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The ViSP instrument scientist has supplied a better approximate calibration of the wavelength in the 3 arms, show below in pseudo IDL code.
630nm = 629.495+findgen(1000)*1.285/1000. ; dispersion 1.285 pm /px
397nm = 396.418+findgen(1000)*0.77/1000. ; dispersion 0.77 pm / px
854nm = 853.182+findgen(1000)*1.882/1000. ; dispersion 1.882 pm / px
The ViSP data has incorrect pointing information in the WCS headers. This is not currently correctable.
Lack of ASDF files
Please note that the current release of ViSP data does not contain ASDF files in the data set directories. This is because of a downstream issues caused by bad WCS headers which prevented the ASDF files from being generated. A fix for the issue has been identified and will be implemented in the next calibration run.
Optical & Algorithm Improvements & Mitigations
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A second source of background light is identified to come from within the beam. Mitigation of this source is in progress but likely not until OCP2. An ad-hoc algorithm was developed and used to fit and subtract this background source using PolCal data .
Algorithm Improvements: Gain Correction & Geometry
During testing, issues were noted with both the lamp and solar gain tasks. Additional algorithms for smoothing and filtering were developed and applied. The algorithm for geometric rotation between beams during the dual-beam merge was improved. The algorithm to compute XY shifts between individual modulation states was also updated.
(see below).
Polarization Accuracy:
Spatial scale for demodulation sampling is yet to-be-finalized. The initial “checkerboard” interpolation pattern, seen in earlier releases of ViSP data has now been fixed. However, several other issues create
We are currently several sources of spurious polarized background signals. Further assessment is necessary to find the appropriate trade-off between errors and smoothness. The QUV continuum error levels are at variable levels around 1%.
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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. It affects all ViSP data currently taken. 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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There’s still some residual difference between the normalized spectra, but overall, the normalized spectra look much more spectrally constant. The resulting modulation curves similarly agree much better in overall contrast and uniformity. Ultimately the best way to deal with this issue is to remove the stray light with appropriate optical aperture stops and masking. Work on this topic is ongoing. The algorithm presented above is not a perfect solution. It is only intended to get data “good enough” at this point, and further tuning of the algorithm settings is necessary. You may notice that there are some line artifacts visible. If you do have any questions or if you see any issues with line signals, you are encouraged to ask (DKIST Help Desk.)
Gain Correction
During calibration testing several issues were noted with both the lamp and solar gain tasks.
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The Solar Gain calculation attempted to preserve detector variations through some complicated interpolation steps. Not only was this fundamentally incorrect but the repeated interpolation of narrow solar spectral lines (especially present in 630nm data) left very large spectral residuals. These residuals also affected.
To remove the optical variations from the Lamp Gain we apply a High-Pass Filter (HPF). The detector variations are, by their very nature, at the highest sampling frequency in the image so a HPF with a very high cutoff frequency can successfully remove optical variations from a gain image, leaving detector variations dominant. Tuning the cutoff frequency of the HPF needs to be done on a per wavelength (arm) basis and assessing the best frequencies to use is still being investigated.
To identify and characterize the solar spectrum in the Solar Gain images we need to account for variations in the spectral shape along the slit that occur as a result of physical non-uniformities in the actual slit construction. In other words we can’t simply take the median spectrum along the slit because the true solar spectrum varies along the slit. To compute the “characteristic spectra” we run a moving 1D Gaussian average along the slit. The width of this Gaussian window is an important tunable parameter.
The core gain algorithms are simple: use a filtered Lamp Gain to remove detector variations, and use a Solar Gain (with solar spectrum removed) to remove several other optical variations. There are some important parameters to tune and further work is necessary to determine the best values for each wavelength region. In some/many cases we may be unable to perfectly remove all optical variations.
Geometric Calculations
Further improvements have been made to the geometric calculations: The algorithm for rotation between beams during the dual-beam merge was improved to fix some failures with some datasets. The algorithm to compute XY shifts between individual modulation states was updated.
Efficiency Drop in 854nm (Arm 3) & 397nm (Arm 2) Beam 2
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