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Accessing online seeing data
At night, the current seeing measurement is updated on the ING Weather Station Mast View (see box below "Wind Gust"). The seeing trend is also graphed on the weather Graph View page (below "Humidity"). 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.
If (Solar) "Radiation" is displayed when you load these pages, it probably means that it is still too early for RoboDIMM measurements to be displayed.
If it is night-time, but you find "n.a." (not available) displayed (or a flat line graph), it probably means that no seeing data was taken in the last 5 minutes. In this case you can use the "last RoboDIMM measurement" link in the Query Database page (see referring page) to see when the last datum was taken.
RoboDIMM data are written automatically to a database that has full and free access. The database is linked through the Seeing homepage (RoboData section, Database Query) or, to open a new window, click here. The time in the database is the Local Time at the midpoint of the sample of images taken by RoboDIMM.
The RoboDIMM measurement represents the best atmospheric seeing obtainable over the preceding minutes, at zenith, and unaffected by telescope effects such as dome seeing, tracking or optical aberration.
Interpretation and calibration
The Weather page reads the time of observation from the RoboDIMM database every 5 minutes and uses the average of all FWHM measurements available over the previous 5 minutes. This average includes the 4 simultaneous FWHM estimates provided by RoboDIMM (see below). The value on the weather page is thus up to 5 minutes "out of date" (or older unless you use Reload on your browser). The only significant differences are in the longer term (say, over 20 minutes or more).
The RoboDIMM measurement represents the best atmospheric seeing obtainable over the preceding minutes, at zenith, and unaffected by telescope effects such as dome seeing, tracking or optical aberrations. 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.
Report on Comparisons between RoboDIMM and other seeing monitors at ORM.
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