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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.
Optical & Algorithm Improvements & Mitigations
Data Issues
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Stray Light
Testing had identified multiple sources of stray light in ViSP. Some stray light enters from the sides and top at the end of the camera arms that were open to the environment. Installation of various baffles was done between May and June. These were successful in reducing the stray light from external sources. Data taken through at least OCP 1.4 will be contain stray light from this source.
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.
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 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%.
Please check the quality report for your data set to note any warning flags and fit failures in PolCal fitting outputs. We have seen data sets where certain variables (transmission of polarization calibration optics) are far away from metrology expectations. There is an expectation of higher than nominal cross-talk levels in these data sets.
We also note that relatively low modulation efficiency is seen in one of the two dual beams for the 397 nm and 854 nm channels (likely due to optics in those arms). Assessment is in progress.
Detailed Description of Data Processing & Optical Issues
Stray Light
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Data analysis and testing has identified multiple sources of stray light in ViSP. One set of tests done with showed that a significant part of the stray light enters from the sides and top at the end of the camera arms that were open to the environment. Reduction of stray light to about the same level as during a dark current observation with the GOS dark shutter in the beam was only possible when both the camera arms and camera lenses where completely covered. Installation of various baffles was done between May and June. These were successful in reducing the stray light from external sources. Data taken through at least OCP 1.4 will be contain stray light from this source.
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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
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Correction
During calibration testing several issues were noted with both the lamp and solar gain tasks.
Lamp gains were being used directly. The lamp was designed to be bright, (1% to 15% of the full solar on-disk DKIST beam) but this came at the cost of spatial uniformity. Many with known spatial variation of intensity well above that of the rest of the optical system. Spatial smoothing of the lamp gains have their own strong optical response and thus this response was affecting all downstream data (both PolCal and science)was implemented.
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 possible in the image so a HPF with a very high cutoff frequency can successfully remove all optical variations from a gain image, leaving only detector variations dominant. Tuning the cutoff frequency of the HPF needs to be done on a per wavelength (arm) basis and researching assessing the best frequencies to use is still being investigated.
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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
An A polarimetric efficiency issue with beam 2 / arm3 (currently 854nm) have drop with one of the two beams in both arms 2 and 3 has been noted.
854nm Beam2 (Arm3) has anomalously low modulation efficiency (35% vs 50%). 397nm Beam2 (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%.
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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 background signals. Further assessment is necessary to find the appropriate trade-off between errors and smoothness. The QUV continuum levels are at variable levels around 1%. We are currently investigating different observing techniques to also improve the polarization zero point performance.
Cross-talk Possibilities & PolCal Fitting Residuals
Please check the quality report for your data set to note any failures in PolCal fitting outputs. We have seen data sets where certain variables (transmission of polarization calibration optics) are far away from metrology expectations. For example, in some cases the variable representing the polarizer transmission is fitted to be near 100% transmission when the optic is known to be 91.5% transmission +/-0.3%. This fitting error can create cross-talk of all types through the correlation between fitted variables. The settings for polarization calibration are currently being investigated, and we expect improvements in the demodulation matrix accuracy as the algorithms are tuned.
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