Missing Fried Parameter Value in DKIST Level 1 FITS Files

The Fried Parameter (commonly referred to as r₀) is a measure of the quality of optical transmission through the atmosphere due to random inhomogeneities in the atmosphere's refractive index. A Fried parameter larger than 12-15 centimeters can be considered generally excellent seeing. One measurement is reported per L1 FITS file^*.

Open

Figure 1. Sample plot showing the distribution of the Fried Parameter during the observation which results in a single dataset.

From the start of observations until recently, the Data Center included all values of the Fried parameter in its calculations to create the plot shown in Figure 1., which is abstracted from the Quality Report for a given dataset. This resulted in the inclusion of non physical Fried values, which as well as skewing the statistics of the average and standard deviation, produced plots that conveyed little information because of the expanded Fried Parameter axis.

To combat this, and make the reported Fried Parameter more physically useful, the Data Center has imposed the following conditions to determine whether a Fried Parameter is valid and the ATMOS_R0 key is written into the Level 1 FITS headers.

• AO_LOCK must be TRUE. If the AO_LOCK is FALSE then the Fried Parameter values are likely non-physical.

• The value of r₀ must be < 30cm. Above this number the Fried Parameter is regarded as non-physical, even if AO_LOCK is TRUE.

• OOBSHIFT contains the number of sub apertures for which the shift value is out of bounds. This is a measure of ATMOS_R0 reliability. If the number of out-of-bounds sub-apertures is < 100 then the r₀ is considered reliable.

In order for a ATMOS_R0 to be considered reliable then all these condition must be met. It can happen that for a certain period of accumulation resulting in a Level 1 FITS file, these conditions were not met. Rather than include something that is misleading and likely non-physical, the Data Center has chosen not to write the ATMOS_R0 keyword into the file.

^*Note that all valid values of the Fried parameter are recorded in the dataset ASDF file.