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WFC3 Data Handbook V. 4.0 |
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5.6.1 Full Well DepthConceptually, full well depths can be derived by analyzing images of a rich starfield taken at two significantly different exposure times, identifying bright but still unsaturated stars in the short exposure image, calculating which stars will saturate in the longer exposure and then simply recording the peak value reached for each star in electrons (using a gain that samples the full well depth, of course). In practice, it is also necessary to correct for a ~10% "piling up" effect of higher values being reached at significant levels of over-saturation relative to the value at which saturation and bleeding to neighboring pixels in the column begins. (see WFC3 ISR 2010-10).There is a real and significant large-scale variation of the full well depth on the UVIS CCDs. The variation over the UVIS CCDs is from about 63,000 e- to 72,000 e- with a typical value of about 68,000 e-. There is a significant offset between the two CCDs. The spatial variation may be seen in Figure 5 of WFC3 ISR 2010-10 the full well depth images in fits file format are available for download from:Linearity at low and moderate exposure levels is explored by comparing counts in back-to-back exposures on NGC 1850. Figure 5.8 shows the response of one of the chips, where aperture sums for stars with flux greater than about 2,000 e- in a short exposure (central pixel would be at greater than about 350 e-) show apparently perfect linear response when compared to the counts in the same aperture in an exposure 50 times as long. However, below a level of 2,000 e- the ratio of long to short exposure counts deviates from a linear response. Summed counts of 200 e- in the short exposure these values are ~ 5% lower than expected based on scaling from the corresponding long exposure. These data were acquired in October 2009, five months after launch of WFC3.Figure 5.9 shows data from both UVIS CCD for stars yielding short exposure aperture sums of 500 to 2000 e-. A clear signature appears that is consistent with perfect linearity for stars near the readout amplifiers, with linearly growing losses in the short relative to long exposure with distance from the amplifiers. This is consistent with losses induced by finite charge transfer efficiency in successive parallel shifts in clocking the charge packets off the CCDs.The response of the WFC3 UVIS CCDs remains linear not only up to, but well beyond, the point of saturation. WFC3 ISR 2010-10 shows the well behaved response of WFC3: electrons are clearly conserved after saturation -- in some locations with the need for a minor calibration, as provided in the ISR, in other regions no correction is needed. This result is similar to that of the STIS CCD (Gilliland et al. 1999) the WFPC2 camera (Gilliland, 1994) and ACS (ACS ISR 2004-01). It is possible to easily perform photometry on point sources that remain isolated simply by summing over all of the pixels into which the charge has bled.Figure 5.10 and Figure 5.11 show results for UVIS. Over a range of nearly 7 magnitudes beyond saturation, photometry remains linear to ~ 1% after a simple calibration. For Amp C of UVIS2 the response is sufficiently linear beyond saturation that no correction is required.Figure 5.10: Linearity Analysis for Amp AFigure 5.11: Long vs. Short Exposure Ratios of LinearityUpper panel shows data from the upper panel of Figure 5.10 for Amp A, plus similar data for Amp B. The lower panel shows the same data after applying the corrections given in detail in WFC3 ISR 2010-10, Not only is the mean level appropriately restored independent of degree of over-saturation, but the star-to-star scatter is much reduced.5.6.4 Shutter StabilityThe WFC3-UVIS shutter is a circular, rotating blade divided into two open and two closed quadrants (See Section 2.3.3 of the WFC3 Instrument Handbook for details). Operationally, the shutter mechanism has two distinct modes, based on commanded exposure times. At the shortest commanded exposure time of 0.5 seconds, the shutter motion is continuous during the exposure, rotating from the closed position through the open position and on to the next closed position. For commanded exposure times of 0.7 seconds and longer (0.6 seconds is not allowed), the shutter rotates into the open position, stops and waits for an appropriate amount of time, and then rotates to the closed position.For short exposure times, detector position dependent exposure time (shutter shading), A versus B blade shutter dependence, stability, and timing accuracy were assessed using data taken during SMOV. For a full discussion of the analysis of shutter behavior from on-orbit data see WFC3 ISR 2009-25 and WFC3 ISR 2015-12. No systematic difference in shutter behavior (exposure time, repeatability, etc.) is found when comparing the A and B blades of the shutter. Even at the shortest exposures, measured shutter shading does not exceed ~0.2% across the detector. The small magnitude of this effect means that no correction for shutter shading is necessary in calwf3.5.6.5 FringingAt wavelengths longer than about 650 nm, silicon becomes transparent enough that multiple internal reflections in the UVIS detector can create patterns of constructive and destructive interference, or fringing. Fringing produces wood-grain patterns in response to narrow-band illumination at long wavelengths, see Figure 5.12.Figure 5.12: Fringe FlatsFlat fields from ground tests (see WFC3 ISR 2008-46) have been used to estimate the magnitude of fringing effects, for a continuum light source, in the narrow-band red filters (see Table 5.3 and WFC3 ISR-2010-04). Each column lists a different metric of fringe amplitude, for a control filter (F606W) and for the filters in which fringing effects could be detected in the flat-field data. These metrics can best be understood by examining the histograms (Figure 5.13) of the flat fields shown in Figure 5.12.
Values are given in units of percentage of the normalized flat-field signal level. Each metric is described in the text and graphically represented in Figure 5.13The first data column in the table is simply the root mean square deviation from the mean of the sample, and is indicated by triangles with horizontal error bars in the histograms. Filters/quadrants with rms deviations greater than corresponding values for the control filter (F606W) may be influenced by fringing. The second column is full width at 20% maximum, rather than full width at 50% maximum, because this metric is more effective for bimodal pixel brightness distributions in filters with strong fringing, such as FQ906N (pictured). The third data column gives the separation between histogram peaks, which can be detected in flat-field data for only the five reddest of the twelve filters affected by fringing. Squares in Figure 5.11 mark the histogram peaks. Adjacent fringes were also manually sampled, and the results reported in the final data column.Symbols correspond to fringe amplitude metrics listed in Table 5.3: rms deviation (triangles with error bars), full width at 20% maximum (circles with error bars), and bimodal histogram peaks (squares).Eventually, tools will be created so that users of UVIS data will be able to generate "fringe flats" for any combination of source SED and long-wavelength filter. Monochromatic ground test data have been used to create thickness maps of the UVIS detector (WFC3 TIR 2010-01), and these maps can be used to model the expected fringing response to an arbitrary SED. On orbit calibration data (Programs 11922 and 12091) are being taken in 2010 with the narrow band filters listed in Table 5.3, and these data will be analyzed to evaluate the fringe model solutions and thickness maps.