The Dispersed infrared background in WFC3 G102 and G141 observations

Background removal is one of the, if not the most, important step during the extraction and calibration of slitless observations. Any error in the amount of dispersed background light has an immediate effect on the quality of the extracted spectra. We show in this work how we determined three new independent background components (Zodi background, HeI emission, and “Scatter” Earth light) which have different spectral signature and therefore contribute differently to the overall background of the final observation. We also introduce a generalized implementation of the variable background subtraction described in ISR 2017-01. This approach allows us to account for a temporal variation of both the HeI and “Scatter” light and retain the use of on-the-ramp fitting in CALWF3. We also investigate, using all of the available G102 and G141 archival data, the conditions under which we measure an increased levels of HeI and “scatter” light emission. 1. The dispersed background While the majority of G102 and G141 observations can be seen to have background levels that vary, most of them appear to have the same background structure. This points to the fact that the principle source of background in WFC3 IR slitless observations is due to Copyright c © 2008 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved. Zodiacal light. The Zodiacal background varies in intensity as a function of position on the sky and as a function of time of the year but the Zodiacal spectrum varies only moderately at the wavelengths covered by G102 and G141. Figure 1 shows models of the Zodi and Galactic backgrounds at two different positions on the sky and at two different dates. As a result, we can subtract the dispersed 2D background from many slitless observations by simply scaling and subtracting a model of the dispersed Zodiacal background component. There is however a subset of WFC3 IR slitless observations for which the same Zodiacal light background is a poor fit. This was identified several years ago in direct imaging (WFC3 ISR 2014-03) and led to the effort to create a dispersed grism model of this component (WFC3 ISR 2015-17) which was identified as HeI glow from the Earth atmosphere. We also determined that, due to the changing limb angle between the Earth’s atmosphere and HST, the amount of HeI light varies during the course of an exposure (WFC3 ISR 2017-05). We show examples of a Zodiacal light dominated G102 observation as well as a HeI light dominated one in Figure 2. We also show a row average (after masking spectra) of these two images in Figure 2 to demonstrate why scaling of a Zodiacal light component does not work to correct for a dispersed HeI background. In practice, many observations suffer from a varying ratio of Zodiacal light and HeI light. A method to estimate the amount of Zodiacal and HeI background in a dataset was described in WFC3 ISR 2015-17 but this method did not account for a time variation in the make up of the background while the expsoures were taken. 2. Newly Available Data An increased number of G102 and G141 WFC3 observations have been accumulating in the archive as time goes on. For this reason, we initiated a project to create updated versions of the master G102 and G141 sky backgrounds. Previous versions of the G102 and G141 background models are now several years old (WFC3 ISR 2015-17) and have a few undesired issues: 1) Relatively low signal-to-noise 2) Imprints of the IR Blobs 3) limited to two components 4) Residual artifacts, especially in the HeI components. The Blobs are clearly visible in Zodi components in Figures 3 to 6 in WFC3 ISR 2015-17. In this ISR, we selected data differently for the processes of determining the Zodi components and any time-varying component and this is described in more detail in Sections 3.2, 4.1, and 5.1. 2.1. Use of Flat-fielding WFC3 grism observations are not flat-fielded by default, and the only flat-fielding that takes place while running CALWF3 is the use of a special flat-field that only applies the