Because there is no slit in the WFC3 grism mode, the PSF of the target determines
the spectral resolution. In the case of non-stellar sources, it is the extent of the target in the direction of dispersion that limits the spectral resolution. The height of the software extraction slit is based on the object extent in the cross-dispersion direction of the direct image.
The dispersion of the grisms is well characterized, but in order to set the
wavelength zero-point, it is necessary to know the position of the target in the direct image. The zeroth-order is generally too weak and is also slightly extended in a dispersed image to allow the wavelength zero-point to be set reliably. Given the typical spacecraft jitter, wavelength zero-points to ±
0.5 pixels should be routinely achievable using a direct image taken just before or after the grism image.
A spectral extraction software package, called aXe
, is available to extract, flat-field, wavelength- and flux-calibrate WFC3 grism spectra. Full details can be found at:
The spectral trace and dispersion solutions are a function of source position within
the field of view. These 2-dimensional variations were determined during the ground calibration campaigns and from on-orbit data. The resulting reference and calibration files are used in the extraction software aXe and are also available from the WFC3 area of the aXe website. For bright sources, the multiple spectral orders of the G280, G102 and G141 grisms may extend across the full detector extent. Therefore, a careful selection of the optimum telescope roll angle is required to obtain non-overlapping spectra of faint sources in the vicinity of brighter objects. (i.e., the observer needs to set the orientation of the detector on the sky by using the Visit Orientation Requirements parameter “ORIENT” in the phase II proposal; e.g. ORIENT ~135 degrees aligns the Y axis of the IR detector with North.)
The quality of extracted spectra from single grism exposures can be degraded by
bad pixels (e.g., dead, hot, strong cosmic ray hit). We recommend a dithering strategy for IR grism exposures. The aXe software automatically takes dither steps into account by using the information in the image headers to produce a combined spectrum with cosmic rays and bad pixels removed. This has not worked well for UVIS grism exposures because of the shape of the G280 spectra. CR-SPLIT
exposures are therefore preferred over dithering for cosmic ray removal from G280 data.