This page reports on some exploratory LBT TCS telemetry investigations to see if we can measure primary mirror seeing by using the FWHM of the off-axis guider images.
The first plot attached below includes all LBT guider images from 20211007 through 20220630. Black dots are SX, and green dots are DX. The vertical axis is the primary mirror glass temperature difference relative to local ambient at the mirror cell. The horizontal axis is FWHM of the off-axis guider images as measured by GCS with sextractor. As expected, the best "seeing" (guider image quality) happens when the primary mirrors are near temperature equilibrium with the ambient air. Many things complicate the detailed interpretation of this plot particularly including free atmosphere seeing and other sources of local dome seeing. A number of factors contribute to the total seeing beyond the temperature of the primary mirrors including local turbulence and the temperature of the structure of the telescope and the enclosure. These temperature difference measurements are from the PMC thermistors for glass and ambient air, but the PMC thermcouples might give more accurate differential temperatures (at the expense of more noise). The local ambient measured at the side of the mirror cell is not the same temperature as the external ambient from the weather station on the roof, so the true temperature difference might be underestimated. There are also known issues about some of the guiders being out of focus. This plot might be better analyzed with only images from AGW1 (see additional plots for individual instruments below). The PMC Mirror Ventilation "Offset" parameter has historically been -0.9 degC. It was changed to -1.1 degC during daytime on 20221104.
Data mining from telemetry was done with the python script /lbt/lbto/supportscripts/TelemetrySupport/guidevtemp_dmsextract.py, and the plot was made with the IDL script guidevtemp_dms.pro.
The conventional rule-of-thumb says that 1 degC of temperature difference contribute 0.25 arcsec FWHM of mirror seeing (see Racine, Salmon, Cowley and Sovka, 1991, in PASP 103: pp1020-1032). This plot looks consistent with that rule-of-thumb, but I don't think it allows us to quantify the relationship in a rigorous way. We also see some evidence that the mirror being cooler makes less seeing than when the mirror is warmer (by suppressing convection).
- 27 Oct 2022
- primary temperature difference vs guider fwhm (all instruments):
Two years of data at individual focal stations
The following plots have two full years of data for individual focal stations. These cover Oct 2021 - Jun 2022 and Oct 2020 - Jun 2021 to give us 250k to 550k points per focal station. Red diamonds follow the 3%-ile line in FWHM in bins of temperature difference. Orange triangles follow the 50%-ile line in temperature difference in bins of FWHM.
- Primary temperature difference vs guider FWHM for LUCI1 AGW1 only:
- Primary temperature difference vs guider fwhm for LUCI2 AGW2 only:
- Primary temperature difference vs guider FWHM for MODS1 AGW5 only:
- Primary temperature difference vs guider FWHM for MODS2 AGW6 only:
- Primary temperature difference vs guider FWHM for PEPSIPFU AGW7 only:
- Primary temperature difference vs guider FWHM for PEPSIPFU AGW8 only:
Plots above were made with commands like:
./guidevtemp_dmsextract.py -d 20220701 -e 11 -b 66 -p -f '/luci' -s SX
./guidevtemp_dmsextract.py -d 20220701 -e 11 -b 66 -p -f '/luci' -s DX
which use the DMS-based telemetry extraction to make the csv data files.
The data file must be made one year at a time with commands like:
./guidevtemp_dmsextract.py -d 20220630 -e 11 -b 6600 -n
./guidevtemp_dmsextract.py -d 20210630 -e 11 -b 6600 -n
as merging of the telemetry takes a long time (hours). Then the four resulting .tmp files were manually merged to make the two-year SX and DX files.