WFC3 Instrument Handbook for Cycle 24

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8.5	Extraction and Calibration of Spectra

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:

http://www.stsci.edu/institute/software_hardware/stsdas/axe

The package is also available in STSDAS. An outline for reducing full array and subarray G280 grism observations is given in WFC3 ISR 2011-18.

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 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.

Extraction of WFC3/IR slitless spectra depends on an accurate determination of the diffuse background light that is observed in all grism exposures. The two-dimensional structure in the background of WFC3/IR grism exposures is caused primarily by overlapping grism spectral orders that are vignetted at different locations within the detector field of view and by the spectrum of the diffuse background. To remove this structure, the aXe analysis software (and related pipelines) fit and subtract “master sky” images, which have been generated for each grism from on-orbit science exposures. In addition to the zodiacal light background, a 1.083 µm emission line in the Earth’s upper atmosphere often appears in exposures obtained while the spacecraft is outside of the earth’s shadow (see Section 7.9.5). The intensity of this airglow line varies on timescales of an orbit or even a single sample sequence. The single aXe master sky images available for each grism do not take this spectral variation into account. More accurate background subtraction can be achieved by using separate images for each of the background components. See WFC3 ISR 2015-17 for a file containing the component images and an algorithm for applying them to observed WFC3/IR grism data. A scattered light component is included for G141.