STScI Logo

gol2af axe



gol2af -- generates an aperture file from a grism object list


gol2af grism config 


Grism or Prism image filename.
Filename of grism/prism extraction configuration file.
Extraction width multiplicative factor.
(back = no) [boolean]
Whether a BAF will be generated rather than an OAF.
(orient = yes) [boolean]
Whether to use tilted extracted. This is the default. When set to "no" only vertical extraction (along columns) is performed.
(auto_orient = yes) [boolean]
Whether to use the automatic extraction orientation feature. This is enabled by default.
Wavelength to apply MMAG_EXTRACT and MMAG_MARK.
(exclude = no) [boolean]
Do not even list faint objects (mag>max_mag) in the output.
(dmag) [float]
A number to add to the MMAG_EXTRACT and MMAG_MARK vale in the config-file.
(in_gol) [string]
Use this name in preference to the default for the input object list catalog file name.
(out_af) [string]
Use this name in preference to the default for the output aperture file name.


This task generates an Aperture File (AF) using an input Grism Object List (GOL) and a valid Main Configuration File which specifies the length, wavelength calibration and global offsets between direct and slitless images. Any offset between the two images produced by dithering of the telescope between the time the direct and the slitless images were taken is assumed to be properly accounted for in the WCS of both images. The width of the BEAMs is set up to be a given number times the full width at half maximum listed in the GOL. If the exclude parameter is also set these faint objects will not appear in the output GOL at all.

This task can be used to generate both an Object Aperture File and a Background Aperture File. These files have a similar format, but it is often desirable to use different Aperture Files in both cases since the first is used to extract counts from pixels which are known to contain flux from the source while the second can be thought of as a definition of a zone to avoid in the slitless image when computing the background level. In practice, a larger extraction width multiplier should be used when generating the Background Aperture File, so that the background estimation is not contaminated by flux from the object.

The task defines how exactly the spectra will be extracted by the tasks pet2spc or axedrizzle in later stages of the aXe reduction. With orient="yes" the cross-dispersion direction follows the anlge "THETA_IMAGE" specified for each image in the Input Object List. The parameter auto_orient="yes" automatically adjusts the cross-dispersion angle if it falls a too small angle (<35deg) with the trace (highly recommended).

With orient="no" the cross-dispersion direction is, for every object, perpendicular to the trace.

Positive values for mfwhm are used to define the extraction width as mfwhm*'object_width' with object_with either A_IMAGE/B_IMAGE (orient=yes) or their projection perpendicular to the trace (orient=no). Negative values specify the extration width in pixels. A detailed description on the interplay between those three parameters is given in the aXe manual.

When using quantitative contamination scheme such as the gaussian contamination there may exist information on the source brightness at different wavelengths. The parameter lambda_mark specifies the wavelength where the cutoff magnitudes defined in the configuration file should be applied. The algorithm selects the wavlength closest to the wavelength given in lambda_mark and applies the selection there. An interpolation between the source brightness at different wavelengths is not performed.


aXe tasks use default names for input and output files based on the given name of the "grism" image. For this task the default input GOL would be called <grism-rootname>_<science extension>.GOL and the output OAF file would be <grism-rootname>_<science extension>.OAF. If used to create a background aperture file the output would be <grism-rootname>_<science extension>.BAF.


1. Create an object aperture file or the objects in the grism/prism image test_grism_1.fits. The cross-dispersion direction is along the angle "THEAT_IMAGE" if this forms not a too small angle with the trace, and perpendicular to "THETA_IMAGE" otherwise. In the former case the extraction width is 2.0*A_IMAGE, in the latter case 2.0*B_IMAGE.

  ax> gol2af test_grism_1.fits fwhm=2 orient='yes' auto_orient='yes'

2. Create an object aperture file or the objects in the grism/prism image test_grism_1.fits. The cross-dispersion direction is perpendicuar to the object trace, and the extraction width is fixed at 3 pixels for all objects.

  ax> gol2af test_grism_1.fits fwhm=-3.0 orient='no'




Refer to manual for more detailed information about using this aXe task:



Search Form · STSDAS