be applied to the `WCS` and `SIP` keywords. This parameter should be set to ‘No’ (`False`) when the WCS keywords have been carefully set by some other method, and need to be passed through to drizzle ‘as is’, otherwise those updates will be over-written by this update.

  > Note

  > This parameter was preserved in the API for compatibility purposes with existing user processing pipe-lines. However, it has been removed from the TEAL interface because it is easy to have it set to ‘yes’ (especially between consecutive runs of `AstroDrizzle`) with potentially disastrous effects on input image WCS (for example it could wipe-out previously aligned WCS).

  See also

  `astrodrizzle`

  Notes

  Tweakreg supports the use of calibrated, distorted images (such as FLT images for ACS and WFC3, or `_c0m.fits` images for WFPC2) as input images. All coordinates for sources derived from these images (either by this task or as provided by the user directly) will be corrected for distortion using the distortion model information specified in each image’s header. This eliminates the need to run ‘astrodrizzle’ on the input images prior to running `tweakreg`.

  Note

  All calibrated input images must have been updated using `updatewcs` from the `STWCS` package, to include the full distortion model in the header. Alternatively, one can set `updatewcs` parameter to `True` when running either `TweakReg` or `AstroDrizzle` from command line (Python interpreter) the first time on such images.

  This task will use catalogs, and catalog-matching, based on the `xyxymatch` algorithm to determine the offset between the input images. The primary mode of operation will be to extract a catalog of source positions from each input image using either a ‘DAOFIND-like’ algorithm or `SExtractor` (if the user has `SExtractor` installed). Alternatively, the user can provide their catalogs of source positions derived from each input chip.

  The reference frame will be defined either by:

  > - the image with the largest overlap with another input image AND with the largest total overlap with the rest of the input images,
  > - a catalog derived from a reference image specified by the user, or
  > - a catalog of undistorted sky positions (RA/Dec) and fluxes provided by the user.

  For a given observation, the distortion model is applied to all distorted input positions, and the sources from each chip are then combined into a single catalog of undistorted positions.

  The undistorted positions for each observation then get passed to `xyxymatch` for matching to objects from the reference catalog.

  The source lists from each image will generally include cosmic-rays as detected sources, which can at times significantly confuse object identification between images. Observations that include long exposures often have more cosmic-ray events than source objects. As such, isolating the cosmic-ray events in those cases would significantly improve the efficiency of common source identification between images. One such method for trimming potential false detections from each source list would be to set a flux limit to exclude detections below that limit. As the fluxes reported in the default source object lists are provided as magnitude values, setting the `maxflux` or `minflux` parameter value to a magnitude- based limit, and then setting the `ascend` parameter to `True`, will allow for the creations of catalogs trimmed of all sources fainter than the provided limit. The trimmed source list can then be used in matching sources between images and in establishing the final fitting for the shifts.

  A fit can then be performed on the matched set of positions between the input and the reference to produce the ‘shiftfile’. If the user is confident that the solution will be correct, the header of each input image can be updated directly with the fit derived for that image. Otherwise, the ‘shiftfile’ can be passed to AstroDrizzle for aligning the images.

  Note

  Because of the nature of the used algorithm it may be necessary to run this task multiple time until new shifts, rotations, and/or scales are small enough for the required precision.

  New sources (that are not in the reference catalog) from the matched images are added to the reference catalog in order to allow next image to be matched to a larger reference catalog. This allows alignment of images that do not overlap directly with the reference image and/or catalog and it is particularly useful in image registration of large mosaics. Addition of new sources to the reference catalog can be turned off by setting `expand_refcat` to `False` when using an external reference catalog. When an external catalog is not provided (`refcat`='') or when using an external reference catalog with `expand_refcat` set to `True` (assuming `writecat` = `True` and `clean` = `False`), the list of all sources in the expanded reference catalog is saved in a catalog file named `cumulative_sky_refcat_###.coo` where ### is the base file name derived from either the external catalog (if provided) or the name of the image used as the reference image.

  When `enforce_user_order` is `False`, image catalogs are matched to the reference catalog in order of decreasing overlap area with the reference catalog, otherwise user order of files specified in the `file` parameter is used.

  Format of Exclusion Catalog

  The format for the exclusions catalog requires 1 line in the file for every input image, regardless of whether or not that image has any defined exclusion regions. A sample file would look like:

      j99da1emq_flt.fits
      j99da1f2q_flt.fits test_exclusion.reg

  This file specifies no exclusion files for the first image, and only an regions file for SCI,1 of the second image. NOTE: The first file can be dropped completely from the exclusion catalog file.

  In the above example, should an exclusion regions file only be needed for the second chip in the second image, the file would need to look like:

      j99da1emq_flt.fits
      j99da1f2q_flt.fits None test_sci2_exclusion.reg

  The value `None` could also be replaced by `INDEF` if desired, but either string needs to be present to signify no regions file for that chip while the code continues parsing the line to find a file for the second chip.

  Format of Region Files

  The format of the exclusions catalogs referenced in the ‘exclusions’ file defaults to the format written out by DS9 using the ‘DS9/Funtools’ region file format. A sample file with circle() regions will look like:

      # Region file format: DS9 version 4.1
      # Filename: j99da1f2q_flt.fits[SCI]
      global color=green dashlist=8 3 width=1 font="helvetica 10 normal roman"
      select=1 highlite=1 dash=0 fixed=0 edit=1 move=1 delete=1 include=1 source=1
      image
      circle(3170,198,20)
      ellipse(3269,428,30,10,45) # a rotated ellipse
      box(3241.1146,219.78132,20,20,15) # a rotated box
      circle(200,200,50)  # outer circle
      -circle(200,200,30) # inner circle

  This region file will be interpreted as “find all sources in the image that are inside the four regions above but not inside the region -circle(200,200,30)”. Effectively we will instruct `TweakReg` to find all the sources inside the following regions:

      circle(3170,198,20)
      ellipse(3269,428,30,10,45) # a rotated ellipse
      box(3241.1146,219.78132,20,20,15) # a rotated box
      annulus(200,200,30,50)  # outer circle(r=50) - inner circle(r=30)

  Examples

  The tweakreg task can be run from either the TEAL GUI or from the command-line using PyRAF or Python. These examples illustrate the various syntax options available.

  Example 1: Align a set of calibrated (`_flt.fits`) images using `IMAGEFIND`, a built-in source finding algorithm based on `DAOPHOT`. Auto-detect the sky sigma value and select sources > 200 sigma. (Auto-sigma is computed from the first input exposure as: `1.5*imstat(image,nclip=3,fields='stddev')`. ) Set the convolution kernel width to `~2x` the value of the PSF FWHM. Save the residual offsets (`dx`, `dy`, `rot`, `scale`, `xfit_rms`, `yfit_rms`) to a text file.

  1. Run the task from PyRAF using the TEAL GUI:

         >>> import drizzlepac
         >>> epar tweakreg

  2. Run the task from PyRAF using the command line while individually specifying source finding parameters for the reference image and input images:

         >>> import drizzlepac
         >>> from drizzlepac import tweakreg
         >>> tweakreg.TweakReg('*flt.fits',
         ...       imagefindcfg={'threshold' : 200, 'conv_width' : 3.5},
         ...       refimagefindcfg={'threshold' : 400, 'conv_width' : 2.5},
         ...       updatehdr=False, shiftfile=True, outshifts='shift.txt')

     or, using `dict` constructor,

         >>> import drizzlepac
         >>> from drizzlepac import tweakreg
         >>> tweakreg.TweakReg('*flt.fits',
         ...       imagefindcfg=dict(threshold=200, conv_width=3.5),
         ...       refimagefindcfg=dict(threshold=400, conv_width=2.5),
         ...       updatehdr=False, shiftfile=True, outshifts='shift.txt')

     Or, run the same task from the PyRAF command line, but specify all parameters in a config file named “myparam.cfg”:

         >>> tweakreg.TweakReg('*flt.fits', configobj='myparam.cfg')

     Alternately, edit the imagefind parameters in a TEAL GUI window prior to running the task:

         >>> tweakreg.edit_imagefindpars()

  3. Help can be accessed via the “Help” pulldown menu in the TEAL GUI. It can also be accessed from the PyRAF command-line and saved to a text file:

         >>> from drizzlepac import tweakreg
         >>> tweakreg.help()

     or

         >>> tweakreg.help(file='help.txt')
         >>> page help.txt

`drizzlepac.tweakreg.``help`(file=None)

  Print out syntax help for running astrodrizzle

  Parameters:

  file : str (Default = None)

  > If given, write out help to the filename specified by this parameter Any previously existing file with this name will be deleted before writing out the help.