Step Arguments

The resample step has the following optional arguments that control the behavior of the processing and the characteristics of the resampled image.

--pixfrac (float, default=1.0)

The fraction by which input pixels are “shrunk” before being drizzled onto the output image grid, given as a real number between 0 and 1.

--kernel (str, default=’square’)

The form of the kernel function used to distribute flux onto the output image. Available kernels are square, gaussian, point, turbo, lanczos2, and lanczos3.

--pixel_scale_ratio (float, default=1.0)

Ratio of output pixel scale to input pixel scale. For imaging data, a value of 0.5 means the output image would have 4 pixels sampling each input pixel. Ignored when pixel_scale or output_wcs are provided.

--pixel_scale (float, default=None)

Absolute pixel scale in arcsec. When provided, overrides pixel_scale_ratio. Ignored when output_wcs is provided.

--rotation (float, default=None)

Position angle of output image’s Y-axis relative to North. A value of 0.0 would orient the final output image to be North up. The default of None specifies that the images will not be rotated, but will instead be resampled in the default orientation for the camera with the x and y axes of the resampled image corresponding approximately to the detector axes. Ignored when pixel_scale or output_wcs are provided.

--crpix (tuple of float, default=None)

0-based coordinates of the reference pixel in the image array in the x, y order. This is the image coordinate to which crval maps to. If crpix is not specified, it will be set to the center of the bounding box of the returned WCS object. When supplied from command line, it should be a comma-separated list of floats. Ignored when output_wcs is provided.

--crval (tuple of float, default=None)

Right ascension and declination of the reference pixel. Automatically computed if not provided. When supplied from command line, it should be a comma-separated list of floats. Ignored when output_wcs is provided.

--output_shape (tuple of int, default=None)

Shape of the image (data array) using “standard” nx first and ny second (opposite to the numpy.ndarray convention - ny first and nx second). This value will be assigned to pixel_shape and array_shape properties of the returned WCS object. When supplied from command line, it should be a comma-separated list of integers nx, ny.

Note

Specifying output_shape is required when the WCS in output_wcs does not have bounding_box property set.

--output_wcs (str, default=’’)

File name of an ASDF file with a GWCS stored under the "wcs" key under the root of the file. The output image size is determined from the bounding box of the WCS (if any). Argument output_shape overrides computed image size and it is required when output WCS does not have bounding_box property set or if pixel_shape or array_shape keys (see below) are not provided.

Additional information may be stored under other keys under the root of the file. Currently, the following keys are recognized:

  • pixel_area: Indicates average pixel area of the output WCS in units of steradians. When provided, this value will be used for updating photometric quantities PIXAR_SR and PIXAR_A2 of the output image. If pixel_area is not provided, the code will attempt to estimate this value from the WCS.

  • pixel_shape: dimensions of the output image in the order (nx, ny). Overrides the value of array_shape if provided.

  • array_shape: shape of the output image in numpy order: (ny, nx).

Note

When output_wcs is specified, WCS-related arguments such as pixel_scale_ratio, pixel_scale, rotation, crpix, and crval will be ignored.

--fillval (str, default=’NAN’)

The value to assign to output pixels that have zero weight or do not receive any flux from any input pixels during drizzling.

--weight_type (str, default=’ivm’)

The weighting type for each input image. If weight_type="ivm" (the default), the scaling value will be determined per-pixel using the inverse of the read noise (VAR_RNOISE) array stored in each input image. If the VAR_RNOISE array does not exist, the variance is set to 1 for all pixels (equal weighting). If weight_type="exptime", the scaling value will be set equal to the measurement time (TMEASURE) found in the image header if available; if unavailable, the scaling will be set equal to the exposure time (EFFEXPTM).

--single (bool, default=False)

If set to True, resample each input image into a separate output. If False (the default), each input is resampled additively (with weights) to a common output.

--blendheaders (bool, default=True)

Blend metadata from all input images into the resampled output image.

--in_memory (boolean, default=True)

Specifies whether or not to load and create all images that are used during processing into memory. If False, input files are loaded from disk when needed and all intermediate files are stored on disk, rather than in memory.

--enable_ctx (boolean, default=True)

Specifies whether or not to compute and store the context array (con) in the datamodel, which is used to track which input images contributed to each pixel in the output image. Setting this to False helps reduce memory usage for very large mosaics.

--enable_err (boolean, default=True)

Specifies whether or not to compute and store the error array in the output model. Setting this to False helps reduce memory usage and output file size for very large mosaics, but the err, var_flat, var_rnoise, and var_poisson arrays will not be computed or reported. If set to False, the report_var flag is ignored.

--report_var (boolean, default=True)

Specifies whether or not to store the variance arrays, namely var_flat, var_rnoise, and var_poisson, in the output model. Setting this to False helps reduce output file size for very large mosaics, but note that the variances are still computed internally if enable_err is True because they are needed to compute the error array.