At night, the most recent seeing measurement is updated on the ING
Weather Station Mast View.
If (Solar) "Radiation" is displayed when you load this page, it probably
means that it is still too early for RoboDIMM measurements to be displayed.
If you are sure that it is after sunset but you find N/A is displayed, it
could mean that no data was taken in the last 5 minutes and you will have
to use the "last RoboDIMM measurement"link in the Query page (see above)
to see when the last data was taken.
There is a plot of these average values against time of observation on the Graph View page, linked from the Mast View page. Note that the minimum value on the graph is 0.4 arcsec, and if no data is available, the trace (in green) will draw a straight line at 0.4 arcsec.
The seeing estimates produced by RoboDIMM are calculated from image motion, using the approximation of Sarazin and Roddier (1990), and based on the Kolmogorov theory of atmospheric turbulence. The calculated values are essentially a prediction of the image width (FWHM) in a "long" exposure (meaning, more than a few seconds), without correction for diffraction, which is insignificant in large telescopes anyway.
By measuring differential image motion from the 4 images formed on the
detector, RoboDIMM simultaneously produces four independent estimates of
the image FWHM. Since all 4 should be in agreement, the data thus allows
a check that the instrument is working as designed.
Note that data taken before 4 Sep 2002 are not corrected for the airmass
of observation, and such data consequently shows a trend in FWHM with respect
to airmass.
In the near future, it is hoped that the FWHM estimates from RoboDIMM can be calibrated against data from the Wave Front Sensor of NAOMI. However, comparing seeing measurements is problematic (see Tokovinin's recent article in PASP, 114, p1134).The finite (10ms) exposure and application of a threshold before centroiding both work to decrease the DIMM estimate while the presence of CCD readout noise tends to increase it. These effects will have to be taken into account in any eventual cross-calibration.
