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A second source of background light is identified to come from within the beam itself and is only seen when the beam is allowed to pass through the slit. This second source cannot be mitigated with external baffles or enclosures, and must be mitigated using other means. It has a different signature (spatial and spectral) at different wavelengths and/or ViSP arms. Analysis of this signal shows it to be additive and mostly unpolarized, much like a dark or background signal. This signal is a much more significant contribution in frames with overall low flux (e.g. 396 nm and 854 nm channels, and the dimmer of the two dual beams). In order to mitigate this, the Data Center is currently using an ad hoc algorithm created by the Polarization Scientist Dave Harrington, that uses the PolCal frames taken at a single slit position. We use the assumption that the modulation should be spectrally constant over the ~1nm bandpass covered within a ViSP camera arm. By normalizing each of the raw PolCal intensity spectra to the mean over all spectral pixels, we get a spectrum compensated for the intensity modulation. Variation in these normalized intensities with wavelength is measure of the stray light impact. Spectral invariance of modulation has been confirmed in each camera arm, and also by comparing the intensity modulation curves of both orthogonally polarized beams recorded strictly simultaneously in the dual beams of each camera. The worst behavior has been observed in the 854nm channel, in one of the two beams as seen below:
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We are working on understanding the response of the DKIST Wavefront Correction System (WFC) to varying atmospheric seeing conditions. The WFC system performance is still in the process of being optimized. The Fried parameter keyword AO___001 within your data set headers provides an estimate of the prevailing seeing condition. Due to technical limitations in the way the estimate is generated by the WFC system, the value provided can be misleading. Therefore, the reconstruction process occasionally fails even if a good Fried parameter is estimated. Furthermore, unrealistic Fried parameters (in the meter range) are estimated whenever the WFC system encounters conditions that are too severe for operation. In that case, a complete image reconstruction is not attempted.
For a movie demonstrating these issues please visit: [LINK]We have encountered unexpected technical issues with the VBI cameras leading to a variety of noise artifacts in the data. This includes an overall dynamic noise pattern in the images which is amplified by the reconstruction process, a vertical stripes pattern in the images, and increased noise at strong gradients in the images as seen in particular in high contrast images (see Ca II K image at [LINK]).
We have developed a variety of algorithms to improve the data quality. These issues continue to be approached from multiple angles to provide further improvements and solutions.In a few rare cases you might notice an overexposure in the G-Band and the Blue Continuum images which worsens during the observing sequence (as the sun rises). If there is a second observe sequence on the observing day, the exposure time will have been adjusted for this second run.
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