IR Grism Sky Images

WFC3 IR Grism Sky Images (Nov 2, 2015)

 

These are sky background images computed for the two WFC3/IR grisms, G102 and G141. The images are derived from on-sky science observations with astronomical sources aggressively masked. Unlike the single sky images provided with the grism calibration files in slitlessutils, separate images are provided for three distinct background components: the nominal zodiacal continuum, the terrestrial He I 10830 Å emission line, and (in v1.0) the bright Earth limb scatter light  (see WFC3 ISR 2014-03 and in WFC3 ISR 2020-04 ). Because these components produce different two-dimensional spatial structure when dispersed through the grism spectral orders, fitting them separately yields a more accurate background subtraction.

The v1.0 sky images (presented in WFC3 ISR 2020-04) represent a substantial improvement over the v0.5 release (presented in WFC3 ISR 2015-17) in several respects:

• Dataset size: v1.0 models are derived from ~10x more individual detector reads, yielding higher signal-to-noise background components.
• Third background component: v0.5 modeled only Zodi and He I, with an approximate scattered light image for G141 only; v1.0 adds a fully independent scattered Earth light component for both G102 and G141 derived from archival data using the same methodology as the Zodi and He I components.
• Improved flat-fields: v1.0 uses custom flat-field reference files corrected for IR Blob artifacts, reducing systematic residuals across the detector, particularly near the left edge. Note that IR Blob imprints remain visible in the Zodi components of both grisms and are documented in WFC3 ISR 2020-04.

 

Full details on the construction of these images and the dispersed IR background are provided in WFC3 ISR 2020-04 (Pirzkal & Ryan) and WFC3 ISR 2015-17 (Brammer, Ryan & Pirzkal). For guidance on applying these files to your data using WFC3 Backsub, see the Recommended Workflow section below. For questions, please contact the STScI helpdesk.

Recommended Workflow

We recommend that users run raw G102 and G141 FITS files through WFC3 Backsub prior to further reductions or spectral extraction with slitlessutils.

WFC3 Backsub is a Python program written specifically to work with these multi-component sky files and perform a simultaneous background fit and subtraction. WFC3 Backsub still relies on and uses calwf3 for bias correction and dark subtraction, but it employs these custom reference files, (not available in the Calibration Reference Data System (CRDS)), to measure and remove the multiple sky components before the final “up-the-ramp” fitting occurs in calwf3.

Like HSTaXe (which is now deprecated), the current STScI-supported tool for wide-field slitless spectroscopy, slitlessutils, does not include support for these multi-component sky files natively. Running WFC3 Backsub first ensures a more accurate background subtraction before passing data to slitlessutils.

The IR Extraction Notebook demonstrates a complete spectral extraction workflow, including running WFC3 Backsub as an early step prior to slitlessutils reduction. A workflow to obtain WFC3 Backsub is also provided in that notebook, which is hosted in the hst_notebooks GitHub repository.

Technical Notes

• The sky images were computed from calibrated WFC3/IR data with appropriate imaging flat-fields divided out. The sky images were produced using an F105W flat for G102 and a F140W flat for G141. See the HST Calibration Reference Data System (CRDS) for the latest IR flat-fields.
• The observed background of a science exposure is modeled as a linear combination of the zodiacal continuum, He I emission line, and scattered Earth light components, whose coefficients are solved for simultaneously. WFC3 Backsub handles all of this and should be run (on raw grism files) before starting any further reduction or analysis.
• An improved scattered Earth light component image is provided for both G141 and G102. The v0.5 release included an approximate scattered light image for G141 only, which captured a simple gradient structure with limited dynamic range. The v1.0 scattered light components are derived from a much larger set of on-sky archival observations, yield higher signal-to-noise models with a more accurate 2D structure, and extend the correction to G102 as well.

Change History

• v1.0 — Updated sky images (~2019), based on WFC3 ISR 2020-04 (Pirzkal & Ryan 2020). Includes three-component background models for both G102 and G141, improved scattered Earth light component, and updated blob-corrected flat-field reference files.
• v0.5 — First release (11/02/15), based on WFC3 ISR 2015-17 (Brammer, Ryan & Pirzkal).