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, was available to extract, flat-field, wavelength- and flux-calibrate WFC3 grism spectra. A separate software package, hstaxe, which is a follow-up to aXe, can now be used to extract and calibrate one-dimensional spectra from WFC3 grism exposures. Python Jupyter notebooks illustrating the extraction and calibration workflow for both UVIS and IR grisms are available in the WFC3 section of the hstaxe Github repository.

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 available on the WFC3 Grism Resources webpage. 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; e.g. ORIENT ~135 degrees aligns the Y axis of the IR detector with North. See Section 6.2.2 of the Phase II Proposal Instructions, which gives detailed information on the relationship between detector coordinates, spacecraft coordinates, and ORIENT, and works out this example for aperture GRISM1024 (ORIENT = 135.3). Using the information in that section, one finds ORIENT ranging from 135.12 to 135.32 for the IR GRISM apertures (used for the grism exposures and matching reference images) and ORIENT = 135.17 for the G280 and G280-REF apertures). Note that any ORIENT requirement must be specified in the proposal PhaseI in the "Special Requirements" section (see also the news section in the Phase I proposal instructions).

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 hstaxe 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. The hstaxe software is used to locate and extract spectra of individual sources from calibrated images and performs wavelength calibration, background subtraction, flat fielding, and absolute flux calibration for the extracted spectra.

A more accurate background subtraction can be achieved by using separate images for each of these background components: zodiacal light, He I emission, and scattered light. These components are shown for G102 and G141 in Figure 8.8 and Figure 8.9, respectively. The He I component is due to a 1.083 µm emission line in the Earth’s upper atmosphere and 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. Scattered light produces a different structure, having bypassed the grism to reach the detector. WFC3 ISR 2015-17 presented a file containing all three component images for G141 and two of the component images for G102 (zodiacal light and He I emission) and an algorithm for applying them to observed WFC3/IR grism data. Images of all three background components for both IR grisms and software to background-subtract grism datasets are now available on the WFC3 Grism Resources webpage. Further discussion of the modeling and removal of background components are given in WFC3 ISR 2017-01 and WFC3 ISR 2020-04.