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Post Cryogenic (NEOWISE) Calibrations | |
o Below we show W1,W2 darks and flats generated using on-orbit data at different times throughout the post-cryogenic mission. A self-consistent calibration method that tracks the change in response of each pixel relative to the overall background was used. More calibrations will be added to this page as the mission proceeds. o Note that the absolute level of the darks using the self-cal method is is very difficult to get right. There is no guarantee that they're physical, although the same method applied to cryogenic data did show stability in the dark level vs time. The "dark" counts (in DEB DN) are actually what you see in the images below minus 128 (the detector bias for W1,W2). The absolute variation is about +/-4 DN. The (relative) spatial variations however are very accurate. o Approximately 2000 frames (~8 scans worth) were used for each run below, although we will probabably use ~6000 frames for pipeline calibrations. It's a challenge to tease out a systematically varying (and evenly time- sampled) background in W1, W2, especially since the detectors are no longer in a thermally controlled environment. o The flat calibrations have an accuracy of ~1%/pixel, where the uncertainty was estimated using a-posteriori methods (hence more believable). The darks have a relative accuracy of ~5%/pixel (w.r.t. their median level). Absolute accuracy is much lower and not quantifiable at this time. o Moon-glow is suspected to have contaminated the Oct 18 W2 flat product. The closest moon-to-frame separation was ~30 degrees. Other calibrations had the moon at >~ 60 degrees. The Oct 18 W1 flat appears unaffected, as expected from cryo-mission analyses. The W2 flat calibration appears stable after Oct 18. o Images in each panel [across all dates] are at the same stretch. The color-bar gives an idea of the variation and level. o Note that the darks and flats below bear no resemblance to the cryogenic (nominal) calibrations. To view these, see: provisonal cals and v3.5 cals.
Dates go from left to right.
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