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WFC3 Grism master sky images (Nov 2, 2015)


These are master sky images computed for the two WFC3/IR grisms, G102 and G141. Observations for the images are on-sky science images with astronomical sources aggressively masked. These images differ from the single images provided with the grism (aXe) calibration files in that separate images are obtained by combining individual reads where the observed background was dominated either by the nominal zodiacal continuum or by the terrestrial He 10830 Å emission line (see Brammer et al. 2014 ). When folded through the grism spectral orders, the distinct background spectra yield different two-dimensional structure in the observed background.

Further information on the construction of these images is provided in WFC3 ISR 2015-17 (Brammer, Ryan & Pirzkal 2015). For questions, please contact the STScI helpdesk.


  • The images provided were computed from calibrated WFC3/IR images with appropriate imaging flat-field images divided out (G102: uc72113oi_pfl.fits [F105W], G141: uc721143i_pfl.fits [F140W]). In order to fit and subtract the master sky images to observed FLT images, the imaging flat has to be either multiplied to the sky image or divided from the science image.
  • The images are intended to be fit simultaneously to an observed science image, i.e., the observed background of a science exposure is a linear combination of the zodi and emission line background components. aXe is not currently able to do the multi-sky fit, so such a fit would have to be performed separately. ISR 2015-16 provides an example algorithm for computing the coefficients of the multi-component fit.
  • Some fraction of the G141 zodi-dominated exposures considered showed some additional excess on the left side of the detector similar to scattered light occasionally seen in the imaging filters. An image approximating the scattered light structure is also provided and can be combined with the G141 sky images.


FITS versions of the images are linked below. All of the FITS files are also available in a tarfile: grism_master_sky_v0.5.tar.gz

Change history:

v0.5 - First release (11/02/15)

LAST UPDATED: 04/07/2022

Please Contact the HST Help Desk with any Questions