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DKIST Data Set Caveats (ViSP / VBI)

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General Caveats

Please note that there are limitations inherent to the Operations Commissioning Phase (shared-risk environment). In the course of the last months we learned a lot more about the instruments and the environment that they are operated in and some prior unknown technical limitations were encountered. For ViSP for example, some of these limitations had an impact on the frame rate and as such the time spent on an individual slit position and map cadences the ViSP can achieve.

In general, summit science operations staff (i.e. resident scientists and science operations specialists) strive to match the requests of any observing proposal the best they can, but there are no guarantees that for example, the lengths of observations, cadences or requested seeing will be 100% consistent with the proposal. If you do have any questions about (summit) science operations and the execution of your observing program(s) you may also contact the DKIST Program Scientist for Operations atritschler@nso.edu or use the DKIST Help Desk.

ViSP Data Set Caveats

These caveats cover all the ViSP datasets released so far through OCP 1.5. There have been many updates to the ViSP pipeline code during the OCP. Many of these have been to fix bugs or update the robustness of key routines such as the geometric correction. Some updates were in response to optical issues discovered during OCP while others were planned as part of ongoing polarization systems calibration work.

Metadata Issues

  • There is a problem with the ViSP CDELT2 keyword (see also the following issue), which is the keyword giving the pixel plate scale along the slit. In the early data that was taken with ViSP this keyword provides a wrong value. We have not yet had the chance to fix it in the files that we made available to you but that is something that may be done in the future. Christian Beck, the ViSP Instrument scientist, has supplied the following correct values for CDELT2.

    • arm 1 630 nm: 0.0298"/pixel

    • arm 2 397 nm: 0.0245"/pixel

    • arm 3 854 nm: 0.0194"/pixel

  • The keywords NAXIS1 and NAXIS2 give the lengths of the spatial and spectral axis of the data array, respectively. However, the keywords CDELT1, CDELT1A, CRPIX1, CRPIX1A, CRVAL1, CRVAL1A, CTYPE1, CTYPE1A, CRDATE1, and CRDATE1A all refer to spectral quantities as opposed to spatial quantities. There may be other keywords that are "transposed" as a result of this that issue. This results in the WCS axis 1 referring to pixel axis 2 in the data array and vice versa.

  • There are issues with the calculated wavelength and dispersion based on the header information in ViSP as illustrated in Figure 1. No attempt is made to correct these at the moment. A wavelength calibration for ViSP data is being investigated for inclusion in a future version of the calibration pipeline.

  • 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

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 (ended 06/17/2022) 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 (see below).

Polarization Accuracy

Spatial scale for demodulation sampling is yet to-be-finalized. 

We are currently investigating several sources of spurious polarized background signals. 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

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 (ended 06/17/2022) will be contain stray light from this source.

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 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:

The algorithm finds a single background unpolarized spectrum, that when subtracted from all the individual modulated spectra, minimizes the spectral variation of the mean-normalized intensities.  These background intensities correlate well with a known stray light optical pathway. An example of the stray light background in the 854nm channel from June is below.  We note that this background is recorded after, and is not impacted by installation of the external baffles discussed above.    

If we then subtract the background signal from all the individual spectra, prior to normalization, then we get the following modulation-normalized spectral shape:

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.)

Efficiency Drop in 854nm (Arm 3) & 397nm (Arm 2)  Beam 2

A polarimetric efficiency 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%.   

Demodulation Sampling

  • 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. 

Current ViSP Polarization Coordinates

In December 2022, Tetsu Anan (tanan@nso.edu), performed an analysis of the current polarization coordinates of the calibrated data from the ViSP instrument. Tetsu compared his analysis to both the Stokes Polarimeter (SP) data from the Hinode Solar Optical Telescope (SOT) and to the Helioseismic and Magnetic Imager (HMI) instrument on the Solar Dynamics Observatory (SDO). The polarization coordinates of Hinode/SOT/SP are detailed in Ichimoto et al. (2008), and is show below together with the derived polarization coordinates of ViSP.

Hinode/SOT/SP is consistent with SDO/HMI and previous instruments such as the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO).

Conclusion 

The signs of Stokes U and V in ViSP are opposite to Hinode/SOT/SP and SDO/HMI. This will be rectified in a future reprocessing to enable more easy comparison to data from other instruments.

References 

Ichimoto, K., Lites, B., Elmore, D. et al. Polarization Calibration of the Solar Optical Telescope onboard Hinode . Sol Phys 249, 233–261 (2008). https://doi.org/10.1007/s11207-008-9169-9

VBI Data Set Caveats

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

Metadata Issues

  • A wrong value is stored in the CRPIX[N] fits header keywords for proposals carried out in OCP 1.2 / 1.3 (February 2022 through April 2022). This should be corrected for runs from April 2022 onward.

Dataset Issues

  • 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.

  • 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. 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.

Using IDL to Read VBI Files

If you want to interact with DKIST data in IDL, you can do this currently using READ_SDO.PRO in SolarSoft. Please note that if you are reading in VBI files using READ_SDO, you will need to use the /USE_SHARED_LIB option.
e.g.

IDL> file = 'atst.ics.vbiBlue.dc.vcc.xfer.74814.100385-1.fits'
IDL> read_sdo, file, index, data, /USE_SHARED_LIB  
% Compiled module: READ_SDO.
atst.ics.vbiBlue.dc.vcc.xfer.74814.100385-1.fits
 ----------------------------------------------------------
| reading atst.ics.vbiBlue.dc.vcc.xfer.74814.100385-1.fits |
 ----------------------------------------------------------

A dedicated READ_DKIST.PRO is being worked on, including a version that will work outside of SolarSoft, and will be available shortly.

Please don't hesitate to contact us if you have questions.

Alisdair Davey
DKIST Data Center Scientist
adavey@nso.edu

Alexandra Tritschler
DKIST Program Scientist for Operations
atritschler@nso.edu

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