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L1 data distributed prior to April 2023 attempted to rotate the telescope’s polarization coordinate frame to the solar frame; however, it only included the parallactic angle (ignoring the P-angle) and applied the rotation in the wrong direction. This led to an incorrect and time-variable polarization reference frame. The revised data products now apply the correct rotation so that the polarization coordinate frame is stable in time and consistent with canonical reference coordinates used by SDO/HMI and Hinode-SP, as shown in the figure below (courtesy of Ichimoto et al. Sol Phys 249, 233–261 (2008). https://doi.org/10.1007/s11207-008-9169-9). The figure references the solar cardinal points where N/S is aligned with the solar rotation axis. The solar rotation axis is misaligned from the geocentric celestial frame by the P angle which varies from +/- the Earth’s obliquity. Additionally, the celestial frame is rotated from the Alt/Az local frame by the parallactic angle. The two combined are sometimes referred to as the solar orientation angle.

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• 854 nm Beam 2 (Arm3) has anomalously low modulation efficiency (35% vs 50%). 397 nm Beam 2 (Arm2) also has reduced efficiency.  It is suspected that this is due to low polarization beam splitter contrast. Optical mitigation likely will be required, and modeling is ongoing. We note that subtraction of the intrinsic stray light (above) improved the modulation efficiency, but it remains at/below 38%.

ViSP Camera Artifacts

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At times, VISP data may be impacted by camera related artifacts. For example, in OCP 1.8 (datasets covering Dec 27-30, 2022), and has also been seen in other wavelengths. The ViSP detectors (which are actively cooled) suffered from a condensation buildup which resulted in the pattern you can see as spots on the right-hand side of the detector (and zoomed in, inset left) in the gain image. These spots have such high central core values that they ruin the segmentation algorithm used to find the two hairlines. Without the hairlines being properly detected, the angle calculation used in determining the overlap of the two beams fails. While the routine for finding the hairlines can be (and is being) hardened against such occurrences, it should be noted that this artifact is also in the science data frames. The camera was subsequently allowed to warm to around 0 degrees C and the condensation disappeared. However, this in turn is likely to slightly increase the noise seen in the images.

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A gain image in the 397 nm channel showing condensation effects. Inset, a zoomed-in image of the spotscondensation buildup within the detector lead to a large number of spot features with high central core values. This is illustrated below.

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Another known artifact is related to the multi-gain architecture of the Zyla detectors, wherein the detector amplifier gain automatically adjusts to the incident flux level. In images with high dynamic range, such as near pores or sunspots and within deep chromospheric lines, the data may exhibit different noise behavior dependent on the signal level. Noise may also be enhanced at the threshold between the gain levels of the detector.

Camera artifacts can influence and/or effect the data calibration quality or the analysis approaches. Please feel free to address particular questions, should they arise, to the DKIST Help Desk.

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VBI Data Set Caveats

The following issues have been found/are being worked on with VBI datasets.

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