The RoboDIMM measurement represents the best atmospheric seeing obtainable on a large telescope over the preceding minutes, observing at zenith and unaffected by real-world effects such as dome seeing, telescope tracking or optical aberrations. It is calculated for an observing wavelength of 500nm.

(a) the measurement takes about 2.5 minutes, so that you can expect to obtain better (or worse) seeing in shorter exposures in the interim (b) the value published on the Met system and Weather page, when runnning live, is updated only every 5 minutes (c) two nearby telescopes will not necessarily measure the same seeing, even when observing the same star at the same time, because they are observing through different parts of the atmosphere and (d) the more variable the seeing the more difficult it is to compare two measurements. From experience, comparing seeing measurements from nearby telescopes is only possible by first averaging measurements over a 15-20 minute period, even in relatively stable conditions.

The seeing estimates produced by RoboDIMM are calculated from image motion, using the approximation from Sarazin and Roddier's DIMM paper (PASP 1990). They are based on the Kolmogorov theory of atmospheric turbulence in the free atmosphere. The calculated values are essentially the expected 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 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. Telescope optical parameters such as the image scale at the focal plane, the entrance subarperture size and separation distance, are the only input values in this calculation, besides the image motion measured by the monitor.

The DIMM method assumes that all of the image motion is caused by Kolmogorov type turbulence. Since it measures differential motion between image spot, telescope movement does not affect the seeing estimate (and trailed images are rejected by RoboDIMM). Exhaustive turbulence analysis using the JOSE camera (1999, MNRAS, 309, 379) has shown that the Kolmogorov theory accurately describes the seeing obtained with the WHT for the great majority of observing time. Non-Kolmogorov turbulence was a negligible component and rarely detected in the samples taken (25 half-nights plus short samples from more than 45 other nights between 1995-99).

For more information, read this report on the Results from the Calibration of ING's RoboDIMM.