Data Set Caveats (OCP 1.2 / 1.3)
The Data Center is aware that there are a number of issues with the ViSP data sets that were taken for you. In this latest run, fixes were made to:
Remove the discontinuous checkerboard pattern observed in the data when performing the field dependent Polarization Calibration. We believe this was because of the interpolation method used when resampling the demodulation matrices. We believe this is fully corrected in the spatial direction but think it may not be properly corrected in the spectral direction. This is being further investigated.
Improve the beam combination to make it more robust. This was particularly noticeable in the 630 nm channel where there was clearly visible ghosting of lines. This has been corrected in this latest run.
In addition, there are issues with the data that have not yet been addressed and some of which may never be able to be corrected:
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The initial calibration runs of ViSP data discovered that there was a strong component of parasitic light observed in all ViSP data taken. Although this has subsequently been mitigated, the issue remains in early data sets and the data center will not undertake a correction for it.
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General Caveats
Please not 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.
In addition, 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
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.
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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 code.
630 630nm = 629.495+findgen(1000)*1.285/1000. ; dispersion 1.285 pm /px
397 397nm = 396.418+findgen(1000)*0.77/1000. ; dispersion 0.77 pm / px
854 854nm = 853.182+findgen(1000)*1.882/1000. ; dispersion 1.882 pm / px
Data Issues
Stray Light
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 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.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]
The Data Center is aware that there are a number of issues with the ViSP data sets that were taken for you. In this latest run, fixes were made to:
Remove the discontinuous checkerboard pattern observed in the data when performing the field dependent Polarization Calibration. We believe this was because of the interpolation method used when resampling the demodulation matrices. We believe this is fully corrected in the spatial direction but think it may not be properly corrected in the spectral direction. This is being further investigated.
Improve the beam combination to make it more robust. This was particularly noticeable in the 630 nm channel where there was clearly visible ghosting of lines. This has been corrected in this latest run.
In addition, there are issues with the data that have not yet been addressed and some of which may never be able to be corrected:
The initial calibration runs of ViSP data discovered that there was a strong component of parasitic light observed in all ViSP data taken. Although this has subsequently been mitigated, the issue remains in early data sets and the data center will not undertake a correction for it.
Please be aware that the V->Q, V->U cross-talk is likely quite large at least in the 630nm channel. The Data Center Calibrations Team are working with the ViSP Instrument Scientist to quantify this, and it will likely be addressed in upcoming pipeline software releases. For the moment, the user will need to apply corrections for this themselves.
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